SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

6.2. BIPOLAR TRANSISTOR: A CONCEPTUAL PICTURE 249

Emitter contact

n+

n+

n+

n+

p

B

E

C

p

p+

p+

p+

p+

p

nepitaxy

n+buried layer

p-type substrate

Base contact

Collector contact

n+

n+

nepitaxy

nepitaxy

n+ p n+

n

EB C

Cross-sectional

view

Base width

Figure 6.3: A schematic of the structure and doping profiles of a bipolar junction transistor along
with a simplified view of the cross-section.

We have shown a state of the art bipolar device. A schematic of the device is shown in
figure 6.1. The device could have a doping of the formn+−p-norp+−n-p.Wewillfocuson
then+−p-ndevice. The emitter is heavily dopedn−type, thep-region forms the base, and the
lowernregion is the collector. The emitter dopingNdeis much larger than the base dopingNab
to ensure that the device has a high current gain, i.e., that a small base current change produces
a large collector current change.
To understand how the device can have gain, let us consider a BJT where the emitter base
junction (EBJ) is forward biased and the base collector junction (BCJ) is reverse biased. This
biasing creates the forward active mode. The band profile of the device is shown in figure 6.4.
Note that the base widthWbis much smaller than the diffusion length of electrons in thep-
type base region. So that when electrons are injected from the emitter, most cross the base
without recombining with holes. The strong electric field these electrons see once they reach the
collector, cause them to be swept away and form the collector current.