EC 25 ELECTRIC CIRCUITS

AND

ELECTRON DEVICES

SEM:II Branch: CSE

Staff-in-Charge: M.UMA SORNA RANI

UNIT I CIRCUIT ANALYSIS TECHNIQUES

INTRODUCTION

Circuit Definitions

• Node – any point where 2 or more circuit elements are connected together

– Wires usually have negligible resistance

– Each node has one voltage (w.r.t. ground)

• Branch – a circuit element between two nodes

• Loop – a collection of branches that form a closed path returning to the same node

without going through any other nodes or branches twice

• Voltage-current characteristic of ideal resistor:

• A Node is a point of connection between two or more circuit elements

• Nodes can be “spread out” by perfect conductors

)()( tiRtv

Kirchoff’s Current Law (KCL)

• The algebraic sum of all currents entering (or leaving) a node is zero

• Equivalently: The sum of the currents entering a node equals the sum of the

currents leaving a node

• Mathematically:

• When applying KCL, the current directions (entering or leaving a node) are based on

the assumed directions of the currents

• Also need to decide whether currents entering the node are positive or negative;

this dictates the sign of the currents leaving the node

• As long all assumptions are consistent, the final result will reflect the actual

current directions in the circuit

Kirchoff’s Voltage Law (KVL)

The algebraic sum of all voltage differences around any closed loop is zero

Equivalently: The sum of the voltage rises around a closed loop is equal to the sum of the

voltage drops around the loop

Mathematically:

Voltage polarities are based on assumed polarities

If assumptions are consistent, the final results will reflect the actual polarities

The algebraic sum of voltages around each loop is zero

Beginning with one node, add voltages across each branch in the loop (if you encounter a

+ sign first) and subtract voltages (if you encounter a – sign first)

Σ voltage drops - Σ voltage rises = 0

Or Σ voltage drops = Σ voltage rises

N

k

k ti1

0)(

N

k

k tv1

0)(

NETWORK THEOREMS

• This chapter introduces important fundamental theorems of network analysis. They are

the

• Superposition theorem

• Thévenin‟s theorem

• Norton‟s theorem

• Maximum power transfer theorem

Superposition Theorem

Used to find the solution to networks with two or more sources that are not in series or

parallel.

The current through, or voltage across, an element in a network is equal to the algebraic

sum of the currents or voltages produced independently by each source.

Since the effect of each source will be determined independently, the number of

networks to be analyzed will equal the number of sources.

The total power delivered to a resistive element must be determined using the total

current through or the total voltage across the element and cannot be determined by a

simple sum of the power levels established by each source.

Thévenin’s Theorem

Any two-terminal dc network can be replaced by an equivalent circuit consisting of a

voltage source and a series resistor.

Thévenin‟s theorem can be used to:

Analyze networks with sources that are not in series or parallel.

Reduce the number of components required to establish the same characteristics at the

output terminals.

Investigate the effect of changing a particular component on the behavior of a network

without having to analyze the entire network after each change.

Procedure to determine the proper values of RTh and ETh

Preliminary

Remove that portion of the network across which the Thévenin equation circuit is to be

found. In the figure below, this requires that the load resistor RL be temporarily removed

from the network.

Mark the terminals of the remaining two-terminal network. (The importance of this step

will become obvious as we progress through some complex networks.)

RTh:

Calculate RTh by first setting all sources to zero (voltage sources are replaced by short

circuits, and current sources by open circuits) and then finding the resultant resistance

between the two marked terminals. (If the internal resistance of the voltage and/or current

sources is included in the original network, it must remain when the sources are set to

zero.)

ETh:

Calculate ETh by first returning all sources to their original position and finding the open-

circuit voltage between the marked terminals. (This step is invariably the one that will

lead to the most confusion and errors. In all cases, keep in mind that it is the open-circuit

potential between the two terminals marked in step 2.)

Draw the Thévenin equivalent circuit with the portion of the circuit previously removed

replaced between the terminals of the equivalent circuit. This step is indicated by the

placement of the resistor RL between the terminals of the Thévenin equivalent circuit.

Norton’s Theorem

Norton‟s theorem states the following:

Any two-terminal linear bilateral dc network can be replaced by an equivalent circuit

consisting of a current and a parallel resistor.

The steps leading to the proper values of IN and RN.

Preliminary steps:

Remove that portion of the network across which the Norton equivalent circuit is found.

Mark the terminals of the remaining two-terminal network.

Finding RN:

Calculate RN by first setting all sources to zero (voltage sources are replaced with short

circuits, and current sources with open circuits) and then finding the resultant resistance

between the two marked terminals. (If the internal resistance of the voltage and/or

current sources is included in the original network, it must remain when the sources are

set to zero.) Since RN = RTh the procedure and value obtained using the approach

described for Thévenin‟s theorem will determine the proper value of RN.

Finding IN :

Calculate IN by first returning all the sources to their original position and then finding

the short-circuit current between the marked terminals. It is the same current that would

be measured by an ammeter placed between the marked terminals.

Conclusion:

Draw the Norton equivalent circuit with the portion of the circuit previously removed

replaced between the terminals of the equivalent circuit.

Maximum Power Transfer Theorem

For loads connected directly to a dc voltage supply, maximum power will be delivered to

the load when the load resistance is equal to the internal resistance of the source; that is,

when: RL = Rint

The maximum power transfer theorem states the following:

A load will receive maximum power from a network when its total resistive value is

exactly equal to the Thévenin resistance of the network applied to the load. That is,

RL = RTh

Series resistors & voltage division

Series: Two or more elements are in series if they are cascaded or connected sequentially

and consequently carry the same current.

The equivalent resistance of any number of resistors connected in a series is the sum of

the individual resistances

Parallel resistors & current division

Parallel: Two or more elements are in parallel if they are connected to the same two nodes and

consequently have the same voltage across them.

The equivalent resistance of a circuit with N resistors in parallel is:

N

n

nNeq RRRRR1

21

v21

22

RR

Rv

v21

11

RR

Rv

Neq RRRR

1111

21

iRR

Ri

21

12

iRR

Ri

21

21

Delta -> Star transformation

Star -> Delta transformation

2

133221

R

RRRRRRRb

)(1

cba

cb

RRR

RRR

)(2

cba

ac

RRR

RRR

)(3

cba

ba

RRR

RRR

1

133221

R

RRRRRRRa

3

133221

R

RRRRRRRc

UNIT II TRANSIENT RESONANCE IN RLC CIRCUITS

INTRODUCTION

The fundamental passive linear circuit elements are the

resistor (R),

capacitor (C)

inductor (L).

These circuit elements can be combined to form an electrical circuit in four distinct ways:

the RC circuit,

the RL circuit,

the LC circuit

the RLC circuit

These circuits exhibit important types of behaviour that are fundamental to analogue

electronics.

RL CIRCUIT

A resistor-inductor circuit (RL circuit), or RL filter or RL network, is one of the simplest

analogue infinite impulse response electronic filters. It consists of a resistor and an inductor,

either in series or in parallel, driven by a voltage source.

The complex impedance ZL (in ohms) of an inductor with inductance L (in henries) is

The complex frequency s is a complex number,

where

j represents the imaginary unit:

j2 = − 1

is the exponential decay constant (in radians per second), and

is the angular frequency (in radians per second).

RC circuit

• The simplest RC circuit is a capacitor and a resistor in series.

• When a circuit composes of only a charged capacitor and a resistor,

• then the capacitor would discharge its energy into the resistor.

• This voltage across the capacitor over time could be found through KCL, where the

current coming out of the capacitor must equal the current going through the resistor.

This results in the linear differential equation

Natural response

• The simplest RC circuit is a capacitor and a resistor in series.

• When a circuit composes of only a charged capacitor and a resistor,

• then the capacitor would discharge its energy into the resistor.

• This voltage across the capacitor over time could be found through KCL,

• where the current coming out of the capacitor must equal the current going through the

resistor.

• This results in the linear differential equation

RLC circuit

Time dependences

Now we study an AC circuit, where the resistor R, coil L, and capacitor C are in series

connection. The circuit is ideal, because the internal resistances of the coil and the capacitor are

ignored. The connections are given in figure 10-1. The power source gives a periodic voltage, u

= ûsint, where the frequency, f = /2, can be adjusted.

Figure 10-1. RLC circuit

When the circuit is closed and the system has stabilized, the current is given by (compare with

eqn. U = RI I = U/R)

),sin()( tZ

ûti

where

22 )1

(C

LRZ

)/1

arctan(R

CL

.

The impedance Z and phase difference (between voltage and current) depend on the frequency

of the source voltage. The angle actually tells how much the current i comes after the total

voltage (source voltage) u. The time dependences of i and u are given in figure 10-2, where >

0.

Resonance

The circuit is said to be in resonance, if the impedance is the same as R, i.e. the effects due to the

inductance and the capacitance cancel each other. In the resonance, the power transferred from

the source to the circuit is in maximum. From figure 10-3 is seen that the requirement for

resonance is

L = 1/C

From the same figure is seen that = 0 and Z = Zmin. The current is given by

I = U/Z = U/R¨= Imax.

Q factor

. The ratio of the inductance L to the resistance R of a coil remains constant for different winding

arrangements in the same volume and shape. It makes sense to define this value as a figure of

merit to distinguish different coil structures. The quality factor Q is defined by this ratio.

The voltage, which is induced by the same current in an inductor scales with the frequency f and

thus the apparent power in the device. The general definition of the quality factor is based on the

ratio of apparent power to the power losses in a device. From this definition, the quality factor of

a coil results to:

with ω = 2πf

RC and RL Transient Analysis Basics

Transient State:

If a network contains energy storage elements, with change in excitation, the current and

voltages change from one state to another state is called transient state. The behavior of the

voltage or current when it is changed from one state to another state is called transient state.

Transient Time:

The time taken for the circuit to change from one steady state to another steady state is

called the transient time.

Natural response:

If we consider a circuit containing storage elements which are independent of sources, the

response depends upon the nature of the circuit, it is called natural response.

Transient response:

The storage elements deliver their energy to the resistances, hence the response changes

with time, gets saturated after sometime, and is referred to the transient response.

Laplace Transform:

The Laplace transform of any time dependent function f(t) is given by F(s).

Where S→A complex frequency given by S=σ + jω

Inverse Laplace Transform:

Inverse Laplace transforms permits going back in the reverse direction i.e. from s domain

to time domain.

Order of a System:

The order of the system is given by the order of the differential equation governing the

system. If the system is governed by nth

order differential equation, than the system is called nth

order system.

Q(s) = a0 sn + a1 s

n-1+ a2 s

n-2 + ……..+an-1 s +an

the order of the system is equal to „n‟.

Initial Value Theorem

The initial value theorem states that if x (t) and x‟ (t) both are laplace transformable, then

Final Value Theorem

The final value theorem states that if x (t) and x‟ (t) both are laplace transformable, then

Driving Point impedance

The ratio of the Laplace transform of the voltage at the port to the laplace transform of

the current at the same port is called driving point impedance.

Transfer Point impedance

The ratio of the voltage transform at one port to the current transform at the other port is

called transfer point impedance.

Resonant Circuit

The circuit that treat a narrow range of frequencies very differently than all other

frequencies are referred to as resonant circuit.

The gain of a highly resonant circuit attains a sharp maximum or minimum at its resonant

frequency.

Resonance

Resonance is defined as a phenomenon in which applied voltage and resulting current are

in phase.

Bandwidth

The Bandwidth is defined as the frequency difference between upper cut-off frequency

(f2) and lower cut-off frequency (f1).

Half Power frequencies

The upper and lower cut-off frequencies are called the half-power frequencies. At these

frequencies the power from the source is half of the power delivered at the resonant frequency.

Selectivity

Selectivity is defined as the ratio of bandwidth to the resonant frequency of resonant

circuit.

Q factor

The quality factor, Q, is the ratio of the reactive power in the inductor or capacitor to the

true power in the resistance in series with the coil or capacitor.

Series Resonance in RLC circuit

In series RLC circuit resonance may be produced by either varying frequency for given

constant values of L and C or varying either L and C or both for a given frequency.

At resonance inductive reactance is equal to the capacitive reactance.

If f < f0 the current I leads the resultant supply voltage V and so the circuit behaves as a

capacitive circuit at the frequencies which are less than f0.

At f = f0, the voltage and current are in phase. The circuit behaves as pure resistive circuit

at the resonant frequency with unit power factor.

If f > f0, the current I lags the resultant supply voltage V and so the circuit behaves as an

inductive circuit at the frequencies which are more than f0.

At resonance series RLC circuit acts as a voltage amplifier.

Series resonance circuit is always driven by a voltage source with very small internal

resistance to maintain high selectivity of the circuit.

Parallel Resonance

A parallel circuit is said to be in resonance when applied voltage and resulting current are

in phase that gives unity power factor condition.

Parallel resonance is also known as Anti resonance.

At anti resonance the parallel resonant circuit acts as current amplifier.

Reactance curves

The graph of individual reactance versus the frequency is called Reactance Curve.

Types of Tuned circuits

Single tuned circuit

Double tuned circuit

Single tuned circuit

In RF circuit design, tuned circuits are generally employed for obtaining maximum

power transfer to the load connected to secondary or for obtaining maximum possible value of

secondary voltage.

A single tuned circuit is used for coupling an amplifier and radio receiver circuits.

Double tuned circuit

In double tuned circuits, a variable capacitor is used at input as well as output side.

With the help of adjustable capacitive reactance, impedance matching is possible if the

coupling is critical, sufficient or above.

It is also possible to adjust phase angle such that impedance at generator side becomes

resistive.

The magnitude matching can be achieved by adjusting mutual inductance to the critical

value, which effectively fulfills maximum power transfer condition.

UNIT III SEMICONDUCTOR DIODES

Review of intrinsic and extrinsic semiconductors

Intrinsic semiconductor

An intrinsic semiconductor is one, which is pure enough that impurities do

not appreciably affect its electrical behavior. In this case, all carriers are created due to thermally

or optically excited electrons from the full valence band into the empty conduction band. Thus

equal numbers of electrons and holes are present in an intrinsic semiconductor. Electrons and

holes flow in opposite directions in an electric field, though they contribute to current in the

same direction since they are oppositely charged. Hole current and electron current are not

necessarily equal in an intrinsic semiconductor, however, because electrons and holes have

different effective masses (crystalline analogues to free inertial masses).

The concentration of carriers is strongly dependent on the temperature. At low

temperatures, the valence band is completely full making the material an insulator. Increasing the

temperature leads to an increase in the number of carriers and a corresponding increase in

conductivity. This characteristic shown by intrinsic semiconductor is different from the behavior

of most metals, which tend to become less conductive at higher temperatures due to increased

phonon scattering.

Both silicon and germanium are tetravalent, i.e. each has four electrons (valence

electrons) in their outermost shell. Both elements crystallize with a diamond-like structure, i.e. in

such a way that each atom in the crystal is inside a tetrahedron formed by the four atoms which

are closest to it. Each atom shares its four valence electrons with its four immediate

neighbours, so that each atom is involved in four covalent bonds.

Extrinsic semiconductor

An extrinsic semiconductor is one that has been doped with impurities to modify the

number and type of free charge carriers. An extrinsic semiconductor is a semiconductor that has

been doped, that is, into which a doping agent has been introduced, giving it different electrical

properties than the intrinsic (pure) semiconductor.

Doping involves adding dopant atoms to an intrinsic semiconductor, which changes the

electron and hole carrier concentrations of the semiconductor at thermal equilibrium. Dominant

carrier concentrations in an extrinsic semiconductor classify it as either an n-type or p-type

semiconductor. The electrical properties of extrinsic semiconductors make them essential

components of many electronic devices.

A pure or intrinsic conductor has thermally generated holes and electrons. However

these are relatively few in number. An enormous increase in the number of charge carriers can

by achieved by introducing impurities into the semiconductor in a controlled manner. The result

is the formation of an extrinsic semiconductor. This process is referred to as doping. There are

basically two types of impurities: donor impurities and acceptor impurities. Donor impurities are

made up of atoms (arsenic for example) which have five valence electrons. Acceptor impurities

are made up of atoms (gallium for example) which have three valence electrons.

The two types of extrinsic semiconductor

N-type semiconductors

Extrinsic semiconductors with a larger electron concentration than hole concentration

are known as n-type semiconductors. The phrase 'n-type' comes from the negative charge of the

electron. In n-type semiconductors, electrons are the majority carriers and holes are the minority

carriers. N-type semiconductors are created by doping an intrinsic semiconductor with donor

impurities. In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsic

semiconductor and lies closer to the conduction band than the valence band.

Arsenic has 5 valence electrons, however, only 4 of them form part of covalent

bonds. The 5th

electron is then free to take part in conduction. The electrons are said to be the

majority carriers and the holes are said to be the minority carriers.

P-type semiconductors

As opposed to n-type semiconductors, p-type semiconductors have a larger hole

concentration than electron concentration. The phrase 'p-type' refers to the positive charge of the

hole. In p-type semiconductors, holes are the majority carriers and electrons are the minority

carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor

impurities. P-type semiconductors have Fermi energy levels below the intrinsic Fermi energy

level. The Fermi energy level lies closer to the valence band than the conduction band in a p-type

semiconductor.

Gallium has 3 valence electrons, however, there are 4 covalent bonds to fill. The 4th

bond therefore remains vacant producing a hole. The holes are said to be the majority carriers

and the electrons are said to be the minority carriers

Theory of PN junction diode

On its own a p-type or n-type semiconductor is not very useful. However when combined very

useful devices can be made. The p-n junction can be formed by allowing a p-type material to

diffuse into a n-type region at high temperatures.

The p-n junction has led to many inventions like the diode, transistors and integrated circuits.

Free electrons on the n-side and free holes on the p-side can initially diffuse across the

junction. Uncovered charges are left in the neighbourhood of the junction. This region is

depleted of mobile carriers and is called the DEPLETION REGION (thickness 0.5 – 1.0 µm).

The diffusion of electrons and holes stop due to the barrier potential difference (p.d across the

junction) reaching some critical value. The barrier potential difference (or the contact potential)

depends on the type of semiconductor, temperature and doping densities.

At room temperature, typical values of barrier p.d. are:

Ge ~ 0.2 – 0.4 V

Si ~ 0.6 – 0.8 V

FORWARD BIAS P-N JUNCTION

When an external voltage is applied to the P-N junction making the P side positive with

respect to the N side the diode is said to be forward biased (F.B). The barrier p.d. is decreased

by the external applied voltage. The depletion band narrows which urges majority carriers to

flow across the junction. A F.B. diode has a very low resistance.

REVERSE BIAS P-N JUNCTION

When an external voltage is applied to the PN junction making the P side negative with

respect to the N side the diode is said to be Reverse Biased (R.B.). The barrier p.d. increases.

The depletion band widens preventing the movement of majority carriers across the junction.

A R.B. diode has a very high resistance.

REVERSE BIAS P-N JUNCTION

Only thermally generated minority carriers are urged across the p-n junction. Therefore the

magnitude of the reverse saturation current (or reverse leakage current) depends on the

temperature of the semiconductor.

When the PN junction is reversed biased the width of the depletion layer increases, however

if the reverse voltage gets too large a phenomenon known as diode breakdown occurs.

I-V CHARACTERISTICS

I-V CHARACTERISTICS

When the diode is F.B., the current increases exponentially with voltage except for a small

range close to the origin.

When the diode is R.B., the reverse current is constant and independent of the applied reverse

bias.

Turn-on or cut-in (threshold) voltage Vγ: for a F.B. diode it is the voltage when the current

increases appreciably from zero.

It is roughly equal to the barrier p.d.:

For Ge, V γ ~ 0.2 – 0.4 V (at room temp.)

For Si, Vγ ~ 0.6 – 0.8 V (at room temp.)

Energy Band structure

The highest electronic energy band in a semiconductor or insulator which can be filled with

electrons. The electrons in the valence band correspond to the valence electrons of the

constituent atoms. In a semiconductor or insulator, at sufficiently low temperatures, the valence

band is completely filled and the conduction band is empty of electrons. Some of the high energy

levels in the valence band may become vacant as a result of thermal excitation of electrons to

higher energy bands or as a result of the presence of impurities. The net effect of the valence

band is then equivalent to that of a few particles which are equal in number and similar in motion

to the missing electrons but each of which carries a positive electronic charge. These “particles”

are referred to as holes.

In solids, the valence band is the highest range of electron energies in which electrons are

normally present at absolute zero temperature.The valence electrons are bound to individual

atoms, as opposed to conduction electrons (found in conductors and semiconductors), which can

move freely within the atomic lattice of the material. On a graph of the electronic band structure

of a material, the valence band is located below the conduction band, separated from it in

insulators and semiconductors by a band gap. In metals, the conduction band has no energy gap

separating it from the valence band.

Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region are

degenerately doped.

Zener diode

A Zener diode is a type of diode that permits current not only in the forward direction

like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown

voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence

Zener, who discovered this electrical property.

A conventional solid-state diode will not allow significant current if it is reverse-biased

below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a

conventional diode is subject to high current due to avalanche breakdown. Unless this current is

limited by circuitry, the diode will be permanently damaged due to overheating. In case of large

forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its

junction built-in voltage and internal resistance. The amount of the voltage drop depends on the

semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially

designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By

contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled

breakdown and allow the current to keep the voltage across the Zener diode close to the Zener

voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage

drop of very nearly 3.2 V across a wide range of reverse currents. The Zener diode is therefore

ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage),

or as a voltage stabilizer for low-current applications.

Zener diode characteristics

A zener diode is much like a normal diode, the exception being is that it is placed in the

circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve

illustrates the operating range for a zener. Note that its forward characteristics are just like a

normal diode.

The zener diode‟s breakdown characteristics are determined by the doping

process. Low voltage zeners (>5V), operate in the zener breakdown range. Those designed to

operate <5 V operate mostly in avalanche breakdown range. Zeners are available with voltage

breakdowns of 1.8 V to 200 V.

Note very small reverse current (before “knee”).

Breakdown occurs @ knee.

Breakdown Characteristics:

• VZ remains near constant

• VZ provides:

-Reference voltage

-Voltage regulation

• IZ escalates rapidly

• IZMAX is achieved quickly

• Exceeding IZMAX is fatal

Zener Diodes – Equivalent Circuit

• Ideal Zener exhibits a constant voltage, regardless of current draw.

• Ideal Zener exhibits no resistance characteristics.

• Zener exhibits a near constant voltage, varied by current draw through the series

resistance ZZ.

• As Iz increases, Vz also increases.

Zener Diodes – Characteristic Curve

• Vz results from Iz.

• Iz thru Zz produce this.

Zener diodes have given characteristics such as;

• Temperature coefficients – describes the % Vz for Temp (0C)

Vz = Vz x T0C

x T %/oC

Power ratings – the zener incurs power dissipation based on Iz and Zz P = I2Z

Power derating factor specifies the reduced power rating for device operating temperatures in

excess of the “rated maximum temperature”.

PD(derated) = PD(max) – (mW/0C)T mW

UNIT IV TRANSISTORS

TRANSISTOR CHARACTERISTICS:

The basic of electronic system nowadays is semiconductor device.

The famous and commonly use of this device is BJTs

(Bipolar Junction Transistors).

It can be use as amplifier and logic switches.

BJT consists of three terminal:

collector : C

base : B

emitter : E

Two types of BJT : pnp and npn

Transistor Construction

• 3 layer semiconductor device consisting:

– 2 n- and 1 p-type layers of material npn transistor

– 2 p- and 1 n-type layers of material pnp transistor

• The term bipolar reflects the fact that holes and electrons participate in the injection

process into the oppositely polarized material

• A single pn junction has two different types of bias:

– forward bias

– reverse bias

• Thus, a two-pn-junction device has four types of bias.

Position of the terminals and symbol of BJT.

• Base is located at the middle and more thin from the level of collector

and emitter

• The emitter and collector terminals are made of the same type of

semiconductor material, while the base of the other type of material

Transistor currents

- The arrow is always drawn on the emitter The arrow always point

toward the n-type

- The arrow indicates the direction of the emitter current:

pnp:E B

npn: B E

IC=the collector current

IB= the base current

IE= the emitter current

Transistor Operation

• The basic operation will be described using the pnp transistor. The operation of the pnp

transistor is exactly the same if the roles played by the electron and hole are interchanged.

• One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased

Forward-biased junction Reverse-biased junction of a pnp transistor

of a pnp transistor

• Both biasing potentials have been applied to a pnp transistor and resulting majority and

minority carrier flows indicated.

• Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type

material.

• A very small number of carriers (+) will through n-type material to the base terminal.

Resulting IB is typically in order of microamperes.

• The large number of majority carriers will diffuse across the reverse-biased junction into

the p-type material connected to the collector terminal.

• Majority carriers can cross the reverse-biased junction because the injected majority

carriers will appear as minority carriers in the n-type material.

• Applying KCL to the transistor :

IE = IC + IB

• The comprises of two components – the majority and minority carriers

IC = ICmajority + ICOminority

• ICO – IC current with emitter terminal open and is called leakage current.

COMMON BASE CONFIGURATION

• Common-base terminology is derived from the fact that the :

- base is common to both input and output of the configuration.

- base is usually the terminal closest to or at ground potential.

• All current directions will refer to conventional (hole) flow and the arrows in all

electronic symbols have a direction defined by this convention.

• Note that the applied biasing (voltage sources) are such as to establish current in the

direction indicated for each branch.

• To describe the behavior of common-base amplifiers requires two set of characteristics:

- Input or driving point characteristics.

- Output or collector characteristics

• The output characteristics has 3 basic regions:

- Active region –defined by the biasing arrangements

- Cutoff region – region where the collector current is 0A

- Saturation region- region of the characteristics to the left of VCB = 0V

• The curves (output characteristics) clearly indicate that a first approximation to the

relationship between IE and IC in the active region is given by

IC ≈IE

• Once a transistor is in the „on‟ state, the base-emitter voltage will be assumed to be

VBE = 0.7V

• In the dc mode the level of IC and IE due to the majority carriers are related by a quantity

called alpha

=

IC = IE + ICBO

E

C

I

I

• It can then be summarize to IC = IE (ignore ICBO due to small value)

• For ac situations where the point of operation moves on the characteristics curve, an ac

alpha defined by

• Alpha a common base current gain factor that shows the efficiency by calculating the

current percent from current flow from emitter to collector.The value of is typical from

0.9 ~ 0.998.

COMMON EMITTER CONFIGURATION

• It is called common-emitter configuration since :

- emitter is common or reference to both input and output terminals.

- emitter is usually the terminal closest to or at ground potential.

• Almost amplifier design is using connection of CE due to the high gain for current and

voltage.

• Two set of characteristics are necessary to describe the behavior for CE ;input (base

terminal) and output (collector terminal) parameters.

E

C

I

I

Input characteristics for a common-emitter NPN transistor

Output characteristics for a common-emitter npn transistor

• For small VCE (VCE < VCESAT, IC increase linearly with increasing of VCE

• VCE > VCESAT IC not totally depends on VCE constant IC

• IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC

• IB=0 A ICEO occur.

• Noticing the value when IC=0A. There is still some value of current flows.

COMMON COLLECTOR CONFIGURATION

• Also called emitter-follower (EF).

• It is called common-emitter configuration since both the

signal source and the load share the collector terminal as a common connection point.

• The output voltage is obtained at emitter terminal.

• The input characteristic of common-collector configuration is similar with common-

emitter. configuration.

• Common-collector circuit configuration is provided with the load resistor connected from

emitter to ground.

• It is used primarily for impedance-matching purpose since it has high input impedance

and low output impedance.

Notation and symbols used with the common-collector configuration:

(a) pnp transistor ; (b) npn transistor

• For the common-collector configuration, the output characteristics are a plot of IE vs VCE for a

range of values of IB.

Limits of Operation

• Many BJT transistor used as an amplifier. Thus it is

important to notice the limits of operations.

• At least 3 maximum values is mentioned in data sheet.

• There are:

a) Maximum power dissipation at collector: PCmax or PD

b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO.

c) Maximum collector current: ICmax

• There are few rules that need to be followed for BJT

transistor used as an amplifier. The rules are:

i) transistor need to be operate in active region!

ii) IC < ICmax

ii) PC < PCmax

FIELD EFFECT TRANSISTOR (FET)

• Field effect devices are those in which current is controlled by the action of an electron

field, rather than carrier injection.

• Field-effect transistors are so named because a weak electrical signal coming in through

one electrode creates an electrical field through the rest of the transistor.

• The FET was known as a “unipolar” transistor.

• The term refers to the fact that current is transported by carriers of one polarity

(majority), whereas in the conventional bipolar transistor carriers of both polarities

(majority and minority) are involved.

The family of FET devices may be divided into :

• Junction FET

• Depletion Mode MOSFET

Enhancement Mode MOSFET

Junction FETs (JFETs

• JFETs consists of a piece of high-resistivity semiconductor material (usually Si) which

constitutes a channel for the majority carrier flow.

• Conducting semiconductor channel between two ohmic contacts – source & drain

• JFET is a high-input resistance device, while the BJT is comparatively low.

• If the channel is doped with a donor impurity, n-type material is formed and the channel

current will consist of electrons.

• If the channel is doped with an acceptor impurity, p-type material will be formed and the

channel current will consist of holes.

N-channel devices have greater conductivity than p-channel types, since electrons have higher

mobility than do holes; thus n-channel JFETs are approximately twice as efficient conductors

compared to their p-channel counterparts

• The magnitude of this current is controlled by a voltage applied to a gate, which is a

reverse-biased.

• The fundamental difference between JFET and BJT devices: when the JFET junction is

reverse-biased the gate current is practically zero, whereas the base current of the BJT is

always some value greater than zero.

Basic structure of JFETs

• In addition to the channel, a JFET contains two ohmic contacts: the source and the drain.

• The JFET will conduct current equally well in either direction and the source and drain

leads are usually interchangeable.

N-channel JFET

• This transistor is made by forming a channel of N-type material in a P-type substrate.

• Three wires are then connected to the device.

• One at each end of the channel.

• One connected to the substrate.

In a sense, the device is a bit like a PN-junction diode, except that there are two wires

connected to the N-type side

• The gate is connected to the source.

• Since the pn junction is reverse-biased, little current will flow in the gate connection.

• The potential gradient established will form a depletion layer, where almost all the

electrons present in the n-type channel will be swept away.

The most depleted portion is in the high field between the G and the D, and the least-depleted

area is between the G and the S.

• Because the flow of current along the channel from the (+ve) drain to the (-ve) source is

really a flow of free electrons from S to D in the n-type Si, the magnitude of this current

will fall as more Si becomes depleted of free electrons.

• There is a limit to the drain current (ID) which increased VDS can drive through the

channel.

• This limiting current is known as IDSS (Drain-to-Source current with the gate shorted to

the source).

• The output characteristics of an n-channel JFET with the gate short-circuited to the

source.

• The initial rise in ID is related to the buildup of the depletion layer as VDS increases.

• The curve approaches the level of the limiting current IDSS when ID begins to be pinched

off.

• The physical meaning of this term leads to one definition of pinch-off voltage, VP , which

is the value of VDS at which the maximum IDSS flows.

• With a steady gate-source voltage of 1 V there is always 1 V across the wall of the

channel at the source end.

• A drain-source voltage of 1 V means that there will be 2 V across the wall at the drain

end. (The drain is ‘up’ 1V from the source potential and the gate is 1V ‘down’, hence the

total difference is 2V.)

• The higher voltage difference at the drain end means that the electron channel is squeezed

down a bit more at this end.

• When the drain-source voltage is increased to 10V the voltage across the channel walls at

the drain end increases to 11V, but remains just 1V at the source end.

• The field across the walls near the drain end is now a lot larger than at the source end.

• As a result the channel near the drain is squeezed down quite a lot.

• Increasing the source-drain voltage to 20V squeezes down this end of the channel still

more.

• As we increase this voltage we increase the electric field which drives electrons along the

open part of the channel.

• However, also squeezes down the channel near the drain end.

• This reduction in the open channel width makes it harder for electrons to pass.

• As a result the drain-source current tends to remain constant when we increase the drain-

source voltage.

• Increasing VDS increases the widths of depletion layers, which penetrate more into

channel and hence result in more channel narrowing toward the drain.

• The resistance of the n-channel, RAB therefore increases with VDS.

• The drain current: IDS = VDS/RAB

• ID versus VDS exhibits a sub linear behavior, see figure for VDS < 5V.

• The pinch-off voltage, VP is the magnitude of reverse bias needed across the p+n junction

to make them just touch at the drain end.

• Since actual bias voltage across p+n junction at drain end is VGD, the pinch-off occur

whenever: VGD = -VP.

JFET: I-V characteristics

MOSFETs and Their Characteristics

• The metal-oxide semiconductor field effect transistor has a gate, source, and drain just

like the JFET.

• The drain current in a MOSFET is controlled by the gate-source voltage VGS.

• There are two basic types of MOSFETS: the enhancement-type and the depletion-type.

• The enhancement-type MOSFET is usually referred to as an E-MOSFET, and the

depletion-type, a D-MOSFET.

The MOSFET is also referred to as an IGFET because the gate is insulated from the channel

n-channel, enhancement-type MOSFET

The p-type substrate makes contact with the SiO2 insulator.

Because of this, there is no channel for conduction between the drain and source terminals.

Unit V SPECIAL SEMICONDUCTOR DIODES

Tunnel diode( Esaki Diode)

• It was introduced by Leo Esaki in 1958.

• Heavily-doped p-n junction

– Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n

junction diode

• Width of the depletion layer is very small

(about 100 A).

• It is generally made up of Ge and GaAs.

• It shows tunneling phenomenon.

• Circuit symbol of tunnel diode is :

Tunnelling Effect

• Classically, carrier must have energy at least equal to potential-barrier height to cross the

junction .

• But according to Quantum mechanics there is finite probability that it can penetrate

through the barrier for a thin width.

• This phenomenon is called tunneling and hence the Esaki Diode is known as Tunnel

Diode.

CHARACTERISTIC OF TUNNEL DIODE

Ip:- Peak Current

Iv :- Valley Current

Vp:- Peak Voltage

Vv:- Valley Voltage

Vf:- Peak Forward Voltage

ENERGY BAND DIAGRAM

Energy-band diagram of pn junction in thermal equilibrium in which both the n and p region are

degenerately doped

AT ZERO BIAS

Simplified energy-band diagram and I-V characteristics of the tunnel diode at zero bias.

- Zero current on the I-V diagram;

- All energy states are filled below EF on both sides of the junction;

AT SMALL FORWARD VOLTAGE

Simplified energy-band diagram and I-V characteristics of the tunnel diode at a slight

forward bias

- Electrons in the conduction band of the n region are directly opposite to the empty states

in the valence band of the p region.

So a finite probability that some electrons tunnel directly into the empty states resulting in

forward-bias tunneling current.

AT MAXIMUM TUNNELING CURENT

Simplified energy-band diagraam and I-V characteristics of the tunnel diode at a forward bias

producing maximum tunneling current.

- The maximum number of electrons in the n region are opposite to the maximum number

of empty states in the p region.

- Hence tunneling current is maximum.

TUNNEL DIODE EQUIVALENT CIRCUIT

• This is the equivalent circuit of tunnel diode when biased in negative resistance region.

• At higher frequencies the series R and L can be ignored.

• Hence equivalent circuit can be reduced to parallel combination of junction capacitance

and negative resistance.

VARACTOR DIODE

A varactor diode is best explained as a variable capacitor. Think of the depletion region as a

variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by reverse

bias voltage changes.

• Junction capacitance is present in all reverse biased diodes because of the depletion

region.

• Junction capacitance is optimized in a varactor diode and is used for high frequencies and

switching applications.

• Varactor diodes are often used for electronic tuning applications in FM radios and

televisions.

• They are also called voltage-variable capacitance diodes.

A Junction diode which acts as a variable capacitor under changing reverse

bias is known as VARACTOR DIODE

A varactor diode is specially constructed to have high resistance under reverse

bias. Capacitance for varactor diode are Pico farad. (10-12 ) range

CT = ЄA / Wd

CT =Total Capacitance of the junction

Є = Permittivity of the semiconductor material

A = Cross sectional area of the junction

WD= Width of the depletion layer

Curve between Reverse bias voltage Vr across varactor diode and total junction capacitance

Ct and Ct can be changed by changing Vr.

Application for Varactor diode

Use of varactor diode in a tuned circuit. Capacitance of the varactor in parallel with the

inductor.(LC circuit)

Silicon Controlled Rectifier (SCR)

Three terminals

anode - P-layer

cathode - N-layer (opposite end)

gate - P-layer near the cathode

Three junctions - four layers

Connect power such that the anode is positive with respect to the cathode - no current will flow

A silicon controlled rectifier is a semiconductor device that acts as a true electronic

switch. it can change alternating current and at the same time can control the amount of power

fed to the load. SCR combines the features of a rectifier and a transistor.

CONSTRUCTION

When a pn junction is added to a junction transistor the resulting three pn junction device

is called a SCR. ordinary rectifier (pn) and a junction transistor (npn) combined in one unit to

form pnpn device. three terminals are taken : one from the outer p- type material called anode a

second from the outer n- type material called cathode K and the third from the base of transistor

called Gate. GSCR is a solid state equivalent of thyratron. the gate anode and cathode of SCR

correspond to the grid plate and cathode of thyratron SCR is called thyristor

WORKING PRINCIPLE

Load is connected in series with anode the anode is always kept at positive potential w.r.t

cathode.

WHEN GATE IS OPEN

No voltage applied to the gate, j2 is reverse biased while j1 and j3 are FB . J1 and J3 is

just in npn transistor with base open .no current flows through the load RL and SCR is cut off. if

the applied voltage is gradually increased a stage is reached when RB junction J2 breakdown .the

SCR now conducts heavily and is said to be ON state. the applied voltage at which SCR

conducts heavily without gate voltage is called Break over Voltage.

WHEN GATE IS POSITIVE W.R.T CATHODE.

The SCR can be made to conduct heavily at smaller applied voltage by applying small

positive potential to the gate.J3 is FB and J2 is RB the electron from n type material start moving

across J3 towards left holes from p type toward right. electrons from j3 are attracted across

junction J2 and gate current starts flowing. as soon as gate current flows anode current increases.

the increased anode current in turn makes more electrons available at J2 breakdown and SCR

starts conducting heavily. the gate looses all control if the gate voltage is removed anode current

does not decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

BREAKOVER VOLTAGE

It is the minimum forward voltage gate being open at which SCR starts conducting

heavily i.e turned on

PEAK REVERSE VOLTAGE( PRV)

It is the maximum reverse voltage applied to an SCR without conducting in the reverse

direction

HOLDING CURRENT

It is the maximum anode current gate being open at which SCR is turned off from on

conditions.

FORWARD CURRENT RATING

It is the maximum anode current that an SCR is capable of passing without destruction

CIRCUIT FUSING RATING

It is the product of of square of forward surge current and the time of duration of the

surge

VI CHARACTERISTICS OF SCR

FORWARD CHARCTERISTICS

When anode is +ve w.r.t cathode the curve between V & I is called Forward

characteristics. OABC is the forward characteristics of the SCR at Ig =0. if the supplied voltage

is increased from zero point A is reached .SCR starts conducting voltage across SCR suddenly

drops (dotted curve AB) most of supply voltage appears across RL

REVERSE CHARCTERISTICS

When anode is –ve w.r.t.cathode the curve b/w V&I is known as reverse characteristics

reverse voltage come across SCR when it is operated with ac supply reverse voltage is increased

anode current remains small avalanche breakdown occurs and SCR starts conducting heavily is

known as reverse breakdown voltage

SCR as a switch

SCR Half and Full wave rectifier

Application

SCR as a static contactor

SCR for power control

SCR for speed control of d.c.shunt motor

Over light detector

UJT unijunction transistor (UJT)

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction.

The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by

lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The

emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter

is open-circuit is called interbase resistance.

There are two types of unijunction transistor:

The original unijunction transistor, or UJT, is a simple device that is essentially a bar of

N type semiconductor material into which P type material has been diffused somewhere

along its length, defining the device parameter η. The 2N2646 is the most commonly

used version of the UJT.The programmable unijunction transistor, or PUT, is a close

cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anode

and a cathode connected to the first and the last layer, and a gate connected to one of the

inner layers. They are not directly interchangeable with conventional UJTs but perform a

similar function. The UJT is biased with a positive voltage between the two bases. This

causes a potential drop along the length of the device. When the emitter voltage is driven

approximately one diode voltage above the voltage at the point where the P diffusion

(emitter) is, current will begin to flow from the emitter into the base region. Because the

base region is very lightly doped, the additional current (actually charges in the base

region) causes conductivity modulation which reduces the resistance of the portion of the

base between the emitter junction and the B2 terminal. This reduction in resistance means

that the emitter junction is more forward biased, and so even more current is injected.

Overall, the effect is a negative resistance at the emitter terminal. This is what makes the

UJT useful, especially in simple oscillator circuits.

Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1970s

and early 1980s because they allowed simple oscillators to be built using just one active device.

Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became

more commonly used.

In addition to its use as the active device in relaxation oscillators, one of the most important

applications of UJTs or PUTs is to trigger thyristors (SCR, TRIAC, etc.). In fact, a DC voltage

can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase

in the DC control voltage. This application is important for large AC current control.

DIAC Diode A.C. switch

A Diac is two terminal , three layer bi directional device which can be switched from its

off state for either polarity of applied voltage.

Construction:

The diac can be constructed in either npn or pnp form.The two leads are connected to p-

regions of silicon separated by an n region. the structure of diac is similar to that of a transistor

differences are

There is no terminal attached to the base layer

The three regions are nearly identical in size. the doping concentrations are identical to

give the device symmetrical properties.

Operation

When a positive or negative voltage is applied across the terminals of Diac only a small

leakage current Ibo will flow through the device as the applied voltage is increased , the leakage

current will continue to flow until the voltage reaches breakover voltage Vbo at this point

avalanche breakdown of the reverse biased junction occurs and the device exhibits negative

resistance i.e current through the device increases with the decreasing values of applied voltage

the voltage across the device then drops to breakback voltage Vw

V- I CHARECTERISTICS OF A DIAC

For applied positive voltage less than + Vbo and Negative voltage less than -Vbo , a

small leakage current flows thrugh the device. Under such conditions the diac blocks flow of

current and behaves as an open circuit. the voltage +Vbo and -Vbo are the breakdown voltages

and usually have range of 30 to 50 volts.

When the positive or negative applied voltage is equal to or greater than tha breakdown

voltage Diac begins to conduct and voltage drop across it becomes a few volts conduction then

continues until the device current drops below its holding current breakover voltage and holding

current values are identical for the forward and reverse regions of operation.

Diacs are used for triggering of triacs in adjustable phase control of a c mains power.

Applications are light dimming heat control universal motor speed control

TRIAC

Triacs are three terminal devices that are used to switch large a.c. currents with a small trigger

signal. Triacs are commonly used in dimmer switches, motor speed control circuits and

equipment that automatically controls mains powered equipment including remote control. The

triac has many advantages over a relay, which could also be used to control mains equipment;

the triac is cheap, it has no moving parts making it reliable and it operates very quickly.

The three terminals on a triac are called „Main Terminal 1‟

(MT1), „Main Terminal 2‟ (MT2) and „Gate‟ (G). To turn on

the triac there needs to be a small current IGT flowing through

the gate, this current will only flow when the voltage between

G and MT1 is at least VGT. The signal that turns on the triac is

called the trigger signal. Once the triac is turned on it will stay

on even if there is no gate current until the current flowing

between MT2 and MT1 fall below the hold current IH.

The triac is always turned fully on or fully off. When the triac is on there is virtually no pd

between MT2 and MT1 so the power dissipated in the triac is low so it does not get hot or waste

electrical power. When the triac is off no current flows between MT2 and MT1 so the power

dissipated in the triac is low so it does not get hot or waste electrical power. This means that

triacs can be small and are very efficient.

Triacs can be used in d.c. circuits in which case when

the triac is triggered it will stay on until power is

removed from the triac. It is easy to calculate the value

of gate resistor needed to turn on a triac using the gate

characteristics and ohms law. The maximum value of

resistor can be found from the voltage across the

resistor (VS - VGT) divided by the gate current IGT. So,

R = (VS - VGT)/ IGT

In a.c. circuits the triac needs to be repeatedly triggered because the triac turns off when the a.c.

current goes from positive to negative or negative to positive as the current become momentarily

zero. The triac is used in mains circuits to control the

amount of power by only turning the triac on for part

of the wave a bit like in pulse width modulation. This

can be done by varying the value of the gate resistor

so that the triac does not turn on until the a.c signal

reaches a particular voltage. The problem with this

first dimmer is that there is a very high voltage across

the variable resistor and it will get hot as there is a lot

of power to dissipate (P=V2/R).

To get round the problem of needing a high power components the variable resistor is usually

connected between MT2 and G so current will only flow through the resistor to trigger the

triac(fig 4), once the triac is on the voltage at MT2 falls to zero so no current flows through the

MT2

MT1

G

VS

0v

R

resistor. The other problem with these circuits is that the minimum power is half the maximum

this is because the highest voltage, which will give the latest trigger point, occurs half way

through each half wave. Using a capacitor can solve this problem, the resistor is adjusted so that

the charging time allows the trigger to happen at any point in the half cycle (fig 5).

PHOTO DIODE

A photodiode is a type of photodetector capable of converting light into either current or

voltage, depending upon the mode of operation.[1]

The common, traditional solar cell used to

generate electric solar power is a large area photodiode.

Photodiodes are similar to regular semiconductor diodes except that they may be either

exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection

to allow light to reach the sensitive part of the device. Many diodes designed for use specifically

as a photodiode will also use a PIN junction rather than the typical PN junction.

A photodiode is a PN junction or PIN structure. When a photon of sufficient energy

strikes the diode, it excites an electron, thereby creating a free electron and a (positively charged

electron hole). If the absorption occurs in the junction's depletion region, or one diffusion length

away from it, these carriers are swept from the junction by the built-in field of the depletion

region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent

is produced.

P-N PHOTODIODE:

The simple p-n diode is illustrated in fig (3-1). The diode junction is reverse-biased, causing

mobile holes (electrons) from the p (n) region toward the n (p) region leaves behind the

immobile negative acceptor ions (positive donor ions), which, in turn, establish an electric field

distribution in the vicinity of the junction called depletion region, as shown in fig (3-1-b).

Because there are no free charges, the resistance in this region is high, so that the voltage drop

across the diode mostly occurs across the junction. When an incident photon is absorbed in the

depletion region after passing through the p-layer, it raises an electron from the valance to the

conduction band the electron is now free to move, and a hole is left in the a valance band. In this

Figure 4 Figure 5

way, free charge carrier pairs, commonly called photocurrents, are created by photon absorption.

These moving carriers then cause current flow through the external circuit.

fig(3-1) (a) p-n diode . (b) Electric field distribution across the diode

In order to generate the electron-hole pair, the incident photon must have energy larger than

that of the band gap Eg between the valance and the conduction bands, that is, h w0 > Eg. In

terms of the cutoff wavelength λc, we have

λc = 1.24 / Eg

With Eg measured in electron volts, the cutoff wavelength is about 1.06µm for silicon and 1.6

µm for germanium, where their band-gap energies are 1.1 and .67 eV, respectively. Just as in

the case of the external photoelectric effect, not all wavelengths lower than λc can generate

photocarrier, as the absorption of photons in the p and n regions is increased at the shorter

wavelengths. After photocarrier are generated in the p and n region, most of the free carriers

will diffused randomly through the diode and recombine before reaching the depletion

junction. The quantum efficiency η for the semiconductor junction diode can then be defined

as the number of electron-hole pairs per incident photon.

Two main factors limit the response time of a photodiode:

1-the transit time of the photocarriers through the depletion region

2-the diffusion time of photocarriers (generated in the depletion region) through the diffusion

region.

Carrier diffusion is inherently a slow process. In order to have a high-speed photodiode, the

carriers should be generated in the depletion region in the high field intensity area or close to it

so that the diffusion times are less than the carrier transit times. This can be accomplished by

increasing the bias voltage, but practical constraints limits the applied bias voltage.

Photovoltaic mode

When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is

restricted and a voltage builds up. This mode exploits the photovoltaic effect, which is the basis

for solar cells – in fact, a traditional solar cell is just a large area photodiode.

Photoconductive mode

In this mode the diode is often reverse biased, dramatically reducing the response time at the

expense of increased noise. This increases the width of the depletion layer, which decreases the

junction's capacitance resulting in faster response times. The reverse bias induces only a small

amount of current (known as saturation or back current) along its direction while the

photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is

linearly proportional to the illuminance (and to the irradiance).[2]

Although this mode is faster, the photoconductive mode tends to exhibit more electronic

noise.[citation needed]

The leakage current of a good PIN diode is so low (< 1nA) that the Johnson–

Nyquist noise of the load resistance in a typical circuit often dominates.

Other modes of operation

Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated

with much higher reverse bias. This allows each photo-generated carrier to be multiplied by

avalanche breakdown, resulting in internal gain within the photodiode, which increases the

effective responsivity of the device.

Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essence

nothing more than a bipolar transistor that is encased in a transparent case so that light can reach

the base-collector junction. The electrons that are generated by photons in the base-collector

junction are injected into the base, and this photodiode current is amplified by the transistor's

current gain β (or hfe). Note that while phototransistors have a higher responsivity for light they

are not able to detect low levels of light any better than photodiodes.[citation needed]

Phototransistors

also have significantly longer response times.

Critical performance parameters of a photodiode include:

Responsivity

The ratio of generated photocurrent to incident light power, typically expressed in A/W

when used in photoconductive mode. The responsivity may also be expressed as a

Quantum efficiency, or the ratio of the number of photogenerated carriers to incident

photons and thus a unitless quantity.

Dark current

The current through the photodiode in the absence of light, when it is operated in

photoconductive mode. The dark current includes photocurrent generated by background

radiation and the saturation current of the semiconductor junction. Dark current must be

accounted for by calibration if a photodiode is used to make an accurate optical power

measurement, and it is also a source of noise when a photodiode is used in an optical

communication system.

Noise-equivalent power

(NEP) The minimum input optical power to generate photocurrent, equal to the rms noise

current in a 1 hertz bandwidth. The related characteristic detectivity (D) is the inverse of

NEP, 1/NEP; and the specific detectivity ( ) is the detectivity normalized to the area

(A) of the photodetector, . The NEP is roughly the minimum detectable

input power of a photodiode.

When a photodiode is used in an optical communication system, these parameters contribute to

the sensitivity of the optical receiver, which is the minimum input power required for the receiver

to achieve a specified bit error ratio.

APPLICATIONS

P-N photodiodes are used in similar applications to other photodetectors, such as

photoconductors, charge-coupled devices, and photomultiplier tubes.

Photodiodes are used in consumer electronics devices such as compact disc players, smoke

detectors, and the receivers for remote controls in VCRs and televisions.

In other consumer items such as camera light meters, clock radios (the ones that dim the display

when it's dark) and street lights, photoconductors are often used rather than photodiodes,

although in principle either could be used.

Photodiodes are often used for accurate measurement of light intensity in science and industry.

They generally have a better, more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed

tomography (coupled with scintillators) or instruments to analyze samples (immunoassay). They

are also used in pulse oximeters.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are

often used for optical communications and in lighting regulation.

P-N photodiodes are not used to measure extremely low light intensities. Instead, if high

sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or

photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision

equipment and laser rangefinding.

P-N photodiodes are used in similar applications to other photodetectors, such as

photoconductors, charge-coupled devices, and photomultiplier tubes.

Photodiodes are used in consumer electronics devices such as compact disc players, smoke

detectors, and the receivers for remote controls in VCRs and televisions.

In other consumer items such as camera light meters, clock radios (the ones that dim the display

when it's dark) and street lights, photoconductors are often used rather than photodiodes,

although in principle either could be used.

Photodiodes are often used for accurate measurement of light intensity in science and industry.

They generally have a better, more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed

tomography (coupled with scintillators) or instruments to analyze samples (immunoassay). They

are also used in pulse oximeters.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are

often used for optical communications and in lighting regulation.

LIGHT EMITTING DIODE (LED)

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as

indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical

electronic component in 1962,[2]

early LEDs emitted low-intensity red light, but modern versions

are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward biased (switched on), electrons are able to

recombine with electron holes within the device, releasing energy in the form of photons. This

effect is called electroluminescence and the color of the light (corresponding to the energy of the

photon) is determined by the energy gap of the semiconductor. An LED is often small in area

(less than 1 mm2), and integrated optical components may be used to shape its radiation

pattern.[3]

LEDs present many advantages over incandescent light sources including lower energy

consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater

durability and reliability. LEDs powerful enough for room lighting are relatively expensive and

require more precise current and heat management than compact fluorescent lamp sources of

comparable output.

Light-emitting diodes are used in applications as diverse as replacements for aviation

lighting, automotive lighting (particularly brake lamps, turn signals and indicators) as well as in

traffic signals. The compact size, the possibility of narrow bandwidth, switching speed, and

extreme reliability of LEDs has allowed new text and video displays and sensors to be

developed, while their high switching rates are also useful in advanced communications

technology. Infrared LEDs are also used in the remote control units of many commercial

products including televisions, DVD players, and other domestic appliances.

SYMBOL OF LED

WORKING PRINCIPLE OF LED

DIODE I-V CURVE

LIQUID CRYSTAL DISPLAY

A liquid crystal display (LCD) is a thin, flat electronic visual display that uses the light

modulating properties of liquid crystals (LCs). LCs do not emit light directly.

They are used in a wide range of applications, including computer monitors, television,

instrument panels, aircraft cockpit displays, signage, etc. They are common in consumer devices

such as video players, gaming devices, clocks, watches, calculators, and telephones. LCDs have

displaced cathode ray tube (CRT) displays in most applications. They are usually more compact,

lightweight, portable, less expensive, more reliable, and easier on the eyes.[citation needed]

They are

available in a wider range of screen sizes than CRT and plasma displays, and since they do not

use phosphors, they cannot suffer image burn-in.

LCDs are more energy efficient and offer safer disposal than CRTs. Its low electrical

power consumption enables it to be used in battery-powered electronic equipment. It is an

electronically-modulated optical device made up of any number of pixels filled with liquid

crystals and arrayed in front of a light source (backlight) or reflector to produce images in colour

or monochrome

Advantages of LCD

Low power is required

Good contrast

Low cost

Disadvantages of LCD

Speed of operation is slow

LCD occupy a large area

LCD life span is quite small, when used on d.c. Therefore, they are used with a.c.

suppliers.

Applications of LCD

Used as numerical counters for counting production items.

Analog quantities can also be displayed as a number on a suitable device. (e.g.) Digital

multimeter.

Used for solid state video displays.

Used for image sensing circuits.

Used for numerical display in pocket calculators.