Question

What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.

Answer 1

Semiconductors are those materials which have a conductivity between conductors (which are generally metals) and nonconductors or insulators (like most ceramics). It can be pure elements, such as silicon or germanium, or can be compounds such as gallium arsenide or cadmium selenite.

Complete step by step solution
Semiconductors allow a little amount of electric flow through them at the room temperature. These are the solids which have conductivity in the range from ${10^{ - 6}}$to ${10^4}$ $oh{m^{ - 1}}{m^{ - 1}}$. They are ideal at zero cover. Silicon and Germanium are common examples of semiconductors.
There are two types of semiconductors-
1.n–type semiconductor
2.p–type semiconductor

a)n–type semiconductor: These semiconductors have Electrons which are having rich impurities. Silicon and germanium belong to group $14$ of the periodic table and each has four valence electrons. Four from five electrons are used in the formation of four covalent bonds with the four neighboring silicon atoms. The fifth electron becomes delocalized. These delocalized electrons increase the conductivity of doped silicon or germanium. The increase in conductivity is due to the negatively charged mobile electron. So semiconductors doped with electrons–rich impurities are called n–type semiconductor.
b)p–type semiconductor: Silicon or Germanium can also be doped with a group $13$ element. The elements of group $13$ contains only three valence electrons. Elements of the $14$ group have 4 valence electrons. The place where the fourth valence electron is absent is called the electron hole. An electron from a neighboring atom can fill the electron hole, but doing so it would leave an electron hole at its original position. It sometimes appears as if the electron hole is moving in the direction which is opposite to that of the electron that filled it. Under the electric field, electrons would move towards the positively charged plate through electronic holes, but it will be as if electron holes are positively charged and are moving towards negatively charged plate. This type of semiconductors are called p–type semiconductors.

Note: A semiconductor material has an electrical conductivity whose values are falling between a conductor, like metallic copper, and an insulator, such as glass. Its resistivity falls as their temperature rises. Metals are the opposite in case of resistivity.