At very low temperatures, semiconductors are like insulators as there are no free carriers in their conduction band.
As temperature is raised, thermal excitation of carriers takes place to the conduction band leading to non-zero conductivity. Such semiconductors are called intrinsic.
The band gap in semiconductors is much smaller than that in insulators, which facilitates thermal excitation in the
former.
For an intrinsic semiconductor, the Fermi level lies in the middle of the forbidden gap.
Electrical properties of semiconductors can be modified by introducing impurities (known as dopants). Dopants can
have excess valence electrons per atom over that of the host material. Such semiconductors are called n-type semiconductors for which the majority carriers are electrons.
In a p-type semiconductor, the dopant atoms have a deficit of valence electrons and in such cases the majority
charge carriers are holes.
A compensated semiconductor is one in which both types of dopants exist.
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