Question

 How does doping increase the conductivity of semiconductors?

Answer 1

 Hint: To answer this question, first of all understand the doping process that is generally used to alter the electrical properties of the semiconductors for various applications. Try to describe semiconductors, it’s types and doping process by giving some examples.

Complete step by step answer:  
Semiconductors are the materials which have a conductivity between conductors (generally metals) and non-conductors or insulators (such as ceramics). Semi-conductors can be compounds such as gallium arsenide or pure elements, such as germanium or silicon. They can be classified as intrinsic and extrinsic semiconductors.
- An intrinsic semiconductor is made to be very pure chemically. It is made up of only a single type of element. Germanium and Silicon are the most common type of intrinsic semiconductor elements.
- Extrinsic semiconductors are semiconductors that are doped with specific impurities and these impurities further modify the electrical properties of the semiconductor. They can be further classified as n-type and p-type semiconductors.
- The conductivity of semiconductors can be improved by introducing a small number of suitable replacement atoms known as impurities. The process of adding impurity atoms to the pure semiconductor or an intrinsic semiconductor is called “doping”.
So, let us see how doping increases the conductivity of a semiconductor by giving an example:
When a pure semiconductor i.s. silicon or germanium, is doped or introduced with a pentavalent impurity or atom such as phosphorus, arsenic, etc. then four electrons out of five valence electrons, bonds with the four electrons of germanium or silicon.
So, the fifth electron of the dopant is set free. Thus, the impurity atom donates a free electron for conduction in the structure and is referred to as the ‘donor’. Since, the number of free electrons increases by the addition of an impurity, it will further help in conduction. By such process, doping increases the conductivity of the semiconductors.
Note: When pure semiconductor is doped with pentavalent atoms, the conduction will be due to the presence of free electrons and this type of semiconductor is called as n-type of semiconductor, while when it is doped with trivalent atoms, the conduction will be due to increase in number of holes in the lattice and this type of semiconductor is called as p-type of semiconductor.