The UJT Stand for Uni Junction Transistor. It has one PN junction and three terminals. As shown in the below figure.
The UJT is consist of a light doped N-Type of silicon bar, there is a heavily doped P-Type of region at one side of the N-Type of bar. The terminal which is connected to the P-Type region is called emitter terminal (E), while the other two terminals which are connected to the opposite ends of the N-Type of bar are called Base 1, Base 2 (B1, B2) as shown in the Figure below.
The Uni Junction Transistor is always operating with forward bias voltages. For this purpose the Emitter Base 1 Junction is always kept forward bias w.r.t Base 2.
When the voltage between the emitter and Base 1 is zero, the Uni Junction Transistor did not conduct and so the N-Type bar act as resistor. But we will see that a small leakage current flows due to reverse bias junction. Now when we increase the emitter voltage step by step, the resistance between emitter and base 1 decrease and also the reverse current decreases, when the emitter voltage is increased to the level that it forward bias the junction and the emitter current start to flow. It is so because the holes of heavily doped P-Type of region are entered in the N-Type bar and recombine with the electrons of lightly doped N-Type bar. In this way the Uni Junction Transistor starts conduction.
The Uni Junction Transistor can be used as a triggering device for SCR and TRAIC, The other application of the Uni Junction Transistor is saw tooth wave generator, Phase control and in timing circuits.
When we switch ON the circuit the DC biasing voltage is applied to the circuit. The capacitor “C” charges to its peak value through the resistor “R”, when the capacitor is charged, it triggers the Uni Junction Transistor and so the Uni Junction Transistor starts conduction. In this way the capacitor “C” discharges through base resistor “RB”. Now when the capacitor is discharge, it start to charge again through the resistor “R” and similarly it will discharge when it trigger the Uni Junction Transistor, so the capacitor charging and discharging take-place and we get saw tooth wave, as shown in the the below Figure.