A unijunction transistor is a three-terminal semiconductor device used in low-power circuits to control or activate other electronic devices. It belongs to the thyristor family and has a single emitter junction. It can be used in relaxation oscillator circuits and trigger circuits with a capacitor. A programmable unijunction transistor, or PUT, is a special type that can regulate the oscillation frequency of a circuit.
A unijunction transistor is a three-terminal solid-state semiconductor device used to control or activate other electronic devices in low-power, low-voltage circuits. The device has a single pn junction, called the emitter junction, between its two base terminals. Unijunction transistors were commonly used by electronic hobbyists in the 1970s to build simple oscillator circuits. In the early 21st century, these transistors were commonly used in relaxation oscillator circuits to control the discharge of a capacitor and to activate a thyristor in alternating current (AC) control circuits.
Previously known as a filament transistor or a double-base diode, the unijunction transistor belongs to the thyristor family, a type of power switching solid-state semiconductor device. A unijunction transistor has a high impedance when it is off, which means that it strongly resists current flow when it is off. When turned on, it has a low impedance, which means that it minimally resists the current flowing through it. Thus, a 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 at the N side of the emitter junction. When more voltage is applied between B1 and B2, the semiconductor material causes the conductivity between the emitter and B1 to increase, so current is easier to flow. As the conductivity between the emitter and B1 increases, the emitter voltage naturally decreases, which can turn off the transistor. This on-and-off effect produces the desired oscillation, or trigger.
One result of these swings is that a unijunction transistor can be used in a trigger circuit along with a capacitor. The capacitor draws current until the emitter voltage across the transistor reaches a certain level, at which time the transistor turns on. The transistor increases its conductivity to ground when it is turned on, so current can flow from the capacitor to ground. The result is a triangular wave oscillation.
A programmable unijunction transistor, or PUT, is a special type of unijunction transistor. A PUT is a pnpn semiconductor device with a gate contact made on 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 regulate the oscillation frequency of the circuit.
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