A N O V E R V I E W O F T H E S U B J E C T

Semiconductor Material & Devices

Usman Ali Khan

Contents

1. Subject Information

2. The study of Electronics

3. History

4. Semiconductor 4. Semiconductor Materials

5. Atomic Structure

Subject Information

Code: EE120Text Book: Electronic Devices & Circuits by Theodore F. Bogart 6th ed.Electronic Devices & Circuits by David A Bell 4th ed.Electronic Devices & Circuits by Floyd Electronic Devices & Circuits by Floyd Electronic Devices & Circuits by Manzar SaeedBasics of Electronic Device by NIIT

Marks distribution

Total Marks: 150 Theory: 100 Practical: 50 Session Marks: 20

• Assignments: 05• Assignments: 05• Quiz: 05• Project + Presentations: 05• Attendance: 05

IntroductionSemiconductor Devices

Building blocks of useful electronic devicesSemiconductor devices include:

DiodesPN junctionPN junctionLight Emitting Diode (LED)Zener DiodeTunnel DiodeVaractor DiodeLaser DiodePhoto Diode

TransistorsBipolar Junction Transistor (BJT)

• NPN BJT• PNP BJT

Junction Field Effect Transistor (JFET)(JFET)

Amplifier FundamentalsSmall Signal Transistor Amplifier

Integrated Circuits (ICs)Analog ICsDigital ICs

Basic Atomic Theory

Every chemical element is composed of atoms

All atoms within a single element have same structure

Every element is unique because the structure of its atoms is uniqueatoms is unique

Atom is composed of three basic particles:Protons (+ive charge)

Neutrons

Electrons (-ive charge)

Nucleus

Silicon Atom

Orbits or Shells K, L, M,N Draw the atomic

structure of Ge (32)

P=14N=14

Valence Shell

Ne( Electrons in nth orbit) = 2n2

+

Sub-shells

Shell Sub-shell CapacityK s 2

Ls 2

p 6p 6

M

s 2

p 6

d 10

N

s 2

p 6

d 10

f 14

Free Electrons

When electrons get enough energy (e.g. from heating), they leave their parent atoms and become free electrons. Flow of free electrons is called current. Therefore more free electrons and more current.

+

Free electrons in (i) conductors (ii) Insulators & (iii) Semiconductors

Valence electrons have more tendency to become free electrons because of less attraction force between nucleus and valence shell

Flow of Free Electrons (Current)

Material containing free electrons

--

Force of repulsion Force of attraction

--

- +

Excess of electrons Lack of electrons

-

Silicon Crystal (Covalent Bonding)

For stability there should be 8 electrons in valence shell

+ *

*

*

*

*

*

*

*

+ +

+

+

+

+

**

**

* ** * +

**

**

* * +

**

**

* *+

**

**

* *

+* * * *

*

*

* *+* *

*

*

+* *

*

*

+*

*

+* * * *

*

*

+* * * *

*

*

+* *

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+*

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Si Crystal

H O L E C U R R E N T

Current in Semiconductors

Usman Ali Khan

Contents

1. Basics

2. Electron Energy

3. Energy Bands3. Energy Bands

4. Temperature & Resistance

5. Holes & Hole Current

Basics:Rupturing of covalent bond

The unit of energy is electronvolt(eV)

Energy acquired by one electron if it is accelerated through potential difference of one volt

1 eV = 1.602 x 10-19 J1 eV = 1.602 x 10-19 J

Valence Electron energy considerably large and need a few amount of energy to release

Electrons in inner shell possess little energy and need a large amount of energy to release

Electrons can lose energy in the form of heat and light

Free electrons can alco lose and fall into valence shell

Important Quantities

Quantity Symbol Unit Unit SymbolCurrent I Ampere A

Voltage V Volt V

Charge Q Coulomb C

Energy W,E Joule J

Electric Field Strength E Volt/meter V/m

Volume V Cubic meter m3

Area A Squared meter m2

Resistance R Ohm Ω

Conductance G Moh, Siemens S

Resistivity ρ Ohm-meter Ω-m

Conductivity σ Siemens/meter S/m

Important Relations

V = IR (Ohm's Law) I = Q/t W = QV R = ρl/A

G = 1/Rσ = 1/ ρ σ = 1/ ρ

Charge on electron = e = 1.602 X 10-19 C Electron energy = 1 eV = 1.602 X 10-19 J

Rupturing of Covalent Bonds

+ Electron Freed

+ +

+

+

+ Electron Freed (Conduction Band)Hole created

Covalent bond ruptured

Valence bandEnergy is supplied in the form of heat to rupture covalent band

Electron Energy

Electrons closer to nucleus are more tightly bound and need more energy to become free

P=14N=14

E1

E2

E3

Therefore:

E1 > E2 > E3

If free electron loses energy and falls back to valence band, this process is called “Annihilation” or “Recombination”

Lost energy emits as light

Energy Bands:Quantum theory explain these bands as

Conduction Band :

Free electrons accommodate there

Valence Band :

Electrons having lesser energy accommodate there

Forbidden band: Forbidden band:

The region between valence and conduction band

No electrons can stay at this energy levet

Energy Bands

eV

Conduction Band(Free Electrons)

Valence Band(Electrons in Valence Shell)

Forbidden Band

(Free Electrons)

Energy Gap

Energy gap is the energy required to rupture covalent bond

Energy Bands for Different Materials

Forbidden Band

Conduction Band

Forbidden Band

Conduction Band

≤0.01eV

Valence BandValence Band

Valence Band

Forbidden Band

Conduction Band

Valence Band

Forbidden Band

Conduction Band

Insulators Conductors

Silicon Germanium

1.1eV0.67eV

Temperature dependent

Temperature & Resistance

dT

dR=αTemp. Coeff. = α

α = -ive

T

I

R

Conductors Semiconductors

R

I

T

α = +iveα = -ive

Holes & Hole Current

+ +++

+ + ++

+ +++

+Hole Movement

Electron Movement

Hole Current Vs Electron Current

The movement of holes and electrons is in opposite directions

There are no holes in pure conductors, they are only created in semiconductors

There are two currents in semiconductors:There are two currents in semiconductors:Hole current (Band ? Charge ?)Free electron current (Band ? Charge ?)

The total current in semiconductor materials is the sum of hole current and electron current

Number of holes = ?

Charge Carriers

Holes are called positive charge carriers

Free electrons are called negative charge carriers

For pure (Intrinsic) semiconductors: Number of positive charge carriers = Number of negative charge carriers

Is there any way to make charge carriers unequal? Is there any way to make charge carriers unequal?

Let hole density be pi (holes/m3) and electron density be ni (electrons/m3) where i denotes intrinsic semiconductor, then:

ni = pi

++

+

+++

-

-

-

-

-

-

Intrinsic Semiconductor

Charge Carriers at Room Temperature

SiliconCarriers/m3

GermaniumCarriers/m3

CopperCarriers/m3

1.5 X 1016 2.4 X 1019 8.4 X 1028

Thank You