A transistor
is a semiconductor device used to amplify or switch electronic signals and
electrical power. It is composed of semiconductor material
usually with at least three terminals for connection to an external circuit. A voltage or current applied to
one pair of the transistor's terminals controls the current through another
pair of terminals. Because the controlled (output) power can be
higher than the controlling (input) power, a transistor can amplify a signal.
Today, some transistors are packaged individually, but many more are found embedded
in integrated circuits.
In the n-p-n
transistor, connected as shown in Fig. 12.4(a), transistor action is
accounted for as follows:
(a) the
majority carriers in the n-type emitter material are electrons
(b) the
base-emitter junction is forward biased to these majority carriers and
electrons cross the junction and appear in the base region
(c) the base
region is very thin and only lightly doped with holes, so some recombination
with holes occurs but many electrons are left in the base region
(d) the
base-collector junction is reverse biased to holes in the base region and
electrons in the collector region, but is forward biased to electrons in the
base region; these electrons are attracted by the positive potential at the
collector terminal
(e) a large
proportion of the electrons in the base region cross the base collector
junction into the collector region, creating a collector current
In the p-n-p
transistor, connected as shown in Fig. 12.4(b), transistor action is
accounted for as
follows:
(a) the
majority carriers in the emitter p-type material are holes
(b) the
base-emitter junction is forward biased to the majority carriers and the holes
cross the junction and appear in the base region
(c) the base
region is very thin and is only lightly doped with electrons so although some
electron hole pairs are formed, many holes are left in the base region
(d) the
base-collector junction is reverse biased to electrons in the base region and
holes in the collector region, but forward biased to holes in the base region;
these holes are attracted by the negative potential at the collector terminal
(e) a large
proportion of the holes in the base region cross the base-collector junction
into the collector region, creating a collector current; conventional current
flow is in the direction of hole movement
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