unijunction

A unijunction transistor (UJT) is a three-terminal semiconductor device with a single P-N junction, used primarily as a switching element and timing component in industrial circuits. Unlike a conventional transistor, which amplifies signals, the UJT acts as a voltage-controlled switch that fires abruptly when its emitter voltage crosses a threshold. This sharp, predictable switching behavior made it invaluable in timing circuits, oscillators, and thyristor triggers before integrated timer chips became dominant.

The device consists of two ohmic contacts called base 1 and base 2, connected to opposite ends of a lightly doped N-type silicon bar, with a P-type emitter junction positioned along the bar between them. When forward bias is applied to the emitter relative to the base material, current remains nearly zero until the emitter voltage reaches the peak voltage, typically 0.5 to 0.7 times the base supply voltage. At this point, the junction switches from high to low resistance almost instantaneously, allowing current to flow until the voltage drops below the valley point.

Common industrial applications

The UJT's primary role has been in generating sawtooth waves for ramp generators, controlling the firing angle of SCRs and triacs in AC power circuits, and creating time-delay functions in machine controllers. A typical circuit uses a capacitor charging through a resistor toward the base voltage; when the emitter voltage reaches threshold, the UJT fires and rapidly discharges the capacitor through the emitter-base 1 path, creating the characteristic sawtooth or relaxation oscillation. Industrial equipment from the 1960s through 1980s relied on this effect for synchronous triggering of power semiconductor devices.

Modern designs have largely replaced discrete UJTs with integrated circuits such as the 555 timer or specialized trigger modules, which offer better temperature stability, lower cost, and easier integration. However, UJTs remain in use in legacy equipment, specialized analog circuits requiring minimal component count, and applications where their negative resistance characteristics are specifically exploited. The term unijunction distinguishes this device from the more common bipolar junction transistor (BJT), which has two junctions and operates on fundamentally different principles.