Transcript Semiconductor Material & Devices

Semiconductor Material &
Devices
AN OVERVIEW OF THE SUBJECT
Usman Ali Khan
Contents
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Subject Information
The study of Electronics
History
Semiconductor
Materials
Atomic Structure
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 Manzar Saeed
Basics 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
Introduction
Semiconductor Devices
Building blocks of useful electronic devices
Semiconductor devices include:
Diodes
PN
junction
Light Emitting Diode (LED)
Zener Diode
Tunnel Diode
Varactor Diode
Laser Diode
Photo Diode
Transistors
Bipolar
Junction Transistor (BJT)
• NPN BJT
• PNP BJT
Junction Field Effect Transistor
(JFET)
Amplifier Fundamentals
Small Signal Transistor Amplifier
Integrated Circuits (ICs)
Analog ICs
Digital 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
Nucleus
Atom is composed of three basic
particles:
Protons
(+ive charge)
Neutrons
Electrons (-ive charge)
Silicon Atom
Orbits or Shells
K, L, M,N
Draw the atomic
structure of Ge (32)
P=14
N=14
Valence Shell
+
Ne( Electrons in nth orbit) = 2n2
Sub-shells
Shell
Sub-shell
Capacity
K
s
2
s
2
p
6
s
2
p
6
d
10
s
2
p
6
d
10
f
14
L
M
N
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.
+
Valence electrons have more tendency to become free
electrons because of less attraction force between nucleus
and valence shell
Free electrons in (i) conductors (ii) Insulators & (iii) Semiconductors
Flow of Free Electrons (Current)
Material containing free electrons
Force of attraction
Force of repulsion
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Excess of electrons
Lack of electrons
Silicon Crystal (Covalent Bonding)
Si Crystal
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For stability there should be 8 electrons in valence shell
Current in Semiconductors
HOLE CURRENT
Usman Ali Khan
Contents
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2.
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Basics
Electron Energy
Energy Bands
Temperature &
Resistance
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 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 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
Conductivity
σ
Siemens/meter
S/m
Important Relations
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V = IR (Ohm's Law)
I = Q/t
W = QV
R = ρl/A
G = 1/R
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
Hole created
Electron Freed
(Conduction Band)
+
Covalent bond ruptured
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Valence band
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Energy 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
E2
Therefore:
P=14
N=14
E3
E1
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:
 The region between valence and conduction band
 No electrons can stay at this energy levet
Energy Bands
eV
Conduction Band
(Free Electrons)
Energy
Gap
Forbidden Band
Valence Band
(Electrons in Valence Shell)
Energy gap is the
energy required to
rupture covalent
bond
Energy Bands for Different Materials
Conduction Band
Conduction Band
Forbidden Band
Forbidden Band
Valence Band
Valence Band
Conductors
Insulators
Conduction Band
Conduction Band
1.1eV
≤0.01eV
Forbidden Band
0.67eV
Forbidden Band
Valence Band
Valence Band
Silicon
Germanium
Temperature dependent
Temperature & Resistance
dR
Temp. Coeff. = α  
dT
α = -ive
α = +ive
I
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R
R
T
Conductors
T
Semiconductors
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Holes & Hole Current
Hole Movement
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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
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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 1019
8.4 X 1028
Thank You