what is Unijunction transistor

Unijunction transistors (UJTs) are three-terminal devices that have only one PN junction. When an aluminum lead is attached at one side of a bar or block of N-doped silicon, the attachment point becomes a P region or emitter. The other two connections are called base1 and base2. With the emitter grounded, the UJT is reverse-biased and no current flows between base1 and base2. As the bias voltage at the emitter increases, current flows when the bias voltage reaches a threshold or peak voltage, at which point the emitter-to-base2 current increases rapidly (negative resistance). When used with an RC timer circuit, UJTs can operate at frequencies up to 1 MHz. A unijunction transistor can be used to trigger a silicon-controlled rectifier (SCR) after a predetermined period of time. Unijunction transistors (UJTs) can be the active device in relaxation oscillators. A relaxation oscillator charges a capacitor gradually. At a preset voltage, the capacitor is discharged rapidly, producing a sawtooth wave. With two bases, a UJT is sometimes called a double-base diode. An integrated circuit can include not only an integrated circuit chip but also a circuit transistor such as a unijunction transistor.