A Unijunction Transistor (UJT) is a three-terminal semiconductor device that operates as a unique switching device. It has only one junction, hence the name "Unijunction." The working principle of a UJT is based on the modulation of its internal properties due to the varying voltage applied to its emitter and base terminals.
The construction of a UJT typically consists of an n-type silicon bar with a p-type material sandwiched between two sections of n-type material. The three terminals of a UJT are:
Emitter (E): This terminal is heavily doped and serves as the main current-carrying terminal.
Base 1 (B1): This terminal is lightly doped and acts as the control terminal.
Base 2 (B2): This terminal is also lightly doped and is used to provide a voltage reference for the device.
The operation of a UJT is primarily governed by the voltage across the two base terminals (B1 and B2). When a positive voltage is applied to the emitter terminal (E) with respect to B2, the emitter-base1 diode junction becomes forward-biased. This allows a small current to flow into the device from the emitter (E).
As this current flows into the UJT, it accumulates in the region near the base 1 terminal. When the accumulated charge reaches a critical level, it triggers an internal voltage drop across the region between base 1 and base 2 (VB1). This internal voltage drop significantly reduces the resistance between the emitter and base 1 (RE).
Once the internal voltage drop (VB1) occurs, the UJT enters its low-resistance state, resulting in a sharp increase in current flowing from the emitter (E) to the B2 terminal. This state is known as the "negative resistance region," where an increase in voltage across the emitter-base1 junction leads to a decrease in resistance.
Applications in Relaxation Oscillators:
Relaxation oscillators are electronic circuits that produce non-sinusoidal waveforms, such as square, triangular, or sawtooth waves. The UJT can be utilized as a key component in relaxation oscillators due to its unique characteristics in the negative resistance region.
In a relaxation oscillator circuit using a UJT, a capacitor (C) is charged through a resistor (R) until it reaches a certain voltage threshold. At this point, the UJT enters its negative resistance region, causing a rapid discharge of the capacitor. The capacitor discharges until it reaches a lower voltage threshold, at which the UJT switches back to its high-resistance state, and the charging process begins again. This cycle repeats, generating a repetitive non-sinusoidal waveform at the output.
The frequency of the generated waveform can be adjusted by changing the values of the resistor (R) and capacitor (C) in the relaxation oscillator circuit. These types of oscillators find applications in various electronic devices, such as timers, pulse generators, waveform generators, and frequency modulators.
In summary, the Unijunction Transistor (UJT) works based on the principle of negative resistance, making it useful in relaxation oscillator circuits for generating non-sinusoidal waveforms with adjustable frequencies.