Semiconductor Material
Semiconductor is one types of elements which have a special class of conductivity between a good conductor and an insulator.
Semiconductor materials can be two classes: one is the single crystal semiconductor (Germanium, Silicon) which has a sequential bonding of one material. And other is the compound semiconductor (Gallium Arsenide, Cadmium Sulphide, Gallium Nitrite, Gallium Arsenide phosphide) which have a bonding structure of two or more materials. Most used semiconductor materials in the solid state electronic devices are Silicon, Germanium and gallium arsenide.
Semiconductor materials are generally formed with covalent bonding. Covalent bonding is the bonding of atoms by the sharing of electrons.
Here are two types of semiconductors:
1.    Intrinsic: It has a very low level of impurities tend to pure elements and it has four valence electrons.
2.     Extrinsic: It contains predefined impurity (n-type or p-type) materials by doping process.
The ability to change the characteristics of a material by adding impurity to a pure semiconductor material is known as doping.
Semiconductor materials are tetravalent. An n-type material is formed by adding impurity elements that have five valance electrons (Phosphorous, Arsenic, Antimony). And a p-type material is formed by adding impurity elements that have three valance electrons (Boron, Gallium, Indium).
An n-type material contains large number of electrons than holes because pentavalent material is added to the pure semiconductor material. For p-type material, it has a number of holes than electrons for the addition of trivalent material. Both p-type and n-type material are electrically neutral.
A table of majority and minority charge carrier for the p-type and n-type material:
Charge carrier
n-type
p-type
Majority
electrons
holes
Minority
holes
Electrons

Energy levels
For the isolated atomic structure of every element, it has valance band, conduction band and an energy gap between them. Insulator has the highest energy gap (Eg>5eV), semiconductor has a special energy gap in between insulator & conductor material and conductor overlaps its conduction band & valance band. There is no energy gap in the conductor element. The higher is the energy gap, the larger is the amount of absorbed energy and it is because of free charge carrier. The energy gap is measured by the electric volts (1eV= 1.6*10-19 J).
Figure 1: Energy gaps (Eg) for insulator, semiconductor and conductor materials.
The electron of the valance band is not reachable to the conduction band for the insulator; in semiconductor, it is reachable to get conduction and electron flows when reached. For Silicon semiconductor Eg=1.1eV.
The difference between semiconductor and conductor material is employed by applying heat on it. With the increase in heat, conductor material shows the resistivity increasing property, on the contrary, the semiconductor material exhibits its conductivity increasing.
Conductor and semiconductor materials have a positive and negative temperature coefficient respectively.
Analyze yourself:
Q1. What is semiconductor?
Q2. Discuss about the doping process.
Q3. Discuss the temperature effect on the semiconductor material.