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What is a Unijunction Transistor?

C. Greason
C. Greason

A unijunction transistor is a three-terminal, solid-state semiconductor device used to control or trigger other electronic devices in low-power, low-voltage circuits. The device has a single p-n junction, called the emitter junction, between its two base terminals. Unijunction transistors were popularly used by electronic hobbyists in the 1970s to build simple oscillator circuits. By the early 21st century, these transistors were commonly used in relaxation oscillator circuits to control the discharge of a capacitor and to trigger a thyristor in alternating current (AC) control circuits.

Previously known as a filamentary transistor or a double-base diode, the unijunction transistor belongs in the thyristor family, a type of power-switching, solid-state semi-conductor device. A unijunction transistor has a high impedance when it is off, which means it strongly resists current flow when it is off. When it is on, it has a low impedance, which means it minimally resists current flowing through it. Thus, a the device can control the flow of current in a circuit.


When its Base 1, or B1, terminal is grounded and a voltage is applied to its Base 2, or B2, terminal, a current is produced on the N side of the emitter junction. As more voltage is applied between B1 and B2, the semi-conductor material causes conductivity between the emitter and B1 to increase, so it is easier for current to flow. When the conductivity between the emitter and B1 increases, the emitter voltage naturally goes back down, which can shut the transistor off. This on-and-off effect produces the desired oscillation, or trigger.

A result of these oscillations is that a unijunction transistor can be used in a triggering circuit along with a capacitor. The capacitor absorbs current until the emitter voltage on the transistor reaches a certain level, at which time the transistor turns on. The transistor increases conductivity to ground when it is turned on, so current can flow from the capacitor to ground. The result is a triangle wave oscillation.

A programmable unijunction transistor, or PUT, is a special kind of unijunction transistor. A PUT is a p-n-p-n semiconductor device with a gate contact made to the n base instead of the p base. A PUT produces an oscillation much like a standard unijunction transistor, except that its emitter voltage is a different ratio of the B2 voltage. A programmable unijunction device can be used in a resonant circuit to adjust the circuit’s oscillating frequency.

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