A unijunction transistor (UJT) is a three-terminal semiconductor device that exhibits a negative resistance characteristic. It is a type of thyristor, which means that it can be used as a switch or as an oscillator. The UJT has a unique structure that consists of a bar of lightly doped n-type material with a heavily doped p-type region at one end and a lightly doped n-type region at the other end. The p-type region is known as the emitter, while the n-type regions are known as the base 1 and base 2. When a voltage is applied to the emitter, a current flows into the base 1 and charges the capacitor formed by the base 1 and base 2 regions. Once the capacitor is charged, the UJT enters its negative resistance region, and the current flowing through the device increases rapidly. This behavior makes the UJT useful in applications such as relaxation oscillators, pulse generators, and timing circuits. The UJT has several advantages over other types of transistors. It is a simple device that is easy to manufacture and has a low cost. It also has a high input impedance, which makes it suitable for use in circuits that require high sensitivity. Additionally, the UJT can be used to trigger other devices such as SCR and TRIAC. Despite its advantages, the UJT has some limitations. It has a low current-carrying capacity and a low voltage gain. It also has a relatively low switching speed, which limits its use in high-frequency applications.
semiconductor, negative resistance, thyristor, oscillator, capacitor
CITATION : "Christopher Jones. 'Unijunction Transistors.' Design+Encyclopedia. https://design-encyclopedia.com/?E=405429 (Accessed on July 16, 2025)"
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