Unijunction Transistors (UJT): Construction, Advantage and Working

Unijunction Transistors (UJT)

Unijunction Transistors (UJT)

A unijunction transistor (UJT) is a three-lead electronic semiconductor device with just one junction that functions solely as an electrically controlled switch.

The development of numerous transistor devices having diverse configurations of N-type and P-type materials is used to leverage the behaviour of semiconductor materials. Devices with distinctive behaviours and uses are created as a result of the physical arrangement of the materials in relation to one another. Bipolar junction transistors are the ones mentioned above that have two junctions of P-type and N-type materials (PN). With just one junction of the PN semiconductor materials, it is possible to create other, simpler transistors. Unijunction transistors are what these are known as (UJT).



One base semiconductor material and a distinct kind of emitter semiconductor material are both present in the UJT. There isn't any collector content. Two electrodes are attached to the base material at opposing ends, and one electrode is attached to the emitter. Base 1 (B1) and Base 2 are the names of these (B2). The UJT looks physically identical to a bipolar junction transistor due to the electrode arrangement. In the UJT, there is just one PN junction, and it functions differently.

A silicon n-type bar with minimal doping serves as the base. Its ends are joined by two ohmic contacts, B1 and B2. The name of the device comes from a single PN junction in the emitter, which is of the p-type and is severely doped. When the emitter is open-circuit, the resistance between B1 and B2 is referred to as interbase resistance. The device is typically not symmetrical since a symmetrical unit does not offer the best electrical characteristics for the majority of applications. Instead, the emitter junction is typically placed closer to base-2 (B2) than base-1 (B1)

If there is no potential difference between its emitter and either of its base leads, then the current flowing from B1 to B2 is incredibly little. On the other hand, if a sufficiently high voltage, referred to as the trigger voltage, is given to the emitter of the device, a very large current from the emitter joins the current from B1 to B2, increasing the output current of the B2 transistor.



A UJT's base material behaves between the electrodes like a resistor. When B2 and B1 are in a positive relationship, the voltage steadily decreases as it passes through the base.

The amount of voltage required to be provided to the emitter electrode in order to forward bias the UJT base-emitter junction is determined by positioning the emitter at a precise place along the base material gradient. The junction is forward biassed and current easily flows from the B1 electrode to the E electrode when the applied emitter voltage is greater than the voltage at the gradient point where the emitter is attached. Otherwise, despite considerable leakage, the connection is reverse biassed and no appreciable current flows. The applied emitter voltage can regulate current flow through the device by choosing a UJT with the appropriate bias level for a certain circuit.

The gadget has the unusual property that, when activated, the emitter current grows regeneratively until the emitter power supply limits it.Its characteristic of negative resistance makes it appropriate for use as an oscillator.

A positive voltage between the two bases biases the UJT. This could result in a drop along the device's length. Current will start to flow from the emitter into the base region when the emitter voltage is driven approximately one diode voltage above the voltage at the location where the P diffusion (emitter) is. The increased current (really charges in the base region) induces conductivity modulation, which lowers the resistance of the section of the base between the emitter junction and the B2 terminal because the base region is only very lightly doped. Due to the emitter junction's increased forward bias as a result of the resistance reduction, more current is injected. At the emitter terminal, the result is a negative resistance overall. The UJT is advantageous in simple oscillator circuits because of this.


There are many different types of UJTs with different designs and features. It is outside the scope of this essay to describe each one of them. UJTs generally outperform bipolar transistors in some respects. UJTs are stable across a wide temperature range of operation. The use of UJTs in various circuits can lower the overall component count, saving money and possibly boosting reliability.


They can be found in oscillators, wave shaping circuits, and switching circuits. Although they are less expensive and more often used, four-layered semiconductor thyristors perform the same function as the UJT that was just discussed.

The UJT is not utilized as a linear amplifier. It is utilised at low to moderate frequencies in pulse generation circuits, synchronised or triggered oscillators, and free-running oscillators (hundreds of kilohertz). It is frequently utilised in the triggering circuits for silicon-controlled rectifiers. In the 1960s, its low cost per unit and distinct property made it appropriate for use in a wide range of applications, including oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies. The initial unijunction transistor types are now regarded as being obsolete, but a subsequent multi-layer device, the programmable unijunction transistor, is still commonly accessible.

The TO-92 transistor package is home to the programmable unijunction transistor (PUT) 2N6027 and the general-purpose silicon PN Unijunction Transistor 2N2646, both of which are intended for use in a variety of industrial applications.

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