SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

252 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

C

IC

R 2

B

n

p

n

E

R 1

V 1

V 2

IB

IE

Collector current

Collector

Base

Emitter

Base current (leads to

changes in base charge)

(a)

(b)

Figure 6.5: A schematic showing how the change in base current affects the majority carrier
injection density and the collector current in a bipolar device. (a) A circuit using a bipolar tran-
sistor. (b) The effect of base current variation on the injected minority charge and the collector
current. The collector current is much larger than the base current.

6.3 STATIC CHARACTERISTICS: CURRENT-VOLTAGE

RELATION

We will now develop a model for the current flow in a BJT. Initially we will use a simple
model which captures the essence of the device performance. Later we will discuss secondary
issues. In the bipolar device carriers from the emitter are injected “vertically” across the base