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What are intrinsic and extrinsic semiconductors ?

Intrinsic semiconductors are pure elements such as silicon and Germanium.  Intrinsic semiconductors have an equal number of holes and electrons.  At temperatures above 

absolute zero, some electrons get knocked out of the atoms.  Thus in an intrinsic semiconductor, the number of holes is equal to the number of electrons.

Intrinsic semiconductors have low conductivity.  When voltage is applied, a small current flows due to the holes and electrons.  As the the material is heated, the electrical conductivity of the the intrinsic semiconductor increases.  

Extrinsic Semiconductors are formed when an impurity is added to the intrinsic semiconductor.  In an extrinsic semiconductor, the number of holes is not equal to the number of electrons.  The number of these charge carriers depends on the type and extent of doping.  

When an extrinsic semiconductor is heated, the resistance decreases and the current increases.  The amount of current depends on the temperature and the extent of doping.