I hope that this will make things clearer. As I am not an expert in solid state physics, let me quote Ashcroft and Mermin's Solid State Physics (pp 579-580):
... we conclude that donor impurities introduce additional electronic levels at energies
which are lower than the energy
at the bottom of the conduction band by an amount that is small compared with the energy gap
(Figure 28.12).
A similar argument can be applied to acceptor impurities, whose valence is one less than that of the host atoms (e.g. gallium in germanium). Such an impurity can be represented by the superimposition of a fixed charge
on top of a host atom, along with the presence of one less electron in the crystal. The missing electron can be represented as a bound hole, attracted by the excess negative charge representing the impurity, with a binding energy that is again small on the scale of the energy gap,
. In terms of the electron picture this bound hole will be manifested as an additional electronic level at an energy
lying slightly above the top of the valence band (Figure 28.12). The hole is bound when the level is empty. The binding energy of the hole is just the energy
necessary to excite an electron from the top of the valence band into the acceptor level, thereby filling the hole in the vicinity of the acceptor and creating a free hole in the valence band.
The single most important fact about these donor and acceptor levels is that they lie very close to the boundaries of the forbidden energy region.
It is far easier thermally to excite an electron into the conduction band from a donor level, or a hole into the valence band from an acceptor level, than it is to excite an electron across the entrie energy gap from valence to conduction band. ...
ถามโจทย์ปัญหา
