What is a semiconductor?

A semiconductor is a substance that has an almost filled valence band and nearly empty conduction band with a very small energy gap (1 eV) separating the two. The resistivity of a semiconductor is less than an insulator but more than a conductor.  Semiconductors have a negative temperature coefficient of resistance. When a suitable metallic impurity (e.g. arsenic, gallium, etc.) is added to a semiconductor, its current conducting properties change appreciably. Semiconductors are formed by covalent bonds.

Semiconductor Materials

In general, semiconductor materials fall into one of two class

1)Single-crystal semiconductor

2)Compound semiconductor

Single-crystal semiconductors such as germanium(Ge)and silicon(si)have a repetitive crystal structure, whereas compound semiconductors such as gallium arsenide(GaAs), cadmium sulfide(Cds), gallium nitride(GaN) arsenide phosphide(GaAaP) are constructed of two or more semiconductor materials of different atomic structures.

Materials commonly used in the development of semiconductor devices 


 Silicon (Si)

  Germanium (Ge) 

 Gallium Arsenide (GaAs) 


Properties of semiconductor 

1. The resistivity of a semiconductor is less than an insulator but higher than a conductor.

2 . Semiconductors have a negative temperature coefficient of resistance. In simple words, the resistance of the semiconductors decreases as the temperature increases and vice-versa. For example Germanium

3. Semiconductors act as insulators. As the temperature is increased it works as a conductor.

4. The conductivity of the semiconductors increases when impurities are added. The process of adding impurities to semiconductors is called doping.

5. Semiconductors are materials that have the conductivity between a conductor and an insulator.


Energy Levels

In the atomic structure of each and every isolated atom, there are specific energy levels associated with each shell and orbiting electron, as shown in the figure. The energy levels associated with each shell will be different for each element.


The farther an electron is from the nucleus, the higher the energy state, and an electron that has left its parent atom have a higher energy state than an electron in the atomic structure. We can see the figure "a" that only specific energy levels can exist for the electrons in the atomic structure of an isolated atom. The result is a series of gaps between allowed energy levels where carriers are not permitted and as the atoms of a material are brought closer together to form the crystal lattice structure, there is an interaction between atoms, which will result in the electrons of a particular shell of an atom having slightly different energy levels from electrons in the same orbit of an adjoining atom.


We can be shown the figure b, that the valence electrons in a silicon material can have varying energy levels as long as they fall within the band. There is a minimum energy level associated with electrons in the conduction band and a maximum energy level of electrons bound to the valance shell of the atom. Between the two is an energy gap that electrons bound to the valence must overcome to become a free carrier. The energy gap is different for Ge, Si, and GaAs.

We can say that

 An electron in the valence band of silicon must absorb more energy than one in the valence band of germanium to become a free carrier,Similarly, an electron in the valence band of galium arsenide must gain more energy than one in silicon or germanium to enter the conduction band.

A semiconductor is a substance that has an almost filled valence band and nearly empty conductance band with a very small energy gap (≃ 1eV) separating the two.