Page 21 - Power Electronics Handbook

14 Power semiconductor devices

Crystal
surface

Figure 1.5 A crystal lattice showing the surface effect

1.3.2 Forward and reverse bias
Connecting a battery across the junction in the forward direction, that is,
with its positive terminal connected to the p layer, will decrease the
depletion region, acting with the potential barrier, and cause a cutrent to
flow as given by

where Vj is the voltage across the junction, 1, is the reverse junction
current, q is the electron charge, T the absolute temperature, and k is
Boltzmann’s constant.
When the voltage across the p-n junction is reversed, the battery
potential helps that of the internal depletion barrier, so that only a small
minority current can flow. This leakage current is due to three causes:
(i) Surface contamination, which can be reduced by cleaning the surface;
by coating it with protective material such as glasses or silicon resins;
or by designing the device such that surface fields are lower than
internal fields. This includes bevelling and the use of surface plates,
as described in section 1.3.3.
(ii) Diffusion of minority Carriers from the neutral areas into the
depletion region. Tbis is low for diodes operating below about lWC,
but is more significant for power devices whose junctions frequently
operate above this temperature. If the width of thep and n regions of
a reverse-biased diode are W and W,, as measured from the edge of
the depletion layer; Zp ancf,, are equilibrium concentrations of