Техническое чтение для энергетиков. Бухарова Г.П. - 45 стр.

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47
that lattice scattering does not predominate, the mobility is limited by scattering by
impurity centers. Ionized impurities are very much more effective than are neutral
impurities. In the ionized impurity scattering region, varies as T
3
/
2
THE P-N JUNCTION
If, within the same single crystal, there are adjacent regions of n- and p-type
semiconductor, the resulting boundary is called a "- junction".
An n-type material contains mobile negative charges (conduction electrons),
and an equal concentration of fixed positive charges (holes), and fixed negative
charges (ionized acceptors). With the two regions in contact, the mobile electrons and
holes might be expected to flow out of the n-and p-type regions, respectively, across
the junction because of the concentration gradients for these species. On the other
hand, this flow leaves the n-type region with a net of' positive charge, and the p-type
region with a net of negative charge, thus establishing a field in a direction which op-
poses further flow. At equilibrium this field just balances the effect of the
concentration gradient, as shown in the top of Fig. The net charges appear in the
region immediately adjacent to the junction, and the field appears in this space-charge
region.
The Fermi levels in the p and n regions must be equal at equilibrium, and this
establishes the magnitude of the electrostatic potential difference between the two
regions. Electrons and holes are constantly being generated in both regions and
recombining at an equal rate. Some of the most energetic electrons in the n-region
cross the potential barrier into the p-region and this forward flow of electrons is
designated as If. At equilibrium, an equal number of electrons cross the junction in
the opposite direction, the source of these in the p-region being thermal generation
(I
g
). Thus there is no net electron current across the junction, and likewise, no net
hole current. If a positive potential is applied to thep-region, the effect is as shown in
Fig. 11 (b). The potential barrier between the two regions is lowered, and the forward
currents of both holes and electrons are greatly increased. The currents arising from
the generation of minority carriers remain the same, and so there is a net flow of
current across the junction, with contributions from both holes and electrons. This is
called the condition of "forward bias". If the p-region is made negative with respect
to the -region, the potential barrier becomes much higher, and If drops to a very low
value for both kinds of carriers, A net current flows due to I
g
, but it is much smaller
than in the forward bias. It is seen therefore, that the p-n junction is a rectifier, i.e.,
the current passed varies in the magnitude depending on the polarity of the applied
voltage. The ratio of forward to reverse current may be very high, e.g. in silicon
rectification ratios of 10
6
can be achieved as shown in Fig.
The p-n junction illustrated is one in which the hole concentration on the p side
is approximately the same as the electron concentration on the n side. This need not
be the case, of course. If, say, the n side is much more heavily doped than the p side,
the higher electron concentration gives rise to much higher electron currents across
the junction than hole currents, in the condition of forward bias. The junction thus

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