If an intrinsic semiconductor material is doped with an N-type impurity, not only the number of electrons increases, but the number of holes decreases below that is available in the intrinsic semiconductor. Presence of a large number of electrons increases the rate of recombination, therefore, the concentration of holes decreases. Under thermal Similarly, the concentration of holes falls below the intrinsic level for a P-type semiconductor. equilibrium, the product of the concentration of electrons (one) and concentration of holes (n) is a constant, independent of the amount of donor and acceptor doping level. This is called mass action law.