In the energy band diagram of a material shown below, the open circles and filled circles denote holes and electrons respectively. The material is

Calculation:

The given figure represents a p-type semiconductor:

When one of the silicon atoms (with valence = 4) is replaced by an atom of aluminium (with valence = 3), the aluminium atom can bond covalently with only three silicon atoms. This creates a "missing" electron (a hole) in one of the aluminium-silicon bonds.

With a small expenditure of energy, an electron can be torn from a neighboring silicon-silicon bond to fill this hole, thereby creating a hole in that bond as well. This process continues as the hole migrates through the lattice.

The aluminium atom is known as an acceptor atom because it readily accepts an electron from a neighboring bond in the valence band of silicon.

As the figure suggests, this electron occupies a localized acceptor state that lies within the energy gap, at an average energy interval E_a above the top of the valence band.

By adding acceptor atoms, it is possible to significantly increase the number of holes in the valence band, thereby making the material a p-type semiconductor.

The material shown in the figure is a p-type semiconductor.