Other meanings of Unijunction transistor
Semiconductor electronics
A unijunction transistor is a three-terminal semiconductor device with a single PN junction. Unlike a bipolar junction transistor, it is primarily a switching and triggering device: a voltage applied to its emitter can abruptly switch conduction between two base terminals. Its negative-resistance characteristic made it useful in relaxation oscillators, pulse generators, timing circuits, and thyristor triggering, although modern integrated timers and programmable devices have displaced it in many applications.1
The device consists of a lightly doped semiconductor bar with ohmic contacts at its ends, called base 1 (B1) and base 2 (B2), plus a heavily doped emitter region that forms a single PN junction with the bar.1 In the common construction, the bar is n-type and the emitter is p-type. With an interbase voltage applied, the bar acts as a resistive path; the emitter junction initially remains reverse-biased or only weakly forward-biased. The voltage at which the emitter begins to conduct is the peak voltage, approximately the interbase voltage multiplied by the intrinsic standoff ratio plus the emitter junction’s forward drop. This ratio depends on the bar’s internal resistance distribution and is commonly represented by the Greek letter eta.
Once emitter current reaches the peak-current threshold, injected carriers reduce the resistance near the emitter. Emitter voltage then falls while emitter current rises, producing a negative-resistance region in the characteristic curve. Conduction continues until the current falls below the valley-current threshold, after which the device returns toward its high-resistance state.
The unijunction transistor is most recognizable as the active element in a relaxation oscillator. A capacitor charges through a resistor until the emitter reaches the peak voltage; the UJT then conducts and rapidly discharges the capacitor through B1, creating a sawtooth voltage and a sharp pulse. The cycle repeats when emitter current drops below the valley current. The pulse can be coupled to the gate of a silicon-controlled rectifier or triac, allowing a low-power timing network to control a larger alternating-current load.3
Its useful behavior is defined by a family of emitter characteristic curves rather than by conventional transistor gain. Important parameters include peak voltage, peak current, valley voltage, valley current, interbase resistance, and intrinsic standoff ratio. Trigger timing varies with supply voltage, temperature, device tolerances, and the charging resistor, so practical circuits often include calibration or provide generous timing margins. The device is not normally used as a linear amplifier, and its base terminals are not interchangeable in a circuit designed around a specified polarity.
The unijunction transistor emerged from the development of semiconductor switching devices in the 1950s and became a widely taught discrete component during the following decades. Its simple three-terminal structure offered a comparatively inexpensive way to obtain threshold switching before inexpensive integrated timers and microcontrollers were widespread. The name “transistor” reflects its semiconductor amplification and switching lineage, but its one-junction structure and operating method distinguish it sharply from the two-junction bipolar transistor.2
Typical applications included lamp dimmers, sawtooth generators, pulse-forming circuits, industrial timing controls, and firing circuits for thyristors. Manufacturer data sheets specify maximum interbase voltage, emitter current, power dissipation, and the peak and valley parameters needed for circuit design.3 Because the triggering point is not as precise as that of many integrated circuits, the UJT is best suited to robust threshold and pulse functions rather than accurate frequency references.
The UJT’s negative resistance is a consequence of conductivity modulation, not a sign that the device supplies energy: an external source provides the power, while the device’s internal carrier redistribution changes its incremental resistance. The emitter junction is physically offset from the ends of the bar, so the two sections of the bar form a voltage divider whose ratio establishes the intrinsic standoff voltage. This makes the device’s geometry as important as its nominal resistance.
A related component, the programmable unijunction transistor (PUT), is a four-layer thyristor-like device with an externally settable anode-gate threshold; despite the similar name, it is not the same semiconductor structure as a conventional UJT.1 UJTs also appear in educational demonstrations because their relaxation-oscillator waveform makes threshold switching visible with simple instruments. In contemporary designs, integrated oscillators, comparators, and digitally controlled timers usually provide better stability, repeatability, and integration density.
Parameter names and operating limits vary among manufacturers; circuit designs should use the data sheet for the specific device and polarity.
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