The
Bipolar junction transistor (BJT) is one of the most
used transistors today. It replaced the
triode in the 1960's. The
first BJT had two
indium electrodes fused to a
germanium
substrate. The substrate was at the time called
base so the
connected electrode also got this name. The collector got its
name since it 'collects'
charge carriers, while the emitter
sends out the carriers. A charge carrier is an electron (negative
carrier) or the 'hole' left when an electron leaves the
atom
(since the atom now have more
protons than electrons it gets a
positive charge). This is also why the BJT is called bipolar
(the conducting area has charge carriers of two polarities)
Buildup of the original
germanium transistor
_
| |
e_| |_e
.-(_| |_)-.
| | | |
| | | |
| |___| |
| | |
| | |
C B E
C Collector
B Base(germanium substrate)
E Emitter
e indium electrodes
Modern BJTs are mostly made from
silicon , and consists
either of NPN or PNP
junctions. The N material is
doped with a material with an
excess of negative
charge carriers while the P material has more positive
carriers.
Schematic buildup of a NPN transistor
.-----------------.
C --| N | P | N |-- E
`-----------------'
|
B
The BJT is a current amplifier; A current
flow from the base to the emitter results in a larger flow
from the collector to the emitter. This secondary flow is caused
by recombination of charge carriers in the 'base' area.
The current amplification might range from tens of times(in high
effect transistors) to several hundred times in signal
transistors.
Darlington transistors
easily have several thousand times amplification. The raw current
amplification is called hFE or ß(beta).
The germanium transistor has a base-emitter
voltage drop of
about 0.2v, while the silicon version has a drop of approximately
0.6v. But the silicon transistor has a better temperature and
high
frequency stability and silicon is much cheaper than
germanium. For
microwave frequency transistors
gallium
arsenide is sometimes used. Most transistors today uses an
epitaxial layer buildup.
Electrical symbol:
(European version is similar to the USA symbol
but it has a circle around it)
USA version
NPN
E
_ |
| \|
|/
B --|
|\
| \
C
PNP
C
| /
|/
B --|
|\ _
| |/
| E