eN eNpn n
= ⋅ =
The intrinsic charge carrier concentration of
gives different results in the
literature, depending on the source. These differences are essentially based on slightly
differing values as for example the bandgap energy E
(T) and the effective mass m*.
Also often simplifications are used.
shows representative results from three
Intrinsic charge carrier concentration n
of silicon as a function of the
For the purposes of this book, a carrier concentration n
at 300K of
cm 10 45.1
be used for silicon.
The conductivity can be increased by several orders of magnitude by targeted
admixture with impurities with valences different to that of the semiconductor material
itself. This targeted introduction is called doping and can be achieved via diffusion,
implantation of ions or neutron irradiation. For example, if silicon (a quatrivalent atom) is
doped with phosphorus (a pentavalent atom), the result is a surplus of moving,
negatively charged electrons. Phosphorus is thus a donor of electrons. Similar
behaviour results in a doping process with trivalent atoms as for instance boron, in
which case there is a surplus of moving, positive holes, whichmakes boron an acceptor
of electrons. As long as the temperature of the semiconductor does not reach values at
which the intrinsic charge carrier concentration n
exceeds that of the charge carrier
density of the doping atoms, the behaviour of the semiconductor is determined by the
doping. If, as the temperature rises, n
lies within the range of the doping concentration,
the behaviour of the semiconductor is determined by its intrinsic carrier conduction
density. As this increases alongwith the temperature, there is a risk of thermal runaway
of the semiconductor within this range.