The Unijunction Transistor 2N2646


The Unijunction Transistor (UJT) is a development from the early days of semiconductor technology. It was developed around 1953 in the Bell laboratories.
Like a normal bipolar transistor, the UJT has three terminals on the outside, but in contrast to this, it has only one PN junction internally, like a diode. It is therefore a hybrid of a diode and a transistor and behaves like a controlled diode, which (despite the constant polarity of the applied voltage) can block or conduct like a switch. Because of the two base connections, the UJT is also called a "double base diode".
If a positive control voltage (small against base B1) is applied to the emitter, then initially no relevant change in conduction behavior happens. However, if the control voltage is increased further, then at a certain potential the emitter-base1 voltage suddenly collapses and the emitter current jumps to a certain higher value, the UJT has "fired" so to speak. This behavior is very similar to that of a Thyratron or as semiconductor Thyristor.
It is also important to note that the UJT has a negative differential resistance between the highest and lowest points of the characteristic curve. The latter predestines the UJT as an oscillator. Nevertheless, the UJT is nowadays only a niche product.

Well known UJT like the 2N2646 are still available. I used this UJT type in 1976 to build a very simple electronic drum kit with three UJT 2N2646. Around that time, the UJT was a popular component for sawtooth generators. I still own the three original Motorola 2N2646 from back then.


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Photo and Diagram

Motorola 2N2646
Characteristic Curve
Schematic Diagram
2N2646 PIN
Osc Schematic
UJT Relaxation
Osc Output


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