SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

6.6. SECONDARY EFFECTS IN REAL DEVICES 285

BASE

EMITTER

Emitter

terminal

Base

terminal

Cross-section

p base

n collector

nnnnn

Top view

High power

BJT device

Interdigitated finger

contacts

Figure 6.23: Top view and the cross-section of a typical device using an interdigitated emitter.

There is a voltage drop IR across the base cross-section that becomes increasingly important at
high injections and high frequencies. As a result of this potential drop, the edge of the emitter
may be forward biased but the “core” of the emitter region may not be forward biased. Higher
current densities would thus flow along the edges of the emitter. This effect is called emitter
crowding and, because of it, the high injection effects discussed above can be important even at
low total current values.
Emitter crowding has an adverse effect on power transistors where high current values are
required. It is essential for these transistors that the emitter be properly designed. Computer
simulation techniques are used to study the current flow so that an optimum emitter can be
used. The emitter crowding effects can be suppressed by increasing the perimeter-to-area ratio
of the emitter. This is often done by using long fingers for the emitter and base contacts in the
“interdigitated” approach shown in figure 6.23.

Example 6.2Consider annpnsilicon transistor at 300 K with a base doping of

5 × 1016 cm−^3 and a collector doping of 5× 1015 cm−^3. The width of the base region is 1.0

μm. Calculate the change in the base width asVCBchanges from 1.0 to 5.0 V. Also

calculate how the collector current changes and determine the Early voltage. Assume that

Db=20cm^2 /s,VBE=0.7V,andWbLb.