DMT 121 ELECTRONIC DEVICES

Chapter 1

Introduction to Semiconductor

DMT 121 ELECTRONIC DEVICES

Semiconductor Materials

Semiconductor Materials

• Definition: Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator

Semiconductor Materials

• Single crystal – Germanium (Ge) and Silicon (Si)

• Compound Semiconductor – Gallium Arsenide (GaAs), Cadmium Sulfide (CdS), Gallium Nitride (GaN) and Gallium Arsenide phosphide (GaAsP).

• Mostly used : Ge, Si and GaAs

Semiconductor Materials

• Ge – First discovered. Used as Diode in 1939, transistor in 1947. Sensitive to changes in temperature – suffer reliability problem.

• Si – Introduced in 1954 (as transistor), less sensitive to temperature. Abundant materials on earth. Over the time – its sensitive to issue of speed.

• GaAs – in 1970 (transistor), 5x speed faster than Si. Problem – difficult to manufacture, expensive, had little design support at the early stage.

Periodic Table

• Columns: Similar Valence Structure

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

He

N e

Ar

Kr

Xe

Rn

inert

gase

s

acc

ep

t 1e

acc

ep

t 2e

giv

e u

p 1

e g

ive u

p 2

e g

ive u

p 3

e

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

Semiconductors, Conductors & Insulators

Conductors• Material that easily conducts electrical current.• The best conductors are single-element material (e.g

copper,silver,gold,aluminum,ect.)• One valence electron very loosely bound to the atom- free electron

Insulators• Material that does not conduct electric current under normal

conditions.• Valence electron are tightly bound to the atom – less free electron

Semiconductors• Material between conductors and insulators in its ability to conduct

electric current• in its pure (intrinsic) state is neither a good conductor nor a good

insulator• most commonly use semiconductor- silicon(Si), germanium(Ge), and

carbon(C).• contains four valence electrons

Covalent Bonding & Intrinsic Materials

• Atom = electron + proton + neutron

• Nucleus = neutrons + protons

Protons (positive charge)

Neutrons (uncharged)

Nucleus(core of atom)

Electrons(negative charge)

ATOM

Atomic Structure

No. of electron in each shell

Ne = 2(n)2

n = no of shell.

Covalent Bonding

Covalent bonding of the Silicon atom

Covalent bonding of the GaAs crystal

Intrinsic CarrierTable 1.1

Intrinsic Carriers

Semiconductor Intrinsic Carriers

(per cubic centimeter)

GaAs 1.7 x 106

Si 1.5 x 1010

Ge 2.5 x 1013

• Intrinsic (pure) carriers – The free electrons in a material due to only external causes

• Ge has the highest number of carriers and GaAs has the lowest intrinsic carriers.

• The term intrinsic (pure) is applied to any semiconductor material that has carefully refined to reduce the number of impurities to a very low level – essentially as pure as can be made available through modern technology

Relative Mobility Factor µn

Table 1.2

Relative Mobility Factor

Semiconductor

µn (cm2/V-s)

Si 1500

Ge 3900

GaAs 8500

• Relative mobility – the ability of the free carriers to move throughout the material.

• GaAs has 5X the mobility of free carriers compared to Si, a factor that results in response times using GaAs electronic devices is 5X those of the same device made from Si.

• Ge has more than twice the mobility of electrons in Si, a factor that results in the continued of Ge in high-speed radio frequency applications.

Difference between Conductors & Semiconductors

• Conductors – Resistance increases with the increase in heat, because their vibration pattern about relatively fixed location makes it increasingly difficult for a sustained flow of carriers through the material – positive temperature coefficient.

• Semiconductors – Exhibit an increased level of conductivity with the application of heat. As the temperature rises, an increasing number of valence electron absorb sufficient thermal energy to break the covalent bond and contribute to the number of free carriers – negative temperature effects

Energy Level

Figure: Energy levels: conduction and valence bands of an insulator, a semiconductor, and a conductor.

Extrinsic Materials : n-Type and P-Type Materials

• The characteristics of a semiconductor material can be altered significantly by the addition of a specific purity atoms to relatively pure semiconductor materials – this process is known as doping process

• A semiconductor that has been subjected to the doping process is called an extrinsic materials.

• Extrinsic Materials are n-type material [five valence electrons (pentavalent)] and p-type material [three valence electrons atom (trivalent)]

N-Type Materials

• n-Type material is created by introducing the impurity (bendasing) elements that have five valence electrons (pentavalent).

• There are antimony (Sb), Arsenic (As) and phosphorous (P).

Figure: Antimony impurity in n-type material

Diffused impurities with five valence electrons are called donor atoms

N-Type Materials

• The effect of this doping cause the energy level (called the donor level) appears in the forbidden band with Eg significantly less than intrinsic material.

• This cause less thermal energy to move free electron (due to added impurity) into conduction band at room temperature.

Figure: Effect of donor impurities on the energy band structure

N-Type Material

• Pentavalent atoms is an n-type semiconductor (n stands for the negative charge on electrons).

• The electrons are called the majority carrier in n-type materials.• In n-type material there are also a few holes that are created when

electrons-holes pairs are thermally generated• Holes in n-type materials are called minority carrier.

P-Type Material

• Si or Ge doped with impurities atoms having three valence electrons.

• Mostly used are boron (B), gallium (Ga) and indium (In).• The void (vacancy) is called ‘hole’ represented by small circle

or a ‘+’ sign.

Figure: Boron impurity in p-type material.

Diffused impurities with three valence electrons are called acceptor atoms

P-Type Material

• In p-type materials the hole is the majority carrier and the electron is the minority carrier.

• Holes can be thought as +ve charges because the absence of electron leaves a net +ve charge on the atom.

Electron vs Hole Flow

• With sufficient kinetic energy to break its covalent bond, the electron will fills the void created by a hole, then a vacancy or hole, will be created in the covalent bond that released the electron.

Semiconductor Diode

Diode

• Simple construction of electronic device

• It is a joining between n-type and p-type material (joining one with majority carrier of electron to one with a majority carrier of holes)

Diode @ No Bias (VD=0V)

Forward Bias (VD > 0 V)

Figure: Forward-biased p–n junction. (a) Internal distribution of charge under forward-bias conditions; (b) forward-bias polarity

and direction of resulting current.

Reverse Bias (VD < 0 V)

Figure: Reverse-biased p–n junction. (a) Internal distribution of charge under reverse-bias conditions; (b) reverse-bias polarity and direction of

reverse saturation current.

Diode Characteristics Curve

Figure: Silicon semiconductor diode characteristics.

Ge, Si and GaAs

Figure: Comparison of Ge, Si, and GaAs diodes.

Temperature Effects

Figure: Variation in Si diode characteristics with temperature change.

Ideal Vs Practical

• Semiconductor diode behaves in a manner similar to mechanical switch that can control the current flow between it’s two terminal

• However, semiconductor diode different from a mechanical switch in the sense that it permit the current flow in one direction

Ideal Vs Practical

Figure: Ideal semiconductor diode:(a) forward-biased (b) reverse-biased.

05

0

mA

V

I

VR

D

DF

mA

V

I

VR

D

DR

0

20

(Short circuit equivalent –fwd bias, actual case R ≠ 0)

(Open circuit equivalent – Reverse bias, actual case saturation current Is ≠ 0)

Figure: Ideal versus actual semiconductor characteristics.

Approximate Diode

Resistance Levels

DC or Static Response• Application of dc voltage will result in an operating

point on the characteristic curve will not change with time.

D

DD

I

VR

In general, the higher the current

through a diode, the lower is the

dc resistance level.

Figure: Determining the dc resistance of a diode at a particular operating

point.

Resistance Levels

dd

dd

I

mV

I

Vr

26

Figure: Defining the dynamic or ac resistance.

AC or Dynamic Response

Resistance Levels

d

dav

I

Vr

Figure: Determining the average ac resistance between indicated limits.

Average AC Response

Diode Equivalent Model

dLIMIT

BIASF

dLIMITFBIAS

dFVF

rR

VVI

VrRIV

rIV

7.0

7.0

7.0

][ LIMITFBIAS RrIV R

Example 1

Determine the forward voltage (VF) and forward current [IF]. Alsofind the voltage across the limitingresistor. Assumed rd’ = 10 at the determined value of forward.

VkmARIV

mVmAVrIVV

mAk

VV

rR

VVI

LIMITFR

dFF

dLIMIT

BIASF

LIMIT21.9)1)(21.9(

792)10)(21.9(7.07.0

21.9101

7.0107.0

'

'

Example 2

Determine the Reverse voltage (VR). Alsofind the voltage across the limiting resistor. Assumed IR = 1 µA.

Answer:

VRLIMIT =1mV

VR=4.999V

Diode Testing

• Analog MM (or Ohm meter testing)

Figure: Checking a diode with an ohmmeter.

Diode Testing

• Digital MM

Figure: DMM diode test on a properly functioning diode.

Diode Testing – Defective diode

• Digital MM (Testing Defective Diode)

Diode failed open: get open circuit reading (2.6 V) or ‘OL’

Diode is shorted: get 0 V reading in both forward and reverse bias test.

Diode Notation

Zener Diode

Figure: Characteristics of Zener region.

Figure: Conduction direction: (a) Zener diode (b) semiconductor diode

(c) resistive element.

Zener Zener Region

• The Zener region is in the diode’s reverse-bias region.

• At some point the reverse bias voltage is so large the diode breaks down and the reverse current increases dramatically.

• This maximum voltage is called avalanche avalanche (runtuhan) breakdown (runtuhan) breakdown voltagevoltage

• The current is called avalanche currentavalanche current.