Electronic Devices

This presentation intends to delve into the content outlined in Unit 1 and
Unit 2 of the course syllabus, specifically focusing on Semiconductor Physics
and PN Junctions.
 The rationale for studying and acquainting
oneself with electronic devices.
Studying Electronic Devices serves several
purposes, both theoretical and practical:

Understanding Technology: Electronic devices are ubiquitous in modern society, powering

everything from smartphones to medical equipment. Studying them helps us understand how these devices
work, enabling us to use and interact with technology more effectively.
Comprehensive Understanding: Grasp fundamental components and principles of modern
communication systems.
Design and Analysis Skills : Acquire skills to design, analyse, and optimise electronic circuits and
devices.
Specialisation Opportunities : Explore areas like wireless communication, satellite systems, optical
communication, and digital signal processing.
Stay Updated and Innovate : Keep abreast of advancements and contribute to designing new and
enhancing existing communication systems.
SEMICONDUCTORS AND THEIR TYPES
What are Semiconductors and what are they used for?

Material Properties: Semiconductors are materials that have electrical conductivity between that of conductors and
insulators.
Band Structure: Their conductivity arises from the ability to control the flow of electrical current through manipulation
of their energy bands.
Doping: Adding impurities to semiconductors through a process called doping can alter their electrical properties.
Types: Common semiconductor materials include silicon (Si), germanium (Ge), and compound semiconductors like
gallium arsenide (GaAs).
Applications of Semi
Conductors:
Electronics: Semiconductors form the foundation of modern electronics, including integrated circuits (ICs), transistors,
and diodes.
Computing: They enable the creation of microprocessors, memory chips, and other essential components in computers
and smartphones.
Communication: Semiconductors are vital in telecommunications devices such as cell phones, satellite communication
systems, and networking equipment.
Renewable Energy: They play a key role in solar cells, converting sunlight into electricity in photovoltaic panels.
Lighting: Light-emitting diodes (LEDs), which use semiconductors to emit light efficiently, are widely used in lighting
applications.
Based upon doping, Semiconductors can be classified
into two main types:
Semiconducto
rs

Intrinsic Extrinsic
Semiconductors Semiconductors
 Intrinsic and Extrinsic Semiconductors

Intrinsic Semiconductors: Intrinsic semiconductors are pure semiconducting materials like silicon or germanium with
no intentional impurities added. In these materials, electrons are bound to their atoms, but at room temperature, some
electrons gain enough energy to break free from their bonds, creating electron-hole pairs. This leads to conductivity,
though not as high as in metals. Examples include , Silicon (Si), Germanium (Ge).

Extrinsic Semiconductors: Extrinsic semiconductors are doped with impurities to modify their electrical properties.
There are two types of extrinsic semiconductors based on the type of doping:a. N-type Semiconductors: N-type
semiconductors are doped with elements that have more electrons in their outer shells (Pentavalent impurities) than the
host material. Commonly used dopants for creating N-type semiconductors include phosphorus, arsenic, and antimony.
These impurities introduce excess free electrons into the material, which become the majority charge carriers, thus
increasing conductivity.b. P-type Semiconductors: P-type semiconductors are doped with elements that have fewer
electrons in their outer shells (Trivalent impurities) than the host material. Boron, gallium, and indium are common
dopants for creating P-type semiconductors. These impurities create "holes" or vacancies in the crystal lattice, which act
as positive charge carriers. In P-type semiconductors, holes are the majority charge carriers, contributing to increased
conductivity.
 Carrier Statistics
Carrier statistics basically refers to how electrons and holes, the charge carries in
semiconductors, distribute themselves among available energy states according to
probabilistic laws , such as Fermi-Dirac statistics.
In a semiconductor, electrons can occupy energy levels within the conduction band, while holes can occupy energy
levels within the valence band. The distribution of electrons and holes among these energy levels follows Fermi-Dirac
statistics.
Fermi-Dirac statistics govern the probability of occupation of energy levels by fermions (particles with half-integer spin,
such as electrons and holes) in a system at a given temperature.
At absolute zero temperature (0 Kelvin), all energy levels below the Fermi level are filled with electrons in the valence
band, and all energy levels above the Fermi level are empty in the conduction band.
As temperature increases, some electrons in the valence band gain enough thermal energy to transition to the
conduction band, creating electron-hole pairs.

Fermi Dirac function ,F(E) = 1\ (1+exp[(E-

Ev)kT]

Where E Energy possessed by

electrons,

Ev Fermi energy
 Thermal Equilibrium Carrier Concentration
Thermal equilibrium carrier concentration refers to the balance between the number of free charge carriers
(electrons and holes) in a semiconductor material at a given temperature when there is no net flow of carriers. In
other words, it's the condition where the rate of carrier generation (due to thermal excitation) equals the rate of
carrier recombination (due to thermal motion).
Electrons and Holes: In an intrinsic semiconductor (un doped semiconductor), the number of electrons in the conduction band equals the number of
holes in the valence band. This balance occurs due to the energy distribution of electrons and holes across the energy bands, governed by Fermi-
Dirac statistics.
Carrier Generation and Recombination: At a given temperature, some electrons gain enough thermal energy to move from the valence band to
the conduction band, creating electron-hole pairs. Simultaneously, electron-hole pairs can recombine, where an electron falls back into a hole,
releasing energy (e.g., as heat or light). At thermal equilibrium, the rate of carrier generation equals the rate of carrier recombination.
Intrinsic Carrier Concentration: The concentration of electron-hole pairs that exist in an intrinsic semiconductor at thermal equilibrium is called the
intrinsic carrier concentration(ni). It depends exponentially on temperature and the bandgap energy of the semiconductor material.

The formula for intrinsic carrier concentration is given by:

ni2 = Nc Nv exp[ -Eg / kT]

Where:
Nc is the effective density of states in the conduction band
Nv is the effective density of states in the valence band
Eg is the band gap Energy ( Eg = Ec - Ev)
K is the Boltzmann Constant
T is the temperature in Kelvin
Energy Band Diagrams in Intrinsic
& Extrinsic Semiconductors
Band Diagram in Intrinsic Semiconductor

Conduction
Band

In an Intrinsic Semiconductor, the

fermi energy lies at the mid-band
E gap energy level. This means that
Fi Fermi energy is halfway between
valence band and conduction
band.

Valence
Band
 Band Diagram N Type
Semiconductor
Conduction
Band In n-type semiconductors,
EFn the Fermi level lies closer to
the conduction band due to
E
the presence of extra
d
electrons introduced by
EFi doping, facilitating the
conductivity of the material
Do by providing abundant
nor charge carriers for
Le conduction.
Valence vel
Band
 Band Diagram In p-type Semiconductor

Conduction
In p-type semiconductors,
Band
the Fermi level lies closer
to the valence band due to
the presence of holes
EFi introduced by doping,
Ea
facilitating the conductivity
of the material by
EFp providing charge carriers
Valence (holes) for conduction.
Acceptor
Level Band
Carrier Transport
Phenomena
 Carrier Transport By Drift
An electric field applied to a semiconductor will produce a force on electrons and holes so that they will
experience a net acceleration and net movement, provided there are available energy states in the conduction
and valence bands. This net movement of charge due to an electric field is called drift. The net drift of charge
gives rise to a drift current.
1. Drift Current

The formula For Drift Current is given by :

Average Drift Velocity = Mobility x Electric Field
VdOr=
2. Drift Current Density
The formula for Drift current density foru.E
Holes and Electrons respectively is given by:
1) For Holes,

2) For Electrons , Jdp = Where, q is the charge , p is

3) Total Density, q.p.Vdp the hole carrier
concentration, n is the electron
Jdn = (-q.n). carrier concentration
Vdn

Jdrift = q (un n + up p)
E
Since both electrons and holes contribute to the drift current, the total drift current
density is the sum of the individual electron and hole drift current densities
 Carrier Transport By Diffusion
Diffusion is the process whereby particles flow from a region of high concentration toward a region of low
concentration. If the particles were electrically charged, the net flow of charge would result in a diffusion current.
A. Electron Diffusion Current Density :

It is given by :
2) Hole Diffusion Current Density:
It is given by:
3) Diffusion Constant and Mobility: Where q is the charge, Dn is the
Jn =Constant
Diffusion -q . diffusion
(D) = Mobility(u) constantVoltage(V
x Thermal , dn/dx isT)the
Einstein Equation: Dn .dn/dx electron concentration gradient

Jp = +q .
Dp .dp/dx

Dn/un = Dp/up= VT =
T/1600 = KT/q