**Charge Densities in Extrinsic Semiconductor:**

Charge Densities in Extrinsic Semiconductor – Equation (6.19) provides relationship between the electron and hole concentrations n and p respectively.

If N_{D} is the concentration of donor atoms, there will be N_{D} immobile positive charges per unit volume contributed by donor ions as the donor atoms are ionized. The positive charge density will be N_{D} + p. Similarly as the acceptor atoms are ionized at room temperature there will he N_{A }immobile negative charges per unit volume and total negative charge density will be N_{A} + n, where N_{A }is the concentration of acceptor atoms. Since the semiconductor is electrically neutral, the magnitude of positive charge-density will be equal to the magnitude of negative charge-density.

In N-type semiconductors there is no acceptor doping i.e., N_{A} = 0. Also the number of electrons is much greater than the number of holes i.e., n ≫ p.

i.e., in N-type semiconductors, the free electron concentration is approximately equal to the density of donor atoms.

Now the concentration of holes in N-type semiconductors will be given by the equation

Similarly in case of P-type semiconductors