Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 230

Current is thus transferred by freely moving electrons, abundantly available
within the crystal lattice regardless of the respective temperature of the material.
Due to this, electrical conductors (like metals) are characterised by a low specific
resistance. With rising temperature, the increasing thermal oscillation of the
atomic cores impedes the movement of the electrons. This is why the specific
resistance of metals increases with a rising temperature.
Insulators. Insulators (e.g. rubber, ceramics) are characterised by a valence band
fully filled with electrons, a wide energy gap ( E g > 3 eV) and an empty conduc-
tion band. Hence, insulators possess virtually no freely moving electrons. Only at
very high temperatures (strong "thermal excitation") are a small number of elec-
trons able to overcome the energy gap. Thus, ceramics, for instance, show
conductivity only at very high temperatures.
Semiconductors. In principle, semiconductors (e.g. silicon, germanium, gallium-
arsenide) are insulators with a relatively narrow energy gap (0.1 eV < E g < 3 eV).
Therefore, at low temperatures, a chemically pure semiconductor acts as an insu-
lator. Only by adding thermal energy, electrons are released from their chemical
bond, and lifted to the conduction band. This is the reason why semiconductors
become conductive with increasing temperatures. This is the other way round
compared to metals, where conductivity decreases with rising temperatures. Re-
garding specific resistance, semiconductors are in-between conductors and insula-
tors. Within the transition area between semiconductors and conductors, in case of
very narrow energy gaps (0 eV < E g < 0.1 eV), such elements are also referred to
as metalloids or semi-metals as they may show similar conductivity as metals.
However, unlike "real" metals they are characterised by a reduced conductivity
with decreasing temperatures.
6.1.3 Conduction mechanisms of semiconductors
Intrinsic conductivity. Semiconductors are conductive beyond a certain tempera-
ture level as valence electrons are released from their chemical bonds with in-
creasing temperatures and thus reach the conduction band (intrinsic conductivity).
They become conduction electrons that are able to move freely through the crystal
lattice (i.e. electron conduction).
On the other hand, also the resulting hole inside the valence band can move
through the semiconductor material, since a neighbouring electron can advance to
the hole. Holes thus contribute equally to conductivity (hole conduction). Since
every free electron creates a hole within undisturbed pure semiconductor crystals
both types of charge carriers equally exists.
Intrinsic conductivity is counteracted by recombination, namely the recombina-
tion of a free electron and a positive hole. Despite this recombination the number
of holes and free electrons remains equal since at a certain temperature level al-