Техническое чтение для энергетиков. Бухарова Г.П. - 44 стр.

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46
In short, many of the invaluable electrical appliances without which life would
seem strange and impossible at present can be utilized only because they transform
electric energy into heat.
The production of heat by an electric current is called heating effect. One might
also name its light effect provided the heat in the conductor be great enough to make
it white-hot, so that it gives off light as well as heat. Take the filament of an electric
lamp as an example. We know it to glow because of heat. By the way, were we able
to look Inside a hot electric iron, we should see that its wires were glowing too. A
similar statement could be applied as well to almost any electric heating device. All
of them give off a little light and a lot of heat.
ELECTRICAL CONDUCTIVITY
The conductivity provided by conduction electrons will be determined by the
number of electrons, and the ease of their movement in an applied electric field. The
latter is described by their "mobility", which is the drift velocity of the carriers in
cm/sec in a field of 1 volt/cm.
The temperature dependence of the conductivity of semiconductors is one of
the most striking and characteristic of their properties. In Fig the behavior of some
arsenic-doped samples of silicon
is shown. The principle changes in the conductivity
of a given sample, with temperature, result from changes in carrier concentration,
although the mobilities also vary with temperature. At low temperatures the conduct-
ivity is low, because most of the carriers are frozen out on the donor centers. As the
temperature rises, the degree of ionization of the donors increases, and the rising
carrier concentration, results in a rapidly increasing conductivity. At around 100 the
conductivity reaches a maximum, because of complete ionization of the donors. At
considerably, higher temperature a very steep rise in the conductivity occurs, due to
the onset of an appreciable intrinsic conduction. The drop in conductivity with rising
temperatures, above 100 and below the intrinsic range, is in the region of saturation,
i.e., the carrier concentration is constant and equal to Nd – Na. The reason for the
drop lies in the temperature dependence of the mobility, In this range of temperatures,
the mobility of the carriers decreases with rising temperatures due to "lattice
scattering". The increasing thermal agitation of the lattice leads to a shorter distance
for the carriers to travel between collisions with the lattice, and the carriers travel
faster at higher temperatures, thus shortening the time between collisions; these
factors both serve to decrease the mobility. Theoretically it is expected under certain
assumptions, that in the lattice-scattering range the mobility should go as T
3
/
2
.
Experimental results usually give a somewhat different exponent.
Any sample which shows little change in conductivity over a wide range of
temperatures, is degenerated, because of the high concentration of arsenic, and of
conduction electrons, in this sample. The behavior of p-type samples, doped with
boron for example, is entirely similar to that shown for the n-type materials.
The lattice scattering mentioned above is one of the two principle mechanisms
that limit mobility. At high impurity concentrations, or at temperatures low enough so

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