Uni Junction Transistor(UJT):

• UJT is a semiconductor device that has only one junction and it has three terminals like transistor such as B1, B2 and E, so it is called Uni Junction Transistor. The construction, internal circuit and the symbol of the UJT is given below.

Construction:-

• UJT consists of a lightly doped N-Type Silicon bar with heavily doped P-type region. It has two end terminals B1 and B2 and since the silicon bar is lightly doped, the resistance between B1 and B2 is very high. The heavily doped P region is called emitter and it is designated as E and is closer to B2 region. Resistance between E & B1 is higher than the resistance between E & B2 because E is close to B2.

UJT characteristics :-

• The VI characteristics of UJT along with the circuit arrangement is given below.

• With emitter terminal open and a voltage VBB is applied between the two bases then the voltage drop across RB1 is given by$V_1 = \eta V_{BB}$Where$\eta = \dfrac{R_{B1}}{R_{B1}+R_{B2}} = \dfrac{R_{B1}}{R_{BB0}}$ and RB1 is the Emiter-to-Base1 resistance and RB2 is the Emiter-to-Base2 resistance. This voltage reverse biases the PN junction and the emitter current is zero.

• When a positive voltage VE is applied to the emitter terminal, even then the PN junction is reverse biased and the emitter current is zero until the emitter voltage VE is greater than$0.7 +V_1 =0.7+ \eta V_{BB}$ . This minimum value of the emitter voltage VE for which the emitter current starts to flow is called the firing voltage of UJT.
• After the firing of UJT as the emitter voltage VE is increased the emitter current IE is also increased up to a particular point called Vp (peak voltage). During this period when emitter current IE flows a large number of holes are injected into the junction. These holes are repelled by B2 and attracted by B1. As a result the region between emitter E and B1 terminal starts saturating by holes and the conductivity of this region starts to increase. This phenomenon of increasing conductivity by the insertion of holes is called conductivity modulation. This increased conductivity reduces RB1 and η. This results in a condition where emitter current IE increases and the emitter voltage VE decreases. This causes UJT to exhibit negative resistance characteristic. The region between Vp and Vv has a negative slope. This negative resistance region in the UJT characteristics is employed in relaxation oscillators.
• When a negative voltage VE is applied to the emitter terminal, even then the PN junction is reverse biased and the emitter current is zero.