In use, an appropriate bias voltage is applied between the two bases, with B2 made positive with respect to B1. Because the N-type bar is resistive, a relatively small current will flow through it, and the applied voltage will be distributed evenly along its length. If we start with the Emitter grounded, the junction will be reverse biased and there will be no emitter current. As the emitter voltage increases, there is no change until the junction suddenly becomes forward biased.

At this point, the emitter injects holes into the silicon bar, greatly reducing the effective resistance of the bar between E and B1. This will lower the emitter voltage required to keep the junction forward biased, and will sustain a heavy emitter current. This condition will continue as long as the circuit connected to the emitter can sustain the heavier current flow. The UJT thus behaves like a variation of the SCR, as suggested by the equivalent circuit shown to the right.

      The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.