The original BJT by Bell
Labs in 1947.
Introduction to Microelectronics
BJT (Bipolar Junction Transistor)
The bipolar junction transistor (BJT) was invented and experimentally demonstrated by
Bardeen, Brattain and Shockley in 1947 in Bell Labs, Murray Hills, NJ, and is often regarded as
the birth of modern microelectronics.
Interestingly, the metal-oxide-semiconductor field-effect
transistors (MOSFET), the dominant market leader
experimental demonstration, but it was predicted not to
work due to the oxide-semiconductor interface concerns
people have in those days.
Before 1947, switching and
amplification were done by vacuum tubes, which have
successfully produced radios, radars and very expensive,
large and hot computers with limited capabilities.
Vacuum tubes dissipated a lot of heat due to the high
voltage and current required to emit electron beams from
a hot filament.
They are also very difficult to scale in
Although for large-power applications such
as radar, vacuum tubes (or a more modern version of
traveling-wave tubes (TWT)) are still in use. Engineers
had dreamed for a replacement in the 1940’s, but was
limited by a method that can reliably define a small
geometry (on the order of 2
m, the reason of which will
be clear later).
In 1947, Bardeen and Brattain finally
found that a sharp knife could create a wedge of that
dimension, as shown in Fig. 1.
The main amplification operation of BJT is by injecting carriers from a forward-bias
junction to a reverse-biased region.
We can use BJT as a good switch, because the carrier flow
through the reverse-biased junction is not determined by the voltage across, but by how much is
injected, which is controlled by the forward-bias part
Repeating Eq. (3.21), we know that
BJT is a good small-signal amplifier as well!!
There are two possible ways to implement BJT by the pn junction, NPN and PNP,
denoted by the type of materials in concatenation.
NPN is usually a bit more popular than PNP
due to the higher electron mobility, but sometime we have to use both for the voltage polarities
The terminal names, emitter (E), base (B) and collector (C) whose meaning will be
further explained in the carrier operations, and circuit symbols are shown in Fig. 4.2.
Edwin C. Kan