Page 61 - Power Electronics Handbook

54 Power semiconductor devices

voltages. However, the wider space charge region now allows the electrons
to reach sufficient velocity, if the distance is greater than the mean free
space for electrons at a given temperature, such that, on impact, they have
enough energy to knock out other electrons from their crystal structure.
These new electrons then free further electrons, so that the current builds
up to a high value very quickly. This is known as the avalanche effect and,
as shown in Figure 1.31(b), it has a very sharp knee in its characteristic,
occumng at voltages in excess of about 7V. In between 5V and 7V a
combination of the zener and avalanche effects takes place.
Voltage-reference diodes are available in the range of 1-3OOV by
control of the doping profiles. Diodes below about 10 V are usually made
by alloyed junctions, whilst above this the junctions are diffused into an
epitaxial layer. The tolerance in the breakdown voltage is usually k5%,
but it is possible to select to tighter limits. The zener voltage decreases with
temperature, whilst the avalanche voltage increases with temperature, so
that the temperature coefficient of reference voltage follows a curve similar
to that shown in Figure 1.31(c). By combining zener and avalanche diodes
it is possible therefore to obtain devices with very good temperature
characteristics. The same effect is obtained by using an avalanche diode in
series with a forward-biased conventional diode, since a forward-biased
junction has a negative temperature coefficient of voltage. These devices
are often referred to as stabistors.

CI
r,
lAnode 0
L
L
a
Voltage

(a) characteristic

0
F
F 5 volts B
I I *
(C) (d) Voltage
Figure 1.31 Voltage-reference diode characteristics: (a) symbol; (b) static characteristic;
(c) temperature coefficient of reference voltage; (d) dynamic impedance