Triggering Voltage of Unijunction Transistors

The triggering voltage of a unijunction transistor is the voltage that is needed on the emitter lead to switch the transistor to the on state so that a larger current flows through the base leads.

Below is a graph of the emitter voltage vs. emitter current characteristics curve of a unijunction transistor.

The point before the voltage reaches the triggering voltage Vtrig is the cutoff region. It is called the cutoff region because the voltage hasn't yet reached the necessary or sufficient point in order to turn the transistor on. Only when the voltage reaches the triggering voltage is it high enough to turn the transistor on. The triggering voltage is the critical voltage that a transistor must reach or exceed in order for the UJT to switch on.