3-38
The emitter of the UJT can be viewed as the wiper arm of a variable resistor. If the voltage level on
the emitter is more positive than the voltage gradient level at the emitter-base material contact point, the
UJT is forward biased. The UJT will conduct heavily (almost a short circuit) from base 1 to the emitter.
The emitter is fixed in position by the manufacturer. The level of the voltage gradient therefore depends
upon the amount of bias voltage, as shown in figure 3-43.
Figure 3-43.—Forward bias point on UJT voltage gradient.
If the voltage level on the emitter is less positive than the voltage gradient opposite the emitter, the
UJT is reverse biased. No current will flow from base 1 to the emitter. However, a small current, called
reverse current, will flow from the emitter to base 2. The reverse current is caused by the impurities used
in the construction of the UJT and is in the form of minority carriers.
More than 40 distinct types of UJTs are presently in use. One of the most common applications is in
switching circuits. They are also used extensively in oscillators and wave-shaping circuits.
Q26. The UJT has how many PN junctions?
Q27. The area between base 1 and base 2 in a UJT acts as what type of common circuit component?
Q28. The sequential rise in voltage between the two bases of the UJT is called what?
Q29. What is the normal current path for a UJT?
Field Effect Transistors
Although it has brought about a revolution in the design of electronic equipment, the bipolar
(PNP/NPN) transistor still has one very undesirable characteristic. The low input impedance associated
with its base-emitter junction causes problems in matching impedances between interstage amplifiers.
For years, scientists searched for a solution that would combine the high input impedance of the
vacuum tube with the many other advantages of the transistor. The result of this research is the FIELD-
EFFECT TRANSISTOR (FET). In contrast to the bipolar transistor, which uses bias current between base
and emitter to control conductivity, the FET uses voltage to control an electrostatic field within the
transistor. Because the FET is voltage-controlled, much like a vacuum tube, it is sometimes called the
"solid-state vacuum tube."
