Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 231

ways the same number of electron-hole-pairs are formed as recombine. For every
temperature, there thus exists an equilibrium state with a certain number of free
holes and free electrons. The number of free electron-hole-pairs increases with
rising temperature.
If an external voltage is applied to such a crystal lattice from outside, electrons
move to the positive pole while the holes move to the negative pole. The mecha-
nism of intrinsic conduction inside semiconductors can also be described by the
energy gap model (Fig. 6.2, left).

Intrinsic conduction Extrinsic conduction

Leitungsband

Conduction band Conduction band Conduction band

E

C

Donor atoms

Gene-
ration

Recombi-
nation

E g

Acceptor atoms

E

V

Valence band Valence band Valence band

Valenzband

Fig. 6.2 Energy gap model showing intrinsic conduction (left) and extrinsic conduction
(centre, right) ( E V valence band energy level, E C conduction band energy level, E g energy
gap; according to e.g. /6-1/, /6-2/)

Electron Hole

Extrinsic conduction. In addition to the - low - intrinsic conduction of pure
crystal lattices extrinsic conduction is created by intentional incorporation of for-
eign atoms ("doping"). Such impurities are effective if their number of valence
electrons differs from that of the base material. If for instance the valence electron
number of the incorporated impurities exceeds that of the lattice atom (e.g. in
pentavalent arsenic (As) incorporated into tetravalent silicon (Si) lattice; Fig. 6.3),
the excess electron is only weakly bound to the impurity atom. It thus separates
easily from the impurity atom due to thermal movements within the lattice and
increases the conductivity of the crystal lattice as a freely moving electron. Such
foreign atoms which increase the number of electrons are referred to as donor
atoms. By this the number of electrons exceeds by far that of the holes. In this
case electrons are called majority carriers, whereas the holes constitute the minor-
ity carriers. Since conductivity is mainly created by negatively charged particles,
this type of conduction is referred to as n-conduction.
If the impurities incorporated into the semiconductor material are by contrast
provided with less valence electrons (e.g. trivalent boron (B) or aluminium (Al)
incorporated into tetravalent silicon (Si); Fig. 6.3), these doping atoms tend to
absorb one additional electron from the valence band of the basic material. Such