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