semiconductor physics

1. UNIT-I :
Semiconductor
Devices and
Characteristics
By. G Sambasiva Rao
ECE/Dept.

2. UNIT-I Semiconductor Devices and Characteristics: Introduction to
semiconductor physics: Energy bands in semiconductor, Direct and
indirect band-gap semiconductors, Carrier concentration in
semiconductor, Drift and diffusion current, Hall effect, Mobility and
resistivity, Generation and recombination of carriers; P-N junction
Diode: Formation of P-N junction, Working of diode, I-V characteristics,
Small signal switching models, Avalanche breakdown, Operation and
characteristics of Zener diode, Schottky diode, Tunnel diodes, Varactor
diode, PIN diode.
UNIT II: Applications of Diodes: Diode circuits: Half wave, Full
wave and Bridge rectifiers, Filters, Voltage multiplier, Clipper circuits,
Clamper circuits, Voltage regulator circuit using Zener diode.
UNIT–III: Bipolar Junction Transistors: Introduction, Transistor
construction, Transistor operation, Transistor current components,
Transistor as an amplifier, Common base configuration, Common
emitter configuration, Common collector configuration, Limits of
operation, Transistor specifications.
SYLLABUS

3. UNIT–IV Field Effect Transistors: Junction Field Effect Transistor
(JFET) - Principle of operation, Volt ampere characteristics, Advantages of
JFET over BJT, Introduction to MOSFETs - depletion and enhancement
type MOSFETs, Operation and Volt-ampere characteristics.
UNIT–V BJT and FET Biasing: Need for biasing, Operating point, Load
line analysis, Bias stabilization techniques: Fixed bias, Collector to base
bias, Self-bias, Stabilization against variations in Ico, VBE and β for the
self bias circuit, Bias compensation techniques, Thermal runaway and
Thermal stability. FET Biasing: Biasing techniques: Fixed bias, Source
self-bias, Voltage divider bias.
Text Books
1. Electronic Devices and Circuits – J.Millman, C.C.Halkias, and
Satyabratha Jit Tata McGraw Hill, 2nd Ed., 2007.
2. Neamen, Donald A. Semiconductor physics and devices: basic
principles. New York, NY: McGraw-Hill„ 2012.
SYLLABUS

4.  The goal of electronic materials is to generate and
control the flow of an electrical current.
 Electronic materials include:
1. Conductors: have low resistance which allows
electrical current flow
2. Insulators: have high resistance which
suppresses electrical current flow
3. Semiconductors: can allow or suppress
electrical current flow
Electronic Materials

7. Conductors
 Good conductors have low resistance so electrons flow
through them with ease.
 Best element conductors include:
 Copper, silver, gold, aluminum, & nickel
 Alloys are also good conductors:
 Brass & steel
 Good conductors can also be liquid:
 Salt water

8. Insulators
 Insulators have a high resistance so current does not flow in
them.
 Good insulators include:
 Glass, ceramic, plastics, & wood
 Most insulators are compounds of several elements.
 The atoms are tightly bound to one another so electrons are
difficult to strip away for current flow.

9.  Semiconductors are materials that essentially can be
conditioned to act as good conductors, or good insulators, or
any thing in between.
 Common elements such as carbon, silicon, and germanium
are semiconductors.
 Silicon is the best and most widely used semiconductor.
Semiconductors

10.  The unique capability of
semiconductor atoms is
their ability to link together
to form a physical structure
called a crystal lattice.
 The atoms link together
with one another sharing
their outer electrons.
 These links are called
covalent bonds.
Crystal Lattice Structure
2D Crystal Lattice Structure

11.  If the material is pure semiconductor material like silicon, the
crystal lattice structure forms an excellent insulator since all the
atoms are bound to one another and are not free for current
flow.
 Good insulating semiconductor material is referred to as
intrinsic.
 Since the outer valence electrons of each atom are tightly bound
together with one another, the electrons are difficult to dislodge
for current flow.
 Silicon in this form is a great insulator.
 Semiconductor material is often used as an insulator.
Semiconductors can be Insulators

12.  To make the semiconductor conduct electricity, other atoms
called impurities must be added.
 “Impurities” are different elements.
 This process is called doping.
Doping

13.  An impurity, or element
like arsenic, has 5
valence electrons.
 Adding arsenic (doping)
will allow four of the
arsenic valence
electrons to bond with
the neighboring silicon
atoms.
 The one electron left
over for each arsenic
atom becomes available
to conduct current flow.
Semiconductors can be Conductors

14.  The silicon doped with extra electrons is called an “N
type” semiconductor.
 “N” is for negative, which is the charge of an electron.
 Silicon doped with material missing electrons that produce
locations called holes is called “P type” semiconductor.
 “P” is for positive, which is the charge of a hole.
Types of Semiconductor

17. Carrier Concentration
Carrier Concentration (intrinsic)
Inside a semiconductor, electrons and holes are generated with thermal
energy. The electron and hole concentration remain constant as long as the
temperature remain constant.
At temperature TK , in an intrinsic semiconductor n = p = ni where ni is called
intrinsic concentration.
Also the product

18. Substituting values of n and p in above equation (1)
Substituting values of Nv and Nc

19. Carrier Concentration (Extrinsic)
As in pair production n = p = ni,
Also the product
where ni is called intrinsic concentration.
Suppose ND and NA are concentration of donor atom in n-type
semiconductor and concentration of acceptor atom p-type
semiconductor respectively.
N-type
Since in N-type semiconductor, majority charge carriers are electrons. The
hole concentration 'p' in comparison to electron concentration 'n' may be
ignored. Also each donor atom contribute one electron to the crystal, the
electron concentration in n-type is approximately equal to concentration of
donor atoms,
i.e.,
Using equation no. 1

20. p-type
Since in p-type semiconductor, majority charge carriers are holes. The
electron concentration 'n' in comparison to hole concentration 'p' may be
ignored. Also each acceptor atom contribute one hole to the crystal, the hole
concentration in p-type is approximately equal to concentration of acceptor
atoms,
i.e.,
Using equation no. 1

21. Charged particles move or drift under the
influence of the applied field.
The resulting current is called drift current
drift current

25. Diffusion:
Due to non-uniform carrier concentration in a semiconductor, the
charge carriers moves from a region of higher concentration to a
region of lower concentration. This process is known as diffusion of
charge carriers.

26. When a Magnetic field is applied perpendicular to a
current Carrying Conductor or Semiconductor,
Voltage is developed across the specimen in a
direction perpendicular to both the current and the
Magnetic field.
This phenomenon is called the Hall effect and
voltage so developed is called the Hall voltage.
Let us consider, a thin rectangular slab carrying
Current in the X-direction. If we place it in a Magnetic
field B which is in the y-direction.
Potential difference Vpq will develop between the
faces p and q which are perpendicular to the z-
direction.
Hall Effect

30. VH be the Hall Voltage in equilibrium , the Hall Electric field is
given by