An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. In intrinsic semiconductors the number of excited electrons and the number of holes are equal: n = p.
The conductivity of intrinsic semiconductors can be due to crystal defects or to thermal excitation. In an intrinsic semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band. An example is Hg0.8Cd0.2Te at room temperature.
An indirect gap intrinsic semiconductor is one where the maximum energy of the valence band occurs at a different k (k-space wave vector) than the minimum energy of the conduction band. Examples include Silicon and Germanium. A direct gap intrinsic semiconductor is one where the maximum energy of the valence band occurs at the same k as the minimum energy of the conduction band. Examples include Gallium arsenide.
A layer of i-type semiconductor is used in PIN diodes.
References
Sze, Simon M. (1981). Physics of Semiconductor Devices (2nd ed.). John Wiley and Sons (WIE). ISBN 0-471-05661-8.
Kittel, Ch. (2004). Introduction to Solid State Physics. John Wiley and Sons. ISBN 0-471-41526-X.
