intro to sconductor

8/10/2019 Intro to SConductor

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PHYS 162 - Chapter 1 Introduction To Semiconductors

Prepared By: Syed Muhammad Asad

Semester 102 Page 1

Figure 1 Atomic Structure

Figure 2 Energy Shells

CHAPTER 1

INTRODUCTION TO SEMICONDUCTORS1-1 ATOMIC STRUCTURE

- Atom is the smallest particle of an element that retains the characteristics of that

element.

- It consists of a nucleus at the center and

electronsin orbit around the nucleus.

- The nucleus consists of protons and

neutrons.

- The electrical charge on these 3 particles is

as follows:

o ElectronNegative Charge

o ProtonPositive Charge

o NeutronNo Charge

-

An atom always contains the same number

of electrons and protons.

- For this reason, an atom is always electrically

neutral.

- The number of electrons or protons in an

atom is termed as atomic number.

1.1.1 Electron Shells and Orbits

- Electrons orbit around the nucleus at certain

distances from the nucleus.- The electrons near the nucleus have less

energy than the electrons far from the

nucleus.

- Only discrete (separate and distinct) values

of energy level exist.

Energy Level

- Each discrete distance or orbit corresponds

to a certain energy level.

- These orbits are grouped into energy bands

called shells.

- The difference in energy level within a shell is

smaller than the difference in energy

between shells.

- The shells are numbered n = 1, 2, 3where 1 is closest to the nucleus.

- The number of electrons in each shell is given by =.


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Figure 3 Carbon Atom

Valence Electrons

- The electrons in orbit farthest from the nucleus are called Valence Electrons and the

energy shell is called Valence Shell.

- These electrons have the highest energy and are loosely bounded to the atom.

Ionization

-

When an atom absorbs energy in the form of heat, light or electrical potential, theelectron energies are increased.

-

As a result, the valence electrons which already have higher energies and are loosely

bound to the atom canjump to other orbitswithin the valence shells.

- If these electrons get enough energy, they can escape from the outer shell and the

atoms influence.

- As a result, the atom now is no more neutral.

-

The removal of an electron from the valence shell makes an atom positively chargedas

now there is one extra proton or one less electron in an atom.

-

This process of losing an electron is called Ionization.- The electron produced through ionization is called a Free Electron.

1-2 INSULATORS, CONDUCTORS AND SEMICONDUCTORS

- For the purpose of understanding, an atom can be represented by the valence shell and

a corethat consists of all the inner shells and the nucleus.

1.2.1 Insulators

- Insulator is a material that does not conduct electric

current.- Valence electrons are tightly bounded to the atom

therefore there are no free electrons to conduct

current.

- Examples are wood, plastic, glass, quartz etc.

1.2.2 Conductors

- Conductor is material that easily conducts current.

- Most metals are good conductors like aluminum, copper, gold and iron.

- They have only one electron in the valence shellwhich is loosely bound to the atom.

1.2.3 Semiconductors

- They are materials that are between insulator and conductor in terms of conduction of

current.

- Pure or intrinsic semiconductor in neither good conductor nor good insulator.

- Most common semiconductors are silicon, germanium and carbon.

- They have four electronsin their valence shell.


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Semester 102 Page 3

Figure 4 Energy Gap

Figure 5 Silicon and Germanium Atoms

1.2.4 Energy Bands

-

A shell represents a band of energy levels.

- Free electrons upon ionization, jump from the valence band to the conduction band.

- The difference of energy between valence band and conduction band is called energy

gap.

- Energy gap is the amount of energy an electron must have to jump from valence band

to conduction band.

- Once in the conduction band, an electron is free to move in the material.

- The energy gap is highest in insulators and lowest in conductors.

1.2.5 Silicon and Germanium

- Valence electrons in germanium are in the fourth shell while in silicon in the third shell.

- So valence electrons in germanium are more energetic and require lower energy to

become free.

- This property makes germanium unstable and is not widely used in electronic circuits.

- Silicon is the most widely used semiconductor.


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Semester 102 Page 4

Figure 6 Covalent Bond in Silicon

Figure 7 Energy Band Diagram

1.2.6 Covalent Bonds

-

Figure 6 shows how each silicon

atom is positioned with four

adjacent silicon atoms.

- Each silicon atom shares its four

valence electrons with its four

neighbors.

- This creates eight shared electrons.

- The bond created with this sharing

of electrons is called Covalent

Bond.

1-3 CURRENT IN SEMICONDUCTORS

- Figure 7shows the energy band diagram for an unexcited, intrinsic silicon atom.

-

This condition only occurs at absolute zero.

1.3.1 Conduction Electrons and Holes

- At room temperature, intrinsic silicon has enough heat to produce free electrons.

- A vacancy is left in the valence band when an electron from the valence band jumps to

the conduction band.


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Semester 102 Page 5

Figure 8 Creation of Electron-Hole pairs

- This vacancy is called a hole.

- This creates an electron-hole pair.

- Recombination occurs when the conduction band electron jumps back to the valence

band.

1.3.2 Electron and Hole Current

-

When voltage is applied to silicon, free electrons are attracted towards the positive end.

- This movement of electrons is a type of current and is called Electron Current.

- Similarly valence electron can jump to a nearby hole as the energy difference within the

valence band is small.

- This leaves a hole from where the electron came from.

- This apparent movement of holes is called Hole Current.

Figure 9 Electron Current


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1-4 N-TYPE AND P-TYPE SEMICONDUCTORS

1.4.1 Doping

-

Adding impurity to an intrinsic (pure) semiconductor improves its conductivity. This process is

called Doping.

1.4.2 N-Type Semiconductor

-

Adding pentavalent impurity (5 valence electrons e.g. arsenic, phosphorous, bismuth and

antimony) to silicon produces an n-type semiconductor.

-

The added impurity forms a covalent bond with its four adjacent silicon atoms, leaving one extra

electron.

-

This electron becomes a conduction electron as it is not involved in the covalent bond.

-

This increases the number of free electrons in the conduction band and as a result improves

conductivity.-

As pentavalent atom loses an electron, it is called a Donor Atom.

1.4.3 P-Type Semiconductors

- Adding trivalent impurity(3 valence electrons e.g. boron, indium and gallium) to silicon

produces p-type semiconductor.

-

The added impurity forms a covalent bond with its four adjacent silicon atoms using all the three

valence electrons, leaving one hole in the valence shell.

-

This electron becomes a conduction electron as it is not involved in the covalent bond.

- As trivalent atom can accept an electron to fill its hole, it is called an Acceptor Atom.

Figure 10 Hole Current


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1-5 THE DIODE

- If an n-type and p-type semiconductors are joined together, a junction between the

boundaries of the two regions is created. This junction is called pn-junction.

1.5.1 Formation of Depletion Region

- At the instant of pn-junction formation, the free electrons near the junction in the n

region move across the junction to the pregion.

- There they combine with holes near the junction.

Figure 12 pn-junction (Left) Formation of Depletion Region (Right)

- Due to this combination of electrons with holes, the nregion loses free electrons. This

creates positively charges ions in the nregionnear the junction.

-

Similarly the p region gains electrons and loses holes. This results in creation of

negatively charged ions in the pregionnear the junction.

- This layer of positively charged and negatively charged ions near the junction forms the

Depletion Region.

Figure 11 Pentavalent Impurity in Silicon Atom Figure 12 Trivalent Impurity in Silicon

Atom


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- This creation of depletion region is a one-time process and stops when a balance is

reached.

- The balance is reached because a point comes when the negatively charged ions in the

pregion repel any more free electrons coming from the nregionand the process of

combination stops.

Barrier Potential- The negatively charged and positively charged ions in the depletion region create an

electric field.

- This field is a barrier and must be overcome to move the free electron from the n

region to p region.

- The potential differenceof the electric field across the depletion region is the amount

of voltage requiredto move electrons through the field.

- This potential difference is called Barrier Potential.

- The barrier potential for silicon is 0.7V and for germanium is 0.3V.

1.5.2 Energy Diagram of the PN Junction and Depletion Region

Figure 13 Energy at instant of junction formation (left) and at equilibrium (right)

- The energy levelsof the valence and conduction band in an n-type material in slightly

lowerthan the p-type.

- The free electrons of the n-type can easily move across the junction and combine with

the holes of the p-type.

- As the process continues (that is formation of depletion region) the energy level of the

conduction band of the n-type decreases. This is due to the loss of high energy free

electrons.


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Semester 102 Page 9

- Soon there are no more free electrons in the conduction band of the n-type with

enough to cross the junction.

- This the balance point of the junction and the completion of the formation of the

depletion region.

1-6 BIASING A DIODE- As no more free electrons can move through the pn junction, an external source of

energy is requiredto move the free electrons.

- This source of energy is in terms of voltage and is termed as biasing.

- There are two types of bias conditions.

o Forward BiasEnables the flow of current

o Reverse BiasPrevents the flow of current

1.6.1 Forward Bias

- Connecting the negative side of the voltage source to the n-type and positive to the p-

type makes a diode forward biased. This enables the diode to conduct current.

- The mechanism of what happens when a diode is forward biased can be given as follows

o As same charges repel, negative side of the bias-voltage pushes the free

electronsin the n-type towardthe pn junction.

o The bias-voltage gives enough energy to the free electrons to overcome the

barrier potential, enter the p-type and combine with the holes.

o Then these electrons move towards the positive side of the voltage source

through the p-type by jumping to nearby holes in the valence band of the p-

type.

o

In the end, these electrons come out of the p-type and travel through the

external circuitto once again enter into the n-type.

Effect of Forward Bias on Depletion Region

- The depletion region will become narrow.

Effect of Barrier Potential during Forward Bias

-

The effect of barrier potential during forward bias is to develop a voltage drop of 0.7V across the

diode.

1.6.2 Reverse Bias- Connecting the negative side of the voltage source to the p-type and positive to the n-

type makes a diode reverse biased. This prevents any flow of current through the diode.

- The mechanism of what happens when a diode is reverse biased can be given as follows

o As unlike charges attract, positive side of the bias-voltage pulls the free

electronsin the n-type awayfrom the pn junction.

o Then these electrons move towards the positive side of the voltage source

through the external circuit and enter the p-type.


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o These electrons combine with holes and move towards the junction by jumping

to nearby holes.

o This results in the widening of the depletion region.

Reverse Current

- A small number of free electrons in the p-type are pushed towards the junction and

combine with the minority carrier holes in the n-type.- This results in a very small reverse current to flow and is called Reverse Current.

Reverse Breakdown

- Increasing the reverse bias voltage to a value called the breakdown voltagewill rapidly

increase the reverse current.

- The mechanism of what happens when a diode reaches breakdown voltage can be given

as follows

o The high reverse voltage speed up the electrons in the p-type.

o They collide with atoms with enough energy to knock out electrons out of orbit

and into the conduction band. These electrons also knock out more electrons.

o The rapid increase in the number of these conduction electrons is known as

avalanche effect or breakdown.

o This results in the rapid increase in the reverse current, heating and damaging

the diode.

1-7 VOLTAGE-CURRENT CHARACTERISTIC OF A DIODE

- This section will describe the relationship between voltage and current in a diode on a

graphical basis.

1.7.1 V-I Characteristic for Forward Bias

- Current that passes through a diode in forward bias is termed forward current IF.

- If the bias voltage is increased steadily and the corresponding forward

current IF is recorded, then the resulting graph between the bias

voltage and the forward current result is the V-I characteristic for

forward bias.

Figure 14 V-I Characteristic for Forward Bias


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Figure 16 Diode Schematic Symbol

Figure 17

- The forward current increases very slowly as the bias voltage increases

from 0V. (Refer to Figure 15 Point A)

- With an increase in the voltage, there is small increase in the current

(Figure 15 Point B).

- Beyond 0.7V the current starts to increase rapidly with very small

change in the bias voltage.

- This point is where the bias voltage is greater than the barrier

potentialand forward current starts to flow.

Dynamic Resistance

-

Unlike the linear resistance, dynamic resistance of a forward biased diode is not constant.

-

It changes as we move along the V-I curve.

-

It can be expressed as =

.

1.7.2 V-I Characteristic for Reverse Bias

- Current that passes through a diode in reverse bias is

termed reverse current IR.

- If the reverse bias voltage is increased steadily until the

breakdown voltage VBR, there will be very small amount of

reverse current through the diode usually in the range of

A or nA.

- Beyond the breakdown voltage, VBR remains almost

constantbut the reverse current increases rapidly.

- This may result in overheating and damaging the diode.

1-8 DIODE MODELS

- The diode schematic symbol for a general purpose diode is shown in Figure 17.

- The n region is called the cathode while the p region is

called the anode.

Forward Bias Connection

- Positive terminalof the voltage connected to the anodeand negative to the cathode

makes the diode forward biased.

-

The diode drop is indicated as VF and forward current

as IF.

Reverse Bias Connection

- Positive terminal of the voltage connected to the

cathode and negative to the anodemakes the diode

reverse biased.

- In this case I = 0A and the diode drop is equal to the bias voltage.

Figure 15 V-I Characteristic for Reverse Bias


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Figure 18 Ideal Diode Biasing and V-I Characteristic Curve

1.8.1 Diode Approximations

-

A diode can be approximated for electrical analysis in the following three ways.

o Ideal Diode ModelLeast accurate

o Practical Diode ModelGood for analysis

o Complete Diode ModelMost accurate

The Ideal Diode Model

- This is the least accurate of all the diode models.

-

In this model, the diode is modeled as an on/off switch.

- The model parameters are as follows

= 0 , =

= 0, =


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Figure 19 Practical Diode Biasing and V-I Characteristic Curve

The Practical Diode Model

- This model includes the barrier potential.

- When forward biased, it is equivalent to a closed switch in series with a small equivalent

source VFwhile in reverse it acts as an open switch.

- Note that this equivalent source is not an actual voltage source but only represents the

barrier potential.

-

The model parameters are as follows

= 0.7, =

= 0, =

The Complete Diode Model

-

This is the most accurate diode approximation.


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- It includes the barrier potential, forward dynamic resistance rd and reverse internal

resistance rD.

- When the diode is forward biased it acts like a closed switch in series with an equivalent

barrier potential VB and small forward dynamic resistance rd.

- When it is reverse biased, it acts an open switch in parallel with a large internal reverse

resistance rDwhich gives path to the reverse current.

- The model parameters are as follows

= 0.7 + , =

0.7

+

Figure 20 Complete Diode Biasing and V-I Characteristic Curve

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