“Since the initial invention of the point contact transistor some twelve
years ago engineers have continually striven to develop new types with
improved performance. The original point contact transistors were very
noisy, rather deli- cate and it was very difficult to produce
consistently good quality types. When the alloy junction transistor was
invented, therefore, they rapidly became obsolete. Alloy junction
transistors, such as the 0071 and 0C44, make excellent A.F. amplifiers
with comparatively low noise figures and R.F. types may be used at
frequencies of up to about 5Mc /s. Even at medium wave band frequencies
of around 1Mc /s, however, their gain is comparatively low because even
the best of them provide a common emitter current of only about 10
times at these frequencies. To overcome the limitations of the
conventional alloy transistor the alloy diffused type was developed,
examples of this type being the 0C170 and 0C171. In these the
resistivity of the base region, instead of being,, constant, varies
steadily from the emitter to the collector. This results in the charge
carriers being accelerated across the base thus reducing the transit
time and increasing the cut-off frequency. Alloy diffused transistors
make excellent, high gain R.F. amplifiers at M.W. frequencies and they
will operate up to 100Mc Is or more. However, they have very high
saturation voltages which means that a comparatively high collector
voltage must be used and their current gain drops rapidly as the
collector current is reduced making them unsuitable for operation at
low collector current levels. They also make poor A.F. amplifiers and
are thus difficult to use in reflex circuits unless the designer takes
numerous precautions. We may now sum up the last two types. The alloy
junction transistor makes an excellent A.F. transistor down to fairly
low levels of collector current and voltage but is a poor R.F.
amplifier by comparison. The alloy diffused type is an excellent R.F.
transistor which can operate right up to V.H.F. frequencies but is not
very good as an A.F. amplifier or at low collector currents and
voltages. A transistor was required. therefore, which combined and
extended the advantages of both types just mentioned without any of the
disadvantages. An early attempt to achieve this resulted in the surface
barrier transistor or SBT. The SBT was made by electrochemically
etching away the base prior to plating on the collector and emitter
electrodes. This resulted in an extremely narrow base width and, hence,
a very high cut-off frequency. SBT’s operated perfectly at both audio
and radio frequencies and they could also operate at very low power
levels. Unfortunately, however, their common emitter current gain was
always very low and they were limited to collector currents of 5mA and
collector voltages of 5 volts. These disadvantages precluded their
widespread application although they were very useful for some purposes
such as in radio control receivers or in radio receivers designed to
operate from low voltage batteries. Finally the micro alloy transistor,
or MAT, was developed. These are made by a technique similar to that
invented for SBT’s but developed considerably to overcome all the
disadvantages of the latter. MAT’s have extremely high levels of
current gain, as high as 200 in many cases, and they can also operate
at collector currents of up to 50mA and collector voltages . up to
about 12 volt.. Their cutoff frequencies are as high or higher than
those of alloy diffused transistors and yet they make better A.F.
amplifiers than alloy junction types. Furthermore they will operate at
much lower levels of collector current and voltage. The two tables
below compare typical MAT’s with the best alloy junction and alloy
diffused types on the market.”