What's the Difference?

Silicon Controlled Rectifiers (SCR) and Unijunction Transistors (UJT) are both semiconductor devices commonly used in electronic circuits. However, they serve different purposes and have distinct characteristics. SCRs are used as controlled rectifiers, allowing current to flow in one direction only when triggered by a small signal. UJTs, on the other hand, are used as relaxation oscillators and timing circuits, generating a series of pulses at a specific frequency. While SCRs are typically used in power control applications, UJTs are more commonly used in timing and triggering circuits. Overall, both devices play important roles in electronic circuits, but their functions and applications differ significantly.


StructureSilicon Controlled RectifierUnijunction Transistor
OperationThyristor device with three PN junctionsTransistor with one PN junction
TriggeringRequires gate current to turn onTriggered by voltage applied to the emitter
ApplicationsPower control, switching, invertersOscillators, pulse generators

Further Detail


Silicon Controlled Rectifiers (SCR) and Unijunction Transistors (UJT) are both semiconductor devices that play important roles in electronic circuits. While they serve different purposes, they share some similarities in terms of their attributes. In this article, we will compare the attributes of SCR and UJT to understand their differences and similarities.


SCR is a four-layer semiconductor device that acts as a controlled switch for high-power applications. It can handle large currents and voltages, making it suitable for applications such as motor control, power supplies, and lighting control. On the other hand, UJT is a three-terminal semiconductor device that is commonly used in relaxation oscillator circuits. It is known for its ability to generate precise timing pulses, making it useful in applications such as timing circuits and sawtooth waveform generators.


SCR consists of three junctions and four layers of alternating P-type and N-type semiconductor materials. It has an anode, a cathode, and a gate terminal for controlling the flow of current. UJT, on the other hand, has only one PN junction and is made up of a single piece of semiconductor material. It has a base terminal and two emitter terminals, which are used to control the device's operation.


SCR operates in three states: forward blocking, forward conduction, and reverse blocking. In the forward blocking state, the device does not conduct current until a gate signal is applied. Once the gate signal is applied, the SCR enters the forward conduction state, allowing current to flow from the anode to the cathode. In the reverse blocking state, the SCR blocks current flow in the reverse direction. UJT, on the other hand, operates as a voltage-controlled oscillator. It generates a sawtooth waveform at its emitter terminals, which can be used to trigger other devices in a circuit.


SCR is commonly used in high-power applications such as motor control, lighting control, and power supplies. It is also used in voltage regulators and inverters. UJT, on the other hand, is used in timing circuits, pulse generators, and sawtooth waveform generators. It is also used in phase control circuits and relaxation oscillators. Both devices have their own unique set of applications based on their operating characteristics.

Advantages and Disadvantages

  • SCR Advantages:
    • Can handle high currents and voltages
    • Simple structure and operation
    • High reliability and ruggedness
  • SCR Disadvantages:
    • Requires a gate signal to turn on
    • Not suitable for low-power applications
    • Can be prone to false triggering
  • UJT Advantages:
    • Precise timing pulse generation
    • Low cost and simple construction
    • Can be used in relaxation oscillator circuits
  • UJT Disadvantages:
    • Limited applications compared to SCR
    • Not suitable for high-power applications
    • Less rugged and reliable than SCR


In conclusion, SCR and UJT are both semiconductor devices with unique attributes that make them suitable for different applications. While SCR is known for its high-power handling capabilities and reliability, UJT is valued for its precise timing pulse generation and simplicity. Understanding the differences and similarities between these two devices can help engineers choose the right component for their specific application requirements.

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