Bipolar Junction (BJT) transistors
A bipolar junction transistor is formed by joining three sections of semiconductors with
different types of doping. The middle section (base) is narrow and one of the other two
regions (emitter) is heavily doped. Two variants of BJT are possible: NPN and PNP.
Fig. 1
A BJT has three terminals. Six parameters; iC, iB, iE, vB, vE and vC; define the state of the
transistor. However, because BJT has three terminals, KVL and KCL should hold for these
terminals, i.e.,
Fig. 2
Operation of NPN Transistor
For proper operation, the emitter-base junction is forward biased and the collector-base junction
is reverse biased as shown in Fig. 3 When the emitter-base junction is forward biased say 100
electrons are emitted into the base resulting in emitter terminal current IE. Say two electrons
recombine in the base resulting in a base terminal current IB Say 98 electrons find themselves in
the depletion region at the collector-base junction because the base is thin and lightly doped. This
means the transition time is little and very few electrons will recombine because there are few
holes in the base. The 98 electrons are quickly swept across the collector-base junction by the
exposed ions at the junction and reach the collector terminal. This constitutes collector current I C.
Thus IE = IB + IC
, Fig. 3
Operation of PNP Transistor
PNP transistor behaves exactly in the same way as the NPN transistor. The only
difference is that the majority charge carriers are holes and minority charge carriers are
electrons.
Fig. 4
Transistor Configurations:
Common Base (CB) configuration
The base terminal is common between input and output circuits.
A bipolar junction transistor is formed by joining three sections of semiconductors with
different types of doping. The middle section (base) is narrow and one of the other two
regions (emitter) is heavily doped. Two variants of BJT are possible: NPN and PNP.
Fig. 1
A BJT has three terminals. Six parameters; iC, iB, iE, vB, vE and vC; define the state of the
transistor. However, because BJT has three terminals, KVL and KCL should hold for these
terminals, i.e.,
Fig. 2
Operation of NPN Transistor
For proper operation, the emitter-base junction is forward biased and the collector-base junction
is reverse biased as shown in Fig. 3 When the emitter-base junction is forward biased say 100
electrons are emitted into the base resulting in emitter terminal current IE. Say two electrons
recombine in the base resulting in a base terminal current IB Say 98 electrons find themselves in
the depletion region at the collector-base junction because the base is thin and lightly doped. This
means the transition time is little and very few electrons will recombine because there are few
holes in the base. The 98 electrons are quickly swept across the collector-base junction by the
exposed ions at the junction and reach the collector terminal. This constitutes collector current I C.
Thus IE = IB + IC
, Fig. 3
Operation of PNP Transistor
PNP transistor behaves exactly in the same way as the NPN transistor. The only
difference is that the majority charge carriers are holes and minority charge carriers are
electrons.
Fig. 4
Transistor Configurations:
Common Base (CB) configuration
The base terminal is common between input and output circuits.
