What is a Unijunction transistor?

The Unijunction Transistor (UJT) is a three-terminal switching semiconductor device. It contains an n-shaped silicone object bar with a terminal attached to its two ends known as base one and base two. The third terminal is connected to a highly enclosed p-type material attached to the bar section of its length and is known as emitter. As there is only one corrective cross within the device, it is therefore called the 'Unijunction' transistor. The characteristics of the single junction transistor show the wrong resistance if it finds use in resting oscillator circuits for various applications.

Principle of UJT

UJT, also known as the double-base diode, is a 2-layer switch, 3-terminal strong-state (silicon) device. This device has a unique function that when it gets triggered, the emitter current will continue to rise until it is blocked using an outgoing power supply. The low price per unit, combined with its specific features, ensures its use in a large system style. Others include oscillators, pulse turbines, sensor teeth, triggering circuits, phase controls, time circuits, and voltage or modern control devices. The device is well-known, as a tool that absorbs less power under normal operating conditions and provides a useful, high-quality resource in the continuous effort to build relatively efficient systems.

The basic performance of UJT depends on the amount of voltage used. If the voltage used between the emitter terminals and base one is zero, then the UJT does not work. N-type material, therefore, tends to act as an antagonist. As the applied voltage usually increases in the emitter output area, the resistance value also increases, and the device starts operating. Throughout the process, the operation depends entirely on the majority of the carriers. This is the basic goal of UJT.

Construction of UJT

The P-N junction is composed of a lightly doped N-shaped silicon-controlled rod with a P-type material installed on one side in a single junction transistor. The ohmic contacts at both the ends of the silicon rod are called base 1 (B1) and base 2 (B2) which are positive-going and negative-going respectively. The P terminal term is called the emitter current baseline 1. UJT and JFET symbols are the same, except that the outward arrow represents the normal direction of current flow, but the operation is different.

The diagram represents the construction of the UJT.
Construction of UJT
GNU Free Documentation License | https://commons.wikimedia.orgg | Kormoran
The diagram represents the symbol of UJT.
UJT Symbol
GNU Free Documentation License| https://commons.wikimedia.org |
en: Weltanschauung

Working of UJT

The same simplified circuit shows that the n-channel contains two opposing components RB2 and RB1, which are connected in series with the same diode, and D represents the PN junction. In the production process, the emitter PN junction is located near the ohmic channel.

Resistance RB1 is provided between the emitter (E) and base 2 (B1) terminals, and resistance RB1 is provided between the emitter (E) and base 1 (B1) terminals. Since the PN junction is close to B1, the value of RB1 will be smaller than the variable resistance of RB2.

The diagram represents the circuit of the Unijunction transistor.
Circuit of UJT
GNU Free Documentation License| https://commons.wikimedia.org |
en: Weltanschauung

The voltage divider network is made up of a series combination of RB2 and RB1 resistance. When a voltage is applied to each semiconductor device, the power will be equal to the position of the base point near the channel.

The emitter (E) acts as an input when used in a circuit because terminal B2 is grounded. The B1 terminal is forward-biased towards B2, where the voltage (VBB) is applied to all B1 terminals and B2. If the emitter current input is zero, the voltage across the voltage separator of the RB1 circuit is calculated by the following formula.


The single-junction transistor (UJT) will be used as a voltage separation device. If the input is used between the emitter and base 1, the minimum value of the interval will be reduced, that is, RB1 will rise to a higher value. This fact shows that RB1 depends on the current emitter and is flexible. The double-base diode is used with the emitter junction forward-bias simultaneously as JFET is usually used with the reverse bias of the cross gate.

Uses of UJT

Unijunction transistor, for short, is some other strong state three-terminal device that may be utilized in gate pulse, timing circuits, and cause generator programs to replace and control either thyristor and Triacs for AC power management kind of applications.

Unijunction transistor can be used for a variety of applications such as:

  • Switching device
  • Unlocks the Triacs device and SCR's
  • Time circles
  • By controlling the section
  • For sawtooth generators
  • In a simple relaxation oscillator

The characteristics curve of UJT

The characteristics of the unijunction transistor (UJT) are provided below. The graph shows the relationship between emitter current IE on the x-axis and emitter voltage VE on the y-axis. The curve has three loops:

The diagram represents the characteristic curve for UJT.
Curve for UJT
  • Cut-off region
  • Negative Resistance region
  • Saturation

Cut-off region

The cut-off zone is the area where the unijunction transistor (UJT) cannot get enough power to turn on. The applied voltage VE has not yet reached the trigger point, thus keeping the transistor off.

Negative resistance region

When the transistor reaches the triggering voltage, the UJT will turn on. After some time, the voltage applied in the emitter rises, and it reaches a peak-point voltage. Though the current increases (negative resistance), the voltage drops from peak-point voltage to the valley point.


The saturation circuit is where the current and voltage rise when the voltage applied to the emitter terminal increases.

Advantages of UJT

The advantages of unijunction transistor:

  • Low fee
  • Negative resistance
  • Need lower trigger current cost
  • Stable trigger voltage
  • Low power, absorbing device

Common Mistakes

Note that the single junction transistor signal looks very similar to the junction transistor or JFET junction, except that it has a curved arrow representing the emitter (E) input. Although the same about their ohmic channels, JFETs, and UJTs work differently and should not be confused.

The main disadvantage of single-junction transistors is that they cannot provide proper amplification.

Context and Applications

This subject matter is important for exams at undergraduate and graduate levels, mainly for:

  • Bachelor of Technology in the Electrical and Electronics Department
  • Bachelor of Science in Physics
  • Master of Science in Physics
  • Transistor
  • Diode
  • Junction Transistor

Practice problems

Q1. What is the number of terminals in a UJT?

  1. 3
  2. 5
  3. 7
  4. 2

Correct option- (a)

Explanation: A unijunction transistor consists of three terminals. It is a semiconductor tool that displays negative resistance and switching characteristics.

Q2. Which type of waveform is generated by UJT?

  1. Square wave
  2. Saw-tooth
  3. Circular wave
  4. Sine wave

Correct option- (b)

Explanation: Saw-tooth wave generated by UJT. Because it is used for triggering an SCR.

Q3. How many types of junctions are there in UJT?

  1. 1 PN
  2. 2 PN
  3. 3 PN
  4. 4 PN

Correct oprtion- (a)

Explanation: UJTis a 3-lead digital semiconductor tool with only one junction, fully acting as an electrical management transmission. The UJT is not used as a linear amplifier.

Q4. Which type of device is UJT?

  1. Conductor
  2. Capacitor 
  3. Semiconductor
  4. None of these

Correct option-(c)

Explanation:  The Unijunction Transistor (double-base diode) is a three-term switching semiconductor device.

Q5. P-type emitter is _________ dopped in UJT.

  1. very light
  2. very high
  3. medium
  4. none of these

Correct option- (b)

Explanation: UJT has a PN junction so, P is highly doped, and N is lightly doped. Therefore, the resistance between double-base terminals is high.

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