Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 229

These electrons do not produce any conductivity. The most energy-rich energy
band, fully occupied with electrons, is referred to as valence band; the electrons it
contains determine the chemical bond type of the material.
A solid with electrical conductivity requires freely moving electrons. However,
electrons are only able to move freely if they are located in an energy band that is
not fully occupied. For energy reasons, this is only true for the energy band lo-
cated above the valence band. This energy band is thus referred as the conduction
band.
The energy gap E g between the valence band and conduction band is termed as
"band gap" (Fig. 6.1). This energy gap exactly equals the minimum amount of
energy required to transfer one electron from the valence band into the conduction
band.

Conduction band
Energy gap

Conductor Semiconductor Insulator

Valence band

Copper oxide Glass, ceramic

Silicon

Mercury Resin

Amber

Mica

Selenium

Iron

Hard rubber

Germanium

Silver

- 6 - 4 - 2 2 4 6 8 10 12 14 16

10 10 10 1 10 10 10 10 10 10 10 10

Conductor Semiconductor Insulator

Specific resistance in Ω cm

Fig. 6.1 Valence and conduction bands as well as energy gap (left) and specific resistance
(right) of conductors, semiconductors and insulators (also refer to /6-2/,/6-10/)

6.1.2 Conductors, semiconductors and insulators

Conductors, semiconductors and insulators are different in terms of their band
structure and the occupation of their bands with electrons (Fig. 6.1).

Conductors. Within conductors (e.g. metals and their alloys) two different condi-
tions might occur.
− The most energy-rich band (i.e. conduction band) occupied by electrons is not
entirely occupied.
− The most energy-rich band fully occupied with electrons (i.e. valence band)
and the conduction band located on top overlap, so that also a partly covered
band (conduction band) is formed.