Extrinsic semiconductors and doping

The conductivity of a semiconductor can be increased deliberately by doping -- adding a minute, controlled quantity of an impurity, called the dopant, to the pure material. A doped semiconductor, having higher conductivity than the pure (intrinsic) semiconductor, is called an extrinsic semiconductor. There are two types.

i. n-type semiconductor : An n-type semiconductor is obtained by doping a group-14 semiconductor (Si or Ge) with a group-15 element such as P, As, Sb or Bi. Consider silicon doped with phosphorus: the P atoms occupy some of the regular lattice sites of the Si atoms, without disturbing the crystal structure (Fig. 1.26). Phosphorus has five valence electrons, and only four of them are needed to bond with the four neighbouring Si atoms -- the fifth electron is left over, and it enters the conduction band (Fig. 1.27a). An n-type semiconductor therefore carries extra electrons in its conduction band ('n' for the negative charge carriers), and its conductivity is higher than that of pure silicon.

ii. p-type semiconductor : A p-type semiconductor is obtained by doping Si or Ge with a group-13 element such as B, Ga or In. Boron has only three valence electrons, so a B atom occupying a regular Si lattice site (Fig. 1.28) can bond with only three of its four Si neighbours -- the fourth bonding position is an electron vacancy, called a hole. A hole behaves as though it carries a positive charge ('p' for positive): under an applied potential, a neighbouring valence electron hops into the hole and leaves a new hole behind, so the holes effectively travel through the valence band (Fig. 1.27b) in the direction opposite to that of the electrons.

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