Semiconductor can be classified as:-


1)Intrinsic Semiconductor

     An intrinsic(pure) semiconductor, also called an undoped semiconductor or i-type semiconductor. It isd. An indirect band gap intrinsic semiconductor is one in which the maximum energy of the valence band occurs at a different k (k-space wave vector) than the minimum energy of the conduction band.      

 Naturally available elements like silicon and germanium are best examples of an Intrinsic Semiconductor.


2)Extrinsic Semiconductor

An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors, etc.) or optoelectronic applications (light emitters and detectors).
  Extrinsic semiconductor is divided into two types:-

A)p-type Semiconductor

  A p-type semiconductor is a type of semiconductor.When the trivalent impurity is added to an intrinsic or pure semiconductor (silicon or germanium), then it is said to be an p-type semiconductor. Trivalent impurities such as Boron (B), Gallium (Ga), Indium(In), Aluminium(Al) etc are called acceptor impurity.
      P-type semiconductors are made by doping the pure semiconductor material. The amount of impurity added is very small compared to the amount of semiconductor. The exact character of the semiconductor can be changed by varying the amount of 'dopant' that is added.In p type semiconductor the no.of holes is much larger than that of thermally generated electron.
      A p-type semiconductor has more holes than electrons. This allows the current to flow along the material from hole to hole but only in one direction.

                                          

B)n-type Semiconductor

 An N-Type semiconductor is created by doping pentavalent impurities like phosphorus (P), arsenic (As), antimony (Sb), or bismuth (Bi). A pentavalent impurity is called a donor atom because it is ready to give a free electron to a semiconductor.
     In n-type semiconductors the number of electrons is more than the holes, so electrons are measured as majority charge carriers and holes are referred to as minority charge carriers.