Semiconductor introduction

 

Energy bands:

Valance band: Electrons in the outermost orbit of an atom are called as valance electrons. In the normal atom, valance band have electrons of highest energy and the range of energies covered by the valance electrons is called as valance band.

Conduction band: If the valance electrons are loosely coupled with the nucleus, there is availability of free electrons which causes the conduction of heat or electricity.

Forbidden gap: Forbidden gap is the separation or gap between the valance band and conduction band.

Fermi level: Fermi level is used to present the highest electron energy level at absolute zero temperature.

Semiconductors:

Before going to semiconductor directly, let’s understand the classification of solids. Solids are generally classified as conductors, insulators and semiconductors.

Conductors: The materials that are good carrier of heat/electricity are called as conductor materials. Conductors have large number of free electrons and there is no forbidden gap between valance band and conduction band. In short valance band and conduction band are overlapped. Figure “c” shows energy band for conductor material.

Examples: Some examples of conductor materials are metals like copper, iron, steel etc.

Insulator: The materials that are not good carrier of heat/electricity are called as insulator material. Forbidden gap between conduction band and valance band is large (about 7 eV) hence there is no chance of conduction. In rare cases insulators can work as conductor if subjected to very high voltage which is called insulators breakdown. Figure “a” shows energy band for insulator material

Examples: Some examples of insulator materials are rubber, mica, wood, glass etc.

Semiconductors: The materials that have conduction properties in between conductor material and insulator material are called as semiconductor materials. Temperature affects on the conductivity of semiconductor, at very less temperature, semiconductors behaves like insulator. On the other hand if temperature is increased, semiconductor starts to behave like conductors. Forbidden gap between conduction band and valance band is small (about 1 eV). Figure “b” shows energy band for semiconductor material

Examples: Silicon, germanium and carbon are types of semiconductor materials.

 

 Properties of semiconductor:

1)    Conductivity of semiconductor lies in between conductor and insulator.

2)    Semiconductors are negative temperature coefficient material i.e. resistivity decreases with increase in temperature.

3)    By doping (adding impurities), conductivity can be changed drastically.

 

Q: What is Hole in semiconductor?

A: In semiconductor, due to increased temperature, covalent bond breaks and electron enters into conduction band which causes vacancy. This missing electron vacancy is called as hole and have +ve charge.

 

Types of semiconductor:

1)    Intrinsic semiconductors

2)    Extrinsic semiconductors

a)     P-type semiconductors

b)    N-type semiconductors

 

·        Intrinsic semiconductors: semiconductors in “PURE” form are called as intrinsic semiconductors. These type of semiconductors acts as insulator at room temperature.

·        Extrinsic semiconductors: semiconductors in “IMPURE” form are called as extrinsic semiconductors. Impurities like arsenic, gallium are added to extrinsic semiconductors to increase the conductivity and the process of adding impurities is called as “doping”.

·        P-type semiconductor: When small amount of trivalent impurities (indium, gallium) are added to pure semiconductor, it is called as P-type semiconductor. Trivalent impurities are also called as acceptor impurities the holes created can accept the electrons.

·        N-type semiconductor: When small amount of pentavalent impurities (arsenic, antimony) are added to pure semiconductor, it is called as N-type semiconductor. Pentavalent impurities are also called as donor impurities because they donates free electrons.