Unijunction transistors (UJT) - definition
Unijunction transistors (UJT) - definition
Unijunction transistors (Unijunction Transistor, UJT) are semiconductor electronic elements whose operation is based on the characteristics of a single p-n junction, embedded asymmetrically in an n-type (less commonly p-type) semiconductor material. Unlike classical Bipolar transistors or Unipolar transistors, the UJT does not act as an amplifier, but is mainly used as a control element in generators, triggering circuits and relaxation oscillators.
The structure of the UJT comprises one p-n junction and two n-type layer ends, designated B1 and B2 (bases), and a p-type emitter electrode (E). A characteristic feature of Unijunction transistors is the presence of a negative dynamic resistance within a certain operating range, which allows the creation of voltage pulses and switching control in digital and analogue circuits. The emitter current initiates an avalanche of carriers across the junction when the threshold voltage is exceeded, leading to a sudden voltage drop between the emitter and the B1 base - a phenomenon used to trigger Thyristors or generate periodic waveforms.
The UJT is primarily used in simple timing circuits, as a ramp or pulse generator, and in phase and power control circuits. Although it is increasingly being replaced by integrated circuits or microcontrollers in modern applications, its simple design, low power consumption and ability to operate over a wide temperature range mean that it remains an essential component in systems of limited complexity. Parameters relevant to the operation of the UJT include the threshold voltage, the η factor (which characterises the voltage division in the structure) and the resistances between the bases.
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