Unijunction transistor

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UJT N symbol (case) .svg
Circuit symbol, n-channel
UJT P symbol (case) .svg
Circuit symbol, p-channel

The unijunction transistor (abbreviation: UJT ), often also called the double base diode, was developed in the Bell laboratories in 1953 . In electronics, this component is a hybrid of diode and transistor , but in some respects it is similar to a thyristor , like the programmable unijunction transistor (PUT) .

Structure and functionality

Similar to a normal bipolar transistor, the unijunction transistor has three connections, but in contrast to this only one pn junction ( unijunction , dt. "A barrier layer"), like a diode. In practice it behaves like a controlled diode, which can be made blocking or conductive despite the polarity of the applied voltage . Because of the two B connections, the UJT is also referred to as a “double base diode”.

If a small positive control voltage is applied to the emitter compared to the base B 1 , then nothing happens at first. If the control voltage is increased further, then the voltage U EB1 suddenly collapses at a certain potential , at the same time the emitter current jumps to a certain value, the UJT has, so to speak, "ignited". This behavior is very similar to that of a thyristor. The UJT is only extinguished when the emitter current falls below a certain level. The exact ignition and holding current values ​​are decisive parameters of a UJT, which can be found in the respective manufacturer's data sheet. The highest point of the U EB1 - I E characteristic is called the cusp point. The lowest point is called the valley point with the designations U t and I t . The characteristic curve has a negative differential resistance between the cusp point and the valley point . A slow traversing the curve, as with a normal transistor, is not possible with the UJT.

The unijunction transistor is a development from the early days of semiconductor technology. Its application was limited to a few special cases, and with the advent of the integrated circuit at the latest, this type of transistor was considered obsolete. A typical representative of the UJT is the 2N2646, which was offered by various manufacturers.


In analog circuit technology, the UJT was used as a central component of sawtooth generators ( tilt oscillator ) with a capacitor at the emitter connection. It can also be used to control thyristors and triacs . However, it is of little practical importance today; in recent years it has been largely replaced by integrated semiconductor circuits such as the NE555 .

Web links

Commons : Unijunction transistors  - Collection of images, videos and audio files

Individual evidence

  1. Hans-Joachim Fischer, Wolfgang E. Schlegel: transistor and circuit technology. 4th edition. Military publishing house of the GDR, 1988, ISBN 3-327-00362-9 .