Semiconductor
is
a material which has a resistance is in between the conductor and insulators. Resistivity
of semiconductor is 10-4
to 107. We have two types
of semiconductors
are there. One is extrinsic semiconductor and another one is intrinsic semiconductor.
Properties of Semiconductor:
1. Semiconductors
having moderate resistivity.
2. Semiconductor
always obeys negative (-ve) temperature coefficient
3. Valence
electrons of semiconductors are
bounded and some are loosely coupled to their atoms.
4. Semiconductors
requires
energy less than an insulators and more than conductors to remove electrons
from parent atoms.
Types of
semiconductors
We have two types of
semiconductors are there. One is extrinsic semiconductor and another one is intrinsic
semiconductor.
· Extrinsic
Semiconductor
· Intrinsic
Semiconductor
Intrinsic
Semiconductor:
A semiconductor in its purest form is known as the intrinsic
semiconductor. These are the intrinsic semiconductors.
Ex: Pure silicon, Pure
germanium.
In intrinsic semiconductors even at room temperature it can creates
electron-hole pairs. Whenever we applied electric field to the intrinsic
semiconductor then it will start conduction. Conduction takes place because of
movement of electrons and holes opposite direction in conduction band and
valance bands.
The numbers of
conduction electrons are equal to number of holes. The energy band diagram of
intrinsic semiconductor is shown in
below.
 |
| Intrinsic Semiconductor |
Extrinsic Semiconductor
For getting good
conducting properties, we will add some impurity atoms or doping agent is added
to pure semiconductor (intrinsic semiconductor).
The process of adding
impurity to the pure semiconductors
is called “Doping”. Why we add some impurity or doping agent to the pure
semiconductor is “to increase the conducting properties of either free
electrons or free holes in semiconductors.
Extrinsic semiconductors are classified as two
types because depending upon the impurity added to the semiconductors.
- · N-Type Semiconductor
- · P-Type Semiconductor
N-Type Semiconductor
N-Type Semiconductor is
formed when a pentavalent impurity is added to the pure semiconductor (Intrinsic
Semiconductor). The pentavalent impurities are 5th group elements.
Ex: Arsenic, antimony, Phosphorus,
Al, Sb, In etc..
Suppose if we take a antimony atom, each
antimony atom forms the four covalent bands to with their surrounding silicon
or germanium atoms with the help of four electrons. But antimony has 5
electrons so the 5th electron is superfluous and also it is loosely
coupled to the antimony atom. It is easily removed from the conduction band by
applying small electric field. This type of semiconductor is known as N-Type
semiconductor. In this n-type semiconductor electrons are majority and it’s
impurity is called as a “donor impurity”.
Because these are donate free
electrons to the semiconductor materials.
After donating free electron to semiconductor
material it became as a positive donor ion. Therefore this type of
semiconductors electrons are more than holes.
The N-type semiconductor is
electrically neutral. The energy band diagram as shown in
below.
 |
| N-Type Semiconductor |
P-Type Semiconductor
P-Type Semiconductor is
formed when a trivalent impurity is added to the pure semiconductor (Intrinsic
Semiconductor). The trivalent impurities are 3rd group elements.
Ex: Boron, Gallium etc..
Suppose if we take a boron atom, each boron
atom forms the four covalent bands to with their surrounding silicon or
germanium atoms. But one band is left incomplete and also it gives a hole. This
will cause boron and trivalent impurity atoms are called as “Acceptor impurity” atoms cause many
holes are created in silicon or germanium band diagram. These types of
semiconductors are called as a P-type semiconductor. In this type of
semiconductors holes are majority than the electrons.
P-type semiconductor is electrically neutral.
 |
| P-Type Semiconductor |