The Energy Band Model
When a semiconductor is doped, energy states are introduced in the band
gap. If it is doped with donors, the energy states are called donor states.
Because it takes very little energy, much less than the band gap energy,
to free the electron that inhabits the donor state, the states are shown
close to the conduction band. Adding donors, therefore, adds more electrons
to the conduction band (without adding holes to the valence band)
making the semiconductor more conductive.
Acceptor states are introduced into the forbidden gap if the
semiconductor is doped with acceptors. These initially empty states
readily accept an electron to complete its bonds with the four nearest
neighbors in the crystal. When an electron from the valence band transitions
to an acceptor state, it leaves behind a hole. The energy required for an
electron to move to an acceptor state is much less than the band gap energy
so it is shown close to the valence band. Holes are created without