Suppose a semiconductor is doped with a donor impurity.

When a semiconductor is doped with a donor impurity, pentavalent atoms are added to the crystal lattice. Each donor atom provides one extra electron to the conduction band.

As a result, the concentration of electrons ($n_e$) increases significantly.

According to the law of mass action for semiconductors, $n_e n_h = n_i^2$, where $n_h$ is the concentration of holes and $n_i$ is the intrinsic carrier concentration. Since $n_i$ is constant at a given temperature, an increase in $n_e$ leads to a decrease in the concentration of holes ($n_h$).

Thus, the concentration of electrons increases and the concentration of holes decreases.