JAI GURU DEV

Maharishi international residential school

Project : PHYSICS
Topic : SEMI-CONDUCTOR
 By :R K SAI SUTHARSHANAN
Introduction: A semiconductor material has
an electrical conductivity value falling between that of
a conductor , such as metallic copper, and
an insulator, such as glass. Its resistance decreases as
its temperature increases, which behaviour is
opposite to that of a metal. Its conducting properties
may be altered in useful ways by the deliberate,
controlled introduction of impurities ("doping ") into
the crystal structure . Where two differently-doped
regions exist in the same crystal,
a semiconductor junction is created. The behavior
of charge carriers which
include electrons , ions and electron holes at these
junctions is the basis of diodes, transistors and all
modern electronics . Some examples of
semiconductors are silicon , germanium , gallium
arsenide , and elements near the so-called "metalloid
staircase " on the periodic table. After silicon, gallium

2
arsenide is the second most common
semiconductor[citation needed ] and is used in laser
diodes, solar cells, microwave-frequency integrated
circuits and others. Silicon is a critical element for
fabricating most electronic circuits.

SEMI-CONDUCTOR DEVICES: can display a range of

useful properties such as passing current more easily
in one direction than the other, showing variable
resistance, and sensitivity to light or heat. Because the
electrical properties of a semiconductor material can
be modified by doping, or by the application of
electrical fields or light, devices made from
semiconductors can be used for amplification,
switching, and energy conversion .
The conductivity of silicon is increased by adding a
small amount (of the order of 1 in 108) of pentavalent
(antimony , phosphorus , or arsenic ) or trivalent
(boron, gallium, indium) atoms. This process is known
as doping and resulting semiconductors are known as
doped or extrinsic semiconductors. Apart from
doping, the conductivity of a semiconductor can
equally be improved by increasing its temperature.
This is contrary to the behaviour of a metal in which
conductivity decreases with increase in temperature.

3
The modern understanding of the properties of a
semiconductor relies on quantum physics to explain
the movement of charge carriers in a crystal
lattice. Doping greatly increases the number of
charge carriers within the crystal. When a doped
semiconductor contains mostly free holes it is called
"p-type", and when it contains mostly free electrons it
is known as "n-type". The semiconductor materials
used in electronic devices are doped under precise
conditions to control the concentration and regions
of p- and n-type dopants. A single semiconductor
crystal can have many p- and n-type regions; the p-n
junctions between these regions are responsible for
the useful electronic behavior.
Some of the properties of semiconductor materials
were observed throughout the mid 19th and first
decades of the 20th century. The first practical
application of semiconductors in electronics was the
1904 development of the cat’s-whisker devices , a
primitive semiconductor diode used in early radio
receivers. Developments in quantum physics in turn
allowed the development of the transistor in 1947l and
the integrated in 1958.

PROPERTIES: Variable electrical conductivity

4
 Semiconductors in their natural state are poor
conductors because a current requires the flow of
electrons, and semiconductors have
their valence filled, preventing the entry flow of
new electrons. There are several developed
techniques that allow semiconducting materials to
behave like conducting materials, such
as doping or gating. These modifications have two
outcomes: n-type and p-type. These refer to the
excess or shortage of electrons, respectively. An
unbalanced number of electrons would cause a
current to flow through the material.
Heterojunctions
Heterojunctions occur when two differently doped
semiconducting materials are joined together. For
example, a configuration could consist of p-doped
and n-doped germanium . This results in an
exchange of electrons and holes between the
differently doped semiconducting materials. The n-
doped germanium would have an excess of
electrons, and the p-doped germanium would have
an excess of holes. The transfer occurs until
equilibrium is reached by a process

5
 called recombination , which causes the migrating
electrons from the n-type to come in contact with
the migrating holes from the p-type. A product of
this process is charged ions, which result in
an electric field.
Excited electrons
A difference in electric potential on a
semiconducting material would cause it to leave
thermal equilibrium and create a non-equilibrium
situation. This introduces electrons and holes to
the system, which interact via a process
called ambipolar junction . Whenever thermal
equilibrium is disturbed in a semiconducting
material, the number of holes and electrons
changes. Such disruptions can occur as a result of a
temperature difference or photons , which can
enter the system and create electrons and holes.
The process that creates and annihilates electrons
and holes are
called generation and recombination .
Light emission

6
 In certain semiconductors, excited electrons can
relax by emitting light instead of producing heat.
These semiconductors are used in the construction
of light emitting diode and fluorescent quantum .
High thermal conductivity

Semiconductors with high thermal conductivity can

be used for heat dissipation and improving thermal
management of electronics.
Thermal energy conversion
Semiconductors have large thermoelectric making
them useful in thermoelectric power generator, as
well as high thermoelectric figures of merit making
them useful in thermoelectric coolers.

CONCLUSION : The two energy bands in solids are

valence band and the conduction band. Depending
on the gap between the two bands, solids are
classified as conductors, insulators and
semiconductors. Semiconductors are those whose
conductivity can be increased by doping and also by
increasing the temperature. Doping results in P - type
and N - type semiconductor. The P and N can be
fused to form a PN junction diode which is used to
rectify A.C. By sandwiching a P type between two N

7
type and vice versa, transistor is produced which is
used to amplify and sustain oscillation in an oscillator.