SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

6.6. SECONDARY EFFECTS IN REAL DEVICES 283

COMMON BASE:

During breakdown, emitter current

is not affected

COMMON EMITTER:

At very low multiplication, base current

starts to increase, causing the emitter

current to rise, and the collector current

starts to run away

12

10

8

6

4

2

0

0 20 40 60 80

IE = 0 ICBO

IE = 10 mA

IC

(mA)

VBC (V)

12

10

8

6

4

2

0

0 20 40 60 80

ICEO

IB = 0

IB = 0.06 mA

IC

(mA)

VEC (V)

0.05

0.04

0.03

0.02

0.01

Figure 6.22: Avalanche breakdown related characteristics of a bipolar transistor in the common-
base and common-emitter configurations.

help or hinder the carriers injected into the base from the emitter. Of course, if the doping can
be made non-uniform in a controlled manner, it can be exploited to shorten the base transit time.

6.6.5 Avalanche Breakdown ...........................

Just as in the case of thep-ndiode, the avalanche process limits the collector-base voltage
that the transistor can sustain. This then sets the limit on the power that can be obtained by
the transistor. The breakdown due to the impact ionization (avalanching) is reflected in the
I-V characteristics of the transistor in a manner shown in figure 6.22. In the common-base
configuration, the breakdown occurs at a well defined collector-base voltageBVCBO.Onthe
other hand, for the common-emitter configuration, the breakdown is not as sharply reflected
in the device output characteristics. The breakdown in the common-emitter configuration also
occurs at a lower value ofVCEthan it does in the common-base configuration.