SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

256 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

pe(xe = 0) nb(x)

Emitter VBE Base VCB Collector

peo

pbo nco

pco

IE

|EBJ| | CBJ |

depletion

layer

depletion

layer (a)

nbo

Equilibrium density

Wbn

neo

– + –+

xe xc

(b)

p nbo

eo

xe = 0

xb= 0

xb =Wbn x

c = 0

xb

pco

Wb

IB IC

Forward bias Reverse bias

Figure 6.7: A forward active mode BJT. (a) The equilibrium carrier concentrations of electrons
and holes and positions of the junction depletion regions in thenpntransistor. (b) Minority
carrier distributions in the emitter, base, and collector regions.

The profile of the total minority carrier densities (i.e., background and excess) is shown in fig-
ure 6.7b. The electron distribution in the base is almost linear, as can be seen, and is assumed
to be so for some simple applications. Once the excess carrier spatial distributions are known
we can calculate the currents as we did for thep-ndiode. We assume that the emitter-base cur-
rents are due to carrier diffusion once the device is biased. We have, for a device of areaAand