PUT

A programmable unijunction transistor is a three-terminal semiconductor device that acts as a voltage-triggered switch. Unlike a standard transistor, the PUT does not amplify signals; instead, it waits for a threshold voltage at its gate terminal, then rapidly switches its main channel from high resistance to low resistance. This abrupt switching behavior makes it useful for timing circuits, oscillators, and relaxation circuits that need precise, repetitive switching at predetermined voltage levels.

The PUT has three leads: an anode (positive terminal), a cathode (negative terminal), and a gate. The gate voltage sets the switching threshold. When the anode voltage rises above the gate voltage by approximately 0.7 volts, the device enters a conducting state and current flows from anode to cathode. When anode voltage drops below the threshold, it snaps back to its high-resistance state. This negative resistance region is the key to its behavior in many circuits.

PUTs appear in sawtooth wave generators, timing circuits, and pulse-width modulation (PWM) controllers because they switch crisply and predictably. A resistor and capacitor network connected to the gate determines the exact trigger voltage. In sawtooth oscillators, a capacitor charges through a resistor until it reaches the threshold, then discharges rapidly through the PUT, and the cycle repeats. This simple topology made PUTs popular in analog synthesizers and function generators before digital timing circuits became dominant.

The device is sometimes confused with the unijunction transistor (UJT), an older two-terminal device with similar switching properties. The PUT's advantage is that you can adjust the switching voltage using external resistors connected to the gate, whereas UJT thresholds are fixed by the device itself during manufacture. This programmability is where the PUT gets its name.

Typical switching times are in the microsecond range, and they can handle currents from milliamps to a few amps depending on the specific part. PUTs are less common in modern designs because microcontrollers and dedicated timing ICs now handle most tasks, but they remain useful in low-cost, discrete analog circuits where parts counts need to stay minimal and component integration is not an option.