ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 2
c. Classification. The two basic types of transistors are the
negativepositivenegative (NPN) and the positivenegative
positive (PNP). The only difference in symbology between the
two transistors is the direction of the arrow on the emitter.
outward, it is an NPN transistor.
(1) Point contact;
(2) Grown or rategrown junction;
(3) Alloy or fused junction; and
(4) Diffused junction.
Each of these types of manufacture will he discussed briefly in
the following paragraphs.
Pointcontact transistors are now practically obsolete. They
have been replaced by junction transistors, which are superior
to pointcontact transistors in nearly all respects. The
junction transistor generates less noise, handles more power,
provides higher current and voltage gains, and can be mass
produced more cheaply than the pointcontact transistor.
Junction transistors are manufactured in much the same manner as
the PNjunction diode discussed in paragraph 3i(1)(c) on pages
112 and 113 of this task. However, when the PNP or NPN material
is grown, the impurity mixing process must be reversed twice in
order to obtain the two junctions required in a transistor.
Likewise, when the alloyjunction on or the diffused junction
process is used, two junctions must also be created within the
e. Transistor Theory. The proper biasing of a transistor
enables the transistor to be used as an amplifier. To function
in this capacity, the emittertobase junction of the transistor
is forward biased, while the basetocollector junction is