SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

6.3. STATIC CHARACTERISTICS: CURRENT - VOLTAGE RELATION 261

VEB

Common base Common emitter Common collector

E

B

E C

B

E C

B

IE C IC

VBC

IB

VEC

VEC

VEB

IE

VCB

IC

IB

VBC

IB

VEB

VEC

IC

IE

IE4

IE3

IE2

IE1

IE= 0

~0.7 V VBC

0

IC

Cutoff

Saturation

}

Active

IB2

IB1

IB = 0

IB2

IB1

IB= 0

Reverse active

Cutoff

Saturation

Active

VBC= 0

VEC

IC

Common base Common emitter

(a)

(b)

Forward active

Figure 6.9: (a) Three possible configurations under which a BJT can be used in circuits. (b)
A schematic of the current-voltage characteristics of a BJT in the common-base and common-
emitter configuration.

gives the reverse-bias EBJ current. These equations represent two diodes that are coupled to
each other. The Ebers-Moll model is primarily useful to develop a physical description of the
bipolar device.
An important application of the Ebers-Moll model is to find the conditions for the saturation
mode. In the common-emitter mode, the saturation condition is given by

VCE(sub)=VBE+VCB=VBE−VBC (6.3.25)

Note that bothVBEandVBC(=−VCB) are positive.
We also have the current conservation expression:

IE+IB+IC=0 (6.3.26)