UNIPOLAR JUNCTION TRANSISTOR Study Material
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UNIPOLAR JUNCTION TRANSISTOR (UJT)
UJT is a three terminal semiconductor switching device. As the name indicates, it has only one PN junction (i.e. unijunction). UJT can be used to control a large AC power with small gain, and not to be used as amplifier. It exhibits negative resistance characteristics and can be used as an oscillator.
Construction: It consists of lightly doped N-type silicon bar with ohmic contacts at the two ends, called base1 (B1) and base2 (B2). A P-type emitter is diffused nearer to the base2 of the bar, and this forms a PN junction diode with the base as shown in Figure 6.8(a). The P-type region is called emitter (E), since the device contains two base terminals with one PN junction and hence it is also called as double base diode. The resistance (RBB) between the bases (B1 and B2) is called inter-base resis-tance and is very high (5 to 10 kΩ), when emitter is in open condition.
- UJT is used as relaxation oscillator.
- It is widely used as triggering device for SCR and TRIAC.
- It is used in phase control circuits.
- UJTs can also be used to measure magnetic flux.
- It is used in switching circuits
- It is used as sawtooth generator.
- It is used in tuning circuits (TV).