What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.
Semiconductors can be defined as the solids having intermediate range of conductivity, i.e. from 10-6 to 104 ohm-1m-1. They are of two types
1) n – type semiconductors
2) p – type semiconductors.
n- type semiconductors:-
Semiconductor formed by doping with electron rich impurities to increase their conductivity are called n – type semiconductors.
For example, silicon and germanium, each has four valance electrons as they belong to group 14th of the periodic table. Arsenic and phosphorus belong to the 15th group of the table and they have valance electrons equal to 5. When silicon or germanium is doped with phosphorus or arsenic, four electrons of phosphorus or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon and germanium. Since the electrical conductivity of silicon and phosphorus is increased because of negatively charged particle (electron), thus this is known as n – type semiconductor.
P – type semiconductor :-
Semiconductors formed by doping with electron deficient impurities; to increase their conductivity; are called p- type semiconductors. In p – type semiconductors, conductivity increases because of formation of electron holes.
For example – electrical conductivity of Silicon and Germanium is doped with elements, such as Boron, aluminium or gallium having valance elentrons equal to 3. Three valance electrons present in these electrons delocalized. The place from where one electron is missing is called the electron hole or electron valance. When the silicon or gallium is placed under electrical field, electron from neighbouring atoms fill the electron hole, but in doing so another electron hole is created at the place of movement of electron. in the influence of electrical field electrons move towards positively charged plate through electron hole as appearing the electron hole as positively charged and are moving towards negatively charged plate.