Extrinsic or Impure Semiconductors

EXTRINSIC OR IMPURE SEMICONDUCTORS

In a semiconducting material, if the charge carriers originate from impurity atoms which are doped to the original material, then this type of semiconductor is known as extrinsic or impure semiconductor.

It is also known as doped semiconductor.

Extrinsic semiconductor is obtained by adding trivalent or pentavalent impurity atoms to a tetravalent semiconductor. The electrical properties of pure semiconductors can be easily changed even with the addition of very little amount of impurities.

Doping

The addition of impurities to a pure semiconductor is known as doping and added impurity is called as doping agent or dopant.

The addition of impurities increases the number of free electrons and holes in semiconductor and hence increases its electrical conductivity.

Some of the common doping agents are arsenic, antimony, phosphorus, gallium, aluminum and boron. These elements have either five or three valence electrons in the outermost orbit.

Advantages of Extrinsic semiconductors

i. Electrical conductivity is high.

ii. Electrical conductivity can be altered to any desired ΠΟΙ value by controlling of doping concentration.

iii. Electrical conductivity is not a function of temperature.

Types of Extrinsic semiconductors

The extrinsic semiconductors are classified into two types based on the type of impurity added. (Fig 3.6)

(i) n-type semiconductor

(ii) p-type semiconductor