Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 232

foreign atoms are thus referred to as acceptor atoms. They increase the number of
holes (quasi positive charge carriers) and create p-conductivity. Under these con-
ditions deficit electrons (i.e. holes) are majority carriers whereas electrons act as
minority carriers.

Hole

Ge Ge Ge Ge Ge Ge Ge Ge Ge Ge

+

Ge Ge Ge Ge Ge Ge Ge Ge Ge Ge

+ -

Ge Ge As Ge Ge Ge Ge In Ge Ge

Ge Ge Ge Ge Ge Ge Ge Ge Ge Ge

Donor
atoms

-

Ge Ge Ge Ge Ge Ge Ge Ge Ge Ge

Acceptor
atoms

Electron

Electron

Si Si Si Si Si Si Si Si Si Si

Si Si Si Si Si Si Si Si Si Si

-

-

+

Si Si P Si Si Si Si B Si Si

Si Si Si Si Si

Si Si Si Si Si

+

Si Si Si Si Si Si Si Si Si Si

Fig. 6.3 Effects caused by donor atoms (left) and acceptor atoms (right) (various sources)

Hole

Also the above-mentioned context can be shown within the energy gap model
(Fig. 6.2). Within the undoped semiconductor material (Fig. 6.2, left) a certain
equilibrium concentration of free mobile charge carriers is formed due to the re-
current processes of electron-hole-pair creation and recombination. During this
process the number of holes and electrons are equal. Besides temperature, charge
carrier density at equilibrium concentration is determined by the minimum energy
required to release one electron from the valence band, and is thus described by
the energy gap E g . For example for germanium (Ge) the energy gap amounts to
0.75 eV and to 1.12 eV for silicon. Since electron and hole densities remain rela-
tively low, also conductivity of the undoped semi-conductor material is low.
The number of electrons within the conduction band is considerably increased
by the addition of donors (Fig. 6.2, centre). Within the energy gap model, donors
correspond to energy levels scarcely below the conduction band. Adding accep-
tors increases the number of holes inside a valence band (Fig. 6.2, right) accord-
ingly. Within the energy gap model, energy levels of acceptors are a little above
the valence band.
By doping the semiconductor with acceptors (p-doping) and donors (n-doping)
conductivity of semiconductor materials can be controlled across several orders of
magnitudes. However, the product of electron density and hole density of a certain
material is a temperature-dependent material constant. Hence, if for instance the
electron density is increased by the incorporation of donors, the hole density is
automatically reduced. Nevertheless, the conductivity increases. However, both