Unijunction Transistors (UJT) Information

Unijunction Transistors (UJTs) Information

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.


Specifications for unijunction transistors (UJTs) include intrinsic standoff ratio (maximum and minimum), RMS power dissipation, RMS emitter current, peak pulse emitter current, interbase voltage, emitter reverse voltage, and operating and storage junction temperatures. RMS is an abbreviation for root mean square. Unijunction transistors (UJTs) are available in standard packages such as TO-18 and TO-92. TO is an abbreviation for transistor outline.


Unijunction transistors (UJTs) that meet U.S. military specifications are manufactured according to standards described in MIL-STD-750 (Test Method Standard For Semiconductor Devices) and MIL-HDBK-6100 (Military Handbook, List Of Case Outlines And Dimensions For Discrete Semiconductor Devices). Like other components made of semiconductor material, unijunction transistors (UJTs) that are marketed in European Union (EU) nations should comply with the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) Directives.