Let NA be the concentration of acceptors in the material. At 0°K, the acceptor atoms are not ionized and are at the level EA which is very near to Eᵥ.
When the temperature is raised above 0°K, the acceptor atoms get ionized and holes appear in the valence band. With increase in temperature more and more acceptor atoms get ionized and the hole concentration in the valence band increases.
Since transition of electrons to the acceptor levels from the valence band requires an energy EA they can go to acceptor levels and ionize acceptor atoms.
