Doping:

Intrinsic semiconductors show very low conductivity and thus cannot be used practically. Thus, the conductivity of intrinsic semiconductors is increased by adding suitable impurities. Addition of appropriate amount of suitable impurities to elements, such as intrinsic semiconductors is called doping.

Doping is done with electron rich or electron deficient element (impurities) to the intrinsic semiconductors. Doping with electron rich or electron deficient elements creates electronic defects in semiconductors.

If the addition of impurity by melting the 2 compounds & then adding impurity (which is 1 of the melted compound) to the other solid.

 

(a) Doping with electron rich impurities: n-type of semiconductor:

Silicon and/or germanium are doped with electron rich impurities to increase their electrical conductivity. Semiconductors so formed after are called n-type semiconductors.

Silicon and germanium, each has four valence electrons as they belong to 14th group of periodic table. Arsenic and phosphorous belong to 15th group of periodic table and they have valence electrons equal to 5. When silicon or germanium is doped with phosphorous or arsenic, four electrons of phosphorous 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 or germanium.

If  gp. 14 is doped with gp 15 element (i.e. with more No. of valance e^- ) then, n – type semiconductor is formed fue to free e^-. eg when P is doped in si

Ge is doped in Ga

Al is doped with Si

 

Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged particle (electron), thus this is known as n-type of semiconductor.

(b) Doping with electron deficient impurities – p-type semiconductor:

If group 14 is doped with gp 13 element, then vacancies will be created due to lesser no. of valance e^- in gp. 13 elements so, it will become p – type semiconductor.

Electrical conductivity of silicon or germanium is doped with elements, such as Boron, Aluminium or Gallium which belong to group 13th in periodic table also. Elements belong to group 13th have valence electrons equal to 3. Three valence electrons present in these elements make covalent bonds with three electrons present in valence shell out of four of silicon or germanium leaving one electron delocalized. The place from where one electron is missing is called electron hole or electron vacancy.

When the silicon or germanium is placed under electrical field, electron from neighbouring atom 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 filed electron moves toward positively charge plate through electron hole as appearing the electron hole as positively charged and are moving towards negatively charged plate.

Semiconductor formed by the doping with electron deficient impurities; are called p-type semiconductors.

Applications of n-type and p-type semiconductors

• Both n-type and p-type semiconductors are used in making electronic components.
• As diode which is the combination of n-type and p-type semiconductors.
• As integrated circuit (ICs).
• In photoelectric cell

As transistors, to amplify radio and audio signal.