24.2 Bipolar Transistors
693
(a)
V EB
I C
I B
I E
V CB
(b)
V EB
I C
I E
I B
V CE
(c)
V BC
I E
I C
I B
V CE
Fig. 24.4 Basic transistor circuits, named after the common contact: a Common base circuit, b common emitter circuit
and c common collector circuit
Fig. 24.5 pnp transistor in
a base circuit. b Doping
profile and space charges
(abrupt approximation) and
c band diagram for typical
operation conditions
(a)
(b)
(c)
x
N D N A
w B
-x E
x C
0
w
p
n
p
I C
I B
emitter
collector
base
I En
I Ep
I Cn
I Cp
V CB
I E
V EB
E
x
E V
V EB
V CB
24.2.1 Carrier Density and Currents
The modeling of transistors is a complex topic. We treat the transistor on the level of the abrupt junction.
As an approximation, we assume that all voltages drop at the junctions. Series resistances, capacities
and stray capacities and other parasitic impedances are neglected at this point.
The major result is that the emitter–base current from the forward-biased emitter–base diode will be
transferred to the collector. The current flowing from the base contact is small compared to the collector
current. This explains the most prominent property of the transistor, the current amplification.
For the neutral part of the base region of a pnp transistor, the stationary equations for diffusion and
continuity are
0 = D B
∂
2 p
∂x 2 −
p − p B
τ B
(24.1a)
j p = −eD B
∂ p
∂x
(24.1b)
693
(a)
V EB
I C
I B
I E
V CB
(b)
V EB
I C
I E
I B
V CE
(c)
V BC
I E
I C
I B
V CE
Fig. 24.4 Basic transistor circuits, named after the common contact: a Common base circuit, b common emitter circuit
and c common collector circuit
Fig. 24.5 pnp transistor in
a base circuit. b Doping
profile and space charges
(abrupt approximation) and
c band diagram for typical
operation conditions
(a)
(b)
(c)
x
N D N A
w B
-x E
x C
0
w
p
n
p
I C
I B
emitter
collector
base
I En
I Ep
I Cn
I Cp
V CB
I E
V EB
E
x
E V
V EB
V CB
24.2.1 Carrier Density and Currents
The modeling of transistors is a complex topic. We treat the transistor on the level of the abrupt junction.
As an approximation, we assume that all voltages drop at the junctions. Series resistances, capacities
and stray capacities and other parasitic impedances are neglected at this point.
The major result is that the emitter–base current from the forward-biased emitter–base diode will be
transferred to the collector. The current flowing from the base contact is small compared to the collector
current. This explains the most prominent property of the transistor, the current amplification.
For the neutral part of the base region of a pnp transistor, the stationary equations for diffusion and
continuity are
0 = D B
∂
2 p
∂x 2 −
p − p B
τ B
(24.1a)
j p = −eD B
∂ p
∂x
(24.1b)