Page 31 - Power Electronics Handbook

24 Power semiconductor devices

Since the base is lightly doped only a few holes (for n-p-n) or electrons
(for p-n-p) are available to cross into the emitter. The collector-base
region is reverse biased resulting in a small minority carrier flow, which is
holes from collector to base and electrons from base to collector, for an
n-p-n transistor, and vice versa for a p-n-p transistor.
The collector bias is much greater than that of the base, and since the
base region is narrow, the majority carriers which cross the emitter-base
region are diffused across the base and into the edge of the collector
depletion layer. These carriers then reach the collector, resulting in
collector current flow. A small amount of the majority carriers, crossing
from the emitter to the base, combine within the base and this, along with
the small number of minority carriers crossing from the base to the emitter,
results in a low value of base current.
Transistors can be operated in common base, common emitter or
common collector modes. Common emitter is the most popular
configuration, common collector being used in emitter follower applica-
tions. For common emitter the base current ZB is the input and the collector
current IC is the output. The ratio between these two (Zc/ZB) is called the
common emitter forward current transfer ratio hm. In the transistor a low
base-emitter voltage and small base current change results in a large
collector current across the high reverse-biased collector-base region.
There is therefore power gain within the transistor.
Leakage current is caused within the reverse-biased collector-base
junction due to minority carriers. For common emitter this current is
multiplied by the gain of the device and flows in the emitter, when the base
is open circuit. This leakage current is further increased with temperature.

1.5.2 Characteristics
The main ratings of a power transistor are related to current and voltage.
Since the transistor has three terminals, base, collector and emitter, the
ratings are associated with each terminal. The maximum collector current
is described as the maximum continuous current and the peak current.
Since the peak current is only of the order of twice as large as the maximum
continuous value, the transistor does not have a high surge rating. The
maximum current is usually limited by the gain of the device and by the
bonding wires.
There are also several voltage ratings for a transistor, depending on
which junction is being considered and the state of the third terminal. For
is
example VCB~ the breakdown voltage between the collector and base
junctions with the emitter open circuit; VCES is the breakdown voltage
between the collector and emitter with the base short-circuited to the
emitter; VC~, is the collector emitter breakdown voltage with the base
open circuit.
The output characteristic of a bipolar transistor is shown in Figure
l.ll(c). For very low values of collector-emitter voltage the collector does
not gather up electrons which pass through the base region. This process
becomes more efficient as V, increases, giving a rapid rise in collector
current. After the knee of the curve has passed, most of the carriers