Unijunctiontransistor

The Unijunctiontransistor (abbreviation: UJT), often called double-base diode, was developed in 1953 at Bell Laboratories. This component is in electronics, a kind of " hybrid " of diode and transistor, however, is similar in some respects a thyristor.

Design and operation

Much like a normal bipolar transistor of the unijunction transistor has three terminals, in contrast, only a pn junction (English unijunction, dt " a barrier " ), such as a diode. In practice, it behaves as a controlled diode, one can make blocking or conducting despite constant polarity of the applied voltage. Because the two B- terminals of the UJT is also referred to as " double-base diode."

If you put the emitter a little against the base B1 positive control voltage, then nothing happens at first. Increasing further the control voltage, then breaks at a certain potential, the voltage UB1 abruptly together, at the same time, the emitter current jumps to a certain value, the UJT has somehow " ignited by ". This behavior is very similar to that of the thyristor. Deleted is the UJT only when below a certain emitter current. The exact ignition and holding current values ​​are key characteristics of a UJT, which can be obtained from the respective manufacturer's data sheet. The highest point of the ÜB1 IE characteristic called cusp point. The deepest point is, according to nadir with the names and Ut it. intermediate cusp point and valley point the curve has a negative differential resistance. A slow driving through the curve, as in a normal transistor is not possible during the UJT.

The Unijunctiontransistor is a development from the early days of the semiconductor technology. Its use was limited to a few special cases, and with the latest advent of the integrated circuit was this transistor type as obsolete.

A very typical representative of the UJT is the 2N2646, which has been offered by various manufacturers.

Application

In the analog circuitry of the UJT as a central component of sawtooth ( relaxation oscillator ) was used with a capacitor at the emitter terminal. Also one can control him with thyristors and triacs. However, its current practical importance is low, it has been largely replaced in recent years by the semiconductor integrated circuit such as the NE555.