Charge Densities, Mobility and Conductivity

In semiconductor, the electrons and holes are the charge carriers.

Charge Density

The number of charge carriers per unit volume is called charge density.

For Intrinsic semiconductor:

Number of free electrons = Number of holes = Number of Intrinsic charge carries → n_e = n_h = n_i

For Extrinsic semiconductor:

i)                    n-type: Number of free electrons ≫ Number of holes → n_e ≫ n_h

ii)                  p-type: Number of holes ≫ Number of free electrons → n_h ≫ n_e

The semiconductor crystal as a whole remains neutral as the charge of additional charge carriers is just equal and opposite to that of ionised atoms in the lattice.

In extrinsic semiconductors, the minority charge carriers are destroyed by meeting majority charge carriers. Hence, by adding impurities, a large number of current carriers of one type are added, which becomes majority charge carriers, indirectly helps to reduce the intrinsic concentration of minority charge.

Thus at room temperature, the density of majority charge carriers is predominantly due to impurity in the extrinsic semiconductor. The electron and hole concentration in a semiconductor in thermal equilibrium is given by: 

n_i² = n_e x n_h

Mobility of Charge Carries in Semiconductor

SI unit of μ (m²/Vs).

Conductivity in Semiconductor

SI unit of σ = Sm^-1