invention of the
bipolar junction transistor in 1948 by Bardeen,
Brattain, and Shockley paved the way for the subsequent arrival of the
integrated circuit, which revolutionized the semiconductor industry.
The bipolar transistor is a
device consisting of 3
alternating n- and p-type materials referred to as the
Its structure basically consists of two back-to-back diodes, one
between the emitter and base and the other between the base and
are two types of bipolar transistor:
NPN and the
In the NPN transistor, the base is composed of a p-type material
and is sandwiched by an n-type emitter and an n-type collector.
In the PNP, the base is n-type while the emitter and collector
Figure 1. Structure of a planar vertical NPN bipolar
transistor works by yielding a
high collector current when a
relatively small current is forced into its
Since Ib is relatively much smaller than Ic, a small variation in
Ib results in a much larger variation in Ic.
This, in effect, is
with the current gain known as the
of the transistor. The currents going into and out of
the emitter, base, and collector follow Kirchoff's current law:
Since Ic is much greater than Ib, Ie is very close in value to Ic.
In short, a large current flows from the emitter to the collector
of a transistor whenever the base receives some input current.
The transistor is therefore very useful as a switch or as an
the transistor operates (and therefore used) depends greatly on how it
is electrically stimulated, or
The transistor may be operated in three different regions:
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A transistor is said to be
both its base-collector and
base-emitter junctions are
Under this mode, the transistor is already completely 'on', i.e.,
the collector current is already very high and no longer increases
appreciably even if more current is fed into the base.
A transistor is in the
of its junctions are
Under this mode, the transistor is 'off'', i.e., the collector
is very low.
A transistor being used as a switch is operated
between saturation and cut-off regions.
A transistor in the active region exhibits a change in collector
current that is
proportional to the change in base current.
A transistor being used as an amplifier is therefore operated in
The base-emitter junction of a transistor in active region is
forward-biased while its base-collector junction is reverse-biased.
What is a Semiconductor?; p-n Junction; Diode;
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