What are Semiconductors? Properties of Semiconductors? Types of Semiconductors?, what are Intrinsic and Extrinsic semiconductor?.

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Semiconductors are the materials which have a conductivity between Conductors & Insulators. Semiconductors act like an insulator at zero Kelvin.On increasing the temperature, it works as a conductor (hence their name Semi - Conductors)

Properties of Semiconductors-
Resistivity : 10–⁵ to 10⁶ Ohm
Conductivity : 10⁵ to 10–⁶ mho/m
Temperature coefficient of resistance : Negative.

Current Flow : Due to electrons & holes.
Forbidden Energy Gap : of about 1eV.

(1) Semiconductors are smaller in size & possess less weight.
(2) Their resistivity is higher than Conductors but lesser than insulators.
(3) Due to their, exceptional electrical properties, Semiconductors can be modified by doping to make semiconductor devices suitable for 
energy conversion, switch, & amplifiers.
(4) Semiconductors can be insulators at low temperatures & Conductors at high temperatures.
(5) Lesser power losses.
(6) The resistance of semiconductor materials decreases with the increase in temperature & vice versa.

Types of Semiconductors-
Semiconductors con be classified as 
• Intrinsic Semiconductor
• Extrinsic Semiconductor
Intrinsic Semiconductor- 
A semiconductor in its extremely pure form is said to be an intrinsic semiconductor.  Germanium (Ge) & Silicon (Si) are the most common intrinsic semiconductor.

The properties of this pure semiconductor are as follows.
• The electrons & holes are solely created by thermal excitation.
• The number of free electrons is equal to numbers of holes.
• The Conduction capability is small at room temperature.

Extrinsic Semiconductor-
In order to increase the Conduction capablility of intrinsic semiconductor, It is better to add some impurities,this process of adding impurities is called Doping. Now,this doped intrinsic semiconductor is called as an Extrinsic Semiconductor.

There are two types of Extrinsic Semiconductors depending upon the type of impurity added. They are N type semiconductor & P type semiconductor.

N Type Semiconductor-
A small amount of pentavalent impurities such as arsenic,antimony, or phosphorus is added to the pure semiconductor (Germanium or Silicon crystal) to get N type semiconductor.
For example - If arsenic (As) atom is added to the germanium(Ge) atom. Germanium atom has four valence electrons and arsenic has five valence electrons.Arsenic atom form a covalent bond with surrounding four germanium atoms. Thus,
Four valence electrons of arsenic(As) atom form covalent bond with four valence electrons of individual Germanium (Ge) atom & fifth valence electron is left free which loosely bound to the arsenic (As) atom.
This loosely bound electron can be easily excited from the valence band to the Conduction band by the application of electric field or increasing the thermal energy. Thus every Arsenic (As) atom contributes one Conduction electron without creating a hole, the donor atom becomes positively charged ion because it loses one electron.
As a result of doping , the number of free electron for exceeds the number of holes in an N - type semiconductor. So electrons are called majority carriers & hole are called minority carriers.

P Type Semiconductor- 
A small amount of trivalent impurities such as aluminium or boron is added to the pure semiconductor to get P type semiconductor.
For example - If boron(B) atom is added to the germanium (Ge) atom. There valence electrons in boron(B) form covalent bond with four surrounding atoms of germanium(Ge) leaving one bond incomplete which gives rise to a hole. These positively charged holes increase the conductivity of P type semiconductor.
Trivalent impurities such as boron is called acceptor impurity because it accepts free electrons in the place of holes. As each boron atom donates a hole for Conduction, it becomes a negatively charged ion. As the number of holes is very much greater than the number of free electrons in a P-type material, holes are termed as majority carriers and electron as minority carriers.

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