SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

278 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

JC > JKirk(hypothetical)

JC = JKirk

wC

(a)

EC

EF, n

EV

EF, p

(b)

eVBC

|E|

Figure 6.19: (a) Hypothetical electric field profile in the collector if the electric field were to
continue evolving as described earlier afterJC>JKirk. (b) Band diagram corresponding to
JC>JKirkabove.

Now, electrons diffusing across the base must diffuse over a distancewB+ΔwBbefore
they are swept into the collector by the electric field in the depletion region. This effective
extension of the base region is called base widening. The consequence is a rapid increase in
base recombination or a decrease inβforJC>JKirk. It it therefore critical thatJKirkbe
maximized. From equation 6.6.13, we can see that this can be achieved by