Page 33 - Power Electronics Handbook

26 Power semiconductor devices

generated at the base-emitter junction are gathered up by the collector, so
that increasing V, does not appreciably affect IC. This is known as the
saturation region of the characteristic. The larger the value of base current,
the higher the collector current at which saturation occurs. Therefore a
bipolar transistor is essentially a current-operated device. At very large
values of VcE the carriers attain sufficient energy to cause avalanche
breakdown.
There are two gains which are important in bipolar transistors, the large
signal or d.c. gain (hm) and the small signal or a.c. gain (h& given by
equations (1.6) and (1.7).

For switching applications the d.c. gain is the most important, and is
given by the ratio of the d.c. values of the collector and base currents. It
falls off slowly at low values of collector current, as shown in Figure
l.ll(d), and also increases slightly with VcE, which accounts for the slope
in the output characteristic of the transistor. The d.c. current gain falls off
rapidly at high values of collector current, as the saturation resistance line
is approached. The gain also increases with temperature.
For linear applications, and for low-frequency and audio amplifiers, the
a.c. gain is of more relevance. It is the ratio of the change in collector
current for a small change in the base current, and its value is determined
by the magnitude of the steady state collector current.
The bipolar transistor is said to be in saturation when increasing base
current gives no further increase in collector current. The voltage drop
across the external terminals of the device is now known as the saturation
voltage. Data sheets usually give the collector to emitter saturation voltage
(V,sA,) and the base to emitter voltage (VBE(SAT)), and specify the
conditions, such as collector current, base current and temperature, under
which they were measured. At saturation both collector-emitter and
base-emitter junctions are forward biased and the junction voltages
oppose. The net junction voltage appearing across the collector-emitter
forms one part of the saturation voltage, the drop in the saturation
resistance (Ra) forming the other. Data sheets sometimes specify the
value of RCE instead of V-(sAm.
Transistor sustaining voltage is defined as the minimum collector-emitter
breakdown voltage. It can be specified with the base open circuit, short
circuit, or biased in the forward or reverse directions. For example,
V,?(S"S) is the minimum collector-emitter voltage with the base open
circuit. As the voltage across the transistor increases it reaches the
sustaining value, at which point the voltage across the device remains
relatively constant, over a wide current range. Figures 1.12(a) and 1.12(b)
show the measurement of VcEo(sus). Switch S, is initially closed, and then
opened once the current has reached a steady state value. The current
starts to decay and the inductor L1 causes the voltage to rise. At a voltage