Semiconductor Material & Devices

AN OVERVIEW OF THE SUBJECT

Semiconductor Material & Devices

Usman Ali Khan

Contents

1. Subject Information2. The study of

Electronics3. History4. Semiconductor

Materials5. 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 Manzar SaeedBasics of Electronic Device by NIIT

Marks distribution

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

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

IntroductionSemiconductor Devices

Building blocks of useful electronic devicesSemiconductor devices include:

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

TransistorsBipolar Junction Transistor (BJT)

•NPN BJT•PNP BJT

Junction Field Effect Transistor (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 unique

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

Nucleus

Silicon Atom

P=14N=14

Orbits or Shells K, L, M,N

Valence Shell

Ne( Electrons in nth orbit) = 2n2

+

Draw the atomic structure of Ge (32)

Sub-shells

Shell Sub-shell CapacityK s 2

Ls 2

p 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

--

- +

Excess of electrons Lack of electrons

-

Force of repulsion Force of attraction

Silicon Crystal (Covalent Bonding)

+

For stability there should be 8 electrons in valence shell

+

+

+

+

+

**

**

* ** * +

**

**

* * +

**

**

* *+

**

**

* *

+* * * *

*

*

* *+* *

*

*

+* *

*

*

+*

*

+* * * *

*

*

+* * * *

*

*

+* *

*

*

+*

*

Si Crystal

HOLE CURRENT

Current in Semiconductors

Usman Ali Khan

Contents

1. Basics 2. Electron Energy3. Energy Bands4. Temperature &

Resistance5. Holes & Hole

Current

Basics:Rupturing of covalent bondThe unit of energy is electronvolt(eV)Energy acquired by one electron if it is accelerated through potential difference of one volt1 eV = 1.602 x 10-19 JValence 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 releaseElectrons 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/AG = 1/Rσ = 1/ ρCharge on electron = e = 1.602 X 10-19 CElectron energy = 1 eV = 1.602 X 10-19 J

Rupturing of Covalent Bonds

+ +

+

+

+ Electron Freed (Conduction Band)Hole created

Covalent bond ruptured

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

Electron Energy

P=14N=14

E1

E2

E3

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

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 thereValence Band :Electrons having lesser energy

accommodate thereForbidden band:The region between valence and conduction bandNo electrons can stay at this energy levet

Energy Bands

eV

Valence Band(Electrons in Valence Shell)

Forbidden Band

Conduction Band(Free Electrons)

Energy Gap

Energy gap is the energy required to rupture covalent bond

Energy Bands for Different Materials

Valence Band

Forbidden Band

Conduction Band

Valence Band

Forbidden Band

Conduction Band

Valence Band

Forbidden Band

Conduction Band

Valence Band

Forbidden Band

Conduction Band

Insulators Conductors

Silicon Germanium

1.1eV0.67eV

Temperature dependent

≤0.01eV

Temperature & Resistance

T

I

R

Conductors Semiconductors

R

I

dT

dR

T

Temp. Coeff. = α

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: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? 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

Silicon

Carriers/m3

Germanium

Carriers/m3

Copper

Carriers/m3

1.5 X 1016 2.4 X 1019 8.4 X 1028

Thank You