Electronic Devices and Circuit

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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

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Subject Information

Code: EE120

Text 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 Saeed Basics of Electronic Device by NIIT

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Marks distribution

Total Marks: 150 Theory: 100

Practical: 50 Session Marks: 20

• Assignments: 05

• Quiz: 05

• Project + Presentations: 05

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Introduction

Semiconductor Devices

Building blocks of useful electronic devices Semiconductor devices include:

Diodes

PN junction PN junction

Light Emitting Diode (LED) Zener Diode

Tunnel Diode Varactor Diode Laser Diode

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Transistors

Bipolar Junction Transistor (BJT)

• NPN BJT

• PNP BJT

Junction Field Effect Transistor (JFET)

(JFET)

Amplifier Fundamentals

Small Signal Transistor Amplifier

Integrated Circuits (ICs)

Analog ICs Digital ICs

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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

atoms is unique

Atom is composed of three basic particles:

Protons (+ive charge) Neutrons

Electrons (-ive charge)

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Silicon Atom

Orbits or Shells

K, L, M,N _{Draw the atomic}

structure of Ge (32) P=14 N=14 Valence Shell N_e( Electrons in nth orbit) = 2n2

+

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Sub-shells

Shell Sub-shell Capacity

K s 2 L s 2 p 6 p 6 M s 2 p 6 d 10 N s 2 p 6 d 10 f 14

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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

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Flow of Free Electrons (Current)

Material containing free electrons

-Force of repulsion Force of attraction

-- +

Excess of electrons Lack of electrons

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-Silicon Crystal (

Covalent

Bonding)

For stability there should be 8 electrons in valence shell

+

* * * * * * * *

+

+ * * * * * * * * + * * * * * * + * * * * * * + * * * * * * + * * * * * * * * + * * * * + * * * * + * * + * * * * * * + * * * * * * + * * * * + * * Si Crystal

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H O L E C U R R E N T

Current in Semiconductors

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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 _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

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Important Quantities

Quantity Symbol Unit Unit Symbol

Current 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

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Important Relations

V = IR (Ohm's Law) I = Q/t W = QV R = ρl/A G = 1/R σ = 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

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Rupturing of Covalent Bonds

+

Electron Freed

+

Electron Freed (Conduction Band) Hole created

Covalent bond ruptured

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

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Electron Energy

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

P=14 N=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”

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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

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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

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Energy Bands for Different Materials

Forbidden Band Conduction Band Forbidden Band Conduction Band ≤0.01eV

Valence Band Valence Band

Valence Band Forbidden Band Conduction Band Valence Band Forbidden Band Conduction Band Insulators Conductors Silicon Germanium 1.1eV 0.67eV Temperature dependent

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Temperature & Resistance

dT dR = α Temp. Coeff. = α α = -ive T I R Conductors Semiconductors R I T α = +ive α = -ive

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Holes & Hole Current

+ + + + + + + + + + + + + Hole Movement Electron Movement

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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

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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 p_i(holes/m3_{) and electron density be n}

i (electrons/m3)

where i denotes intrinsic semiconductor, then:

n_i = p_i + + + + + + -Intrinsic Semiconductor

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Charge Carriers at Room Temperature

Silicon Carriers/m3 Germanium Carriers/m3 Copper Carriers/m3 1.5 X 1016 _{2.4 X 10}19 _{8.4 X 10}28

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