SCR (Silicon Controlled Rectifier) – Construction, Working Principle, Characteristics and Applications

What is Thyristor (SCR)?

A Silicon Controlled Rectifier (SCR) is a three-terminal, four-layer (PNPN), unidirectional power semiconductor device belonging to the thyristor family, which is used for controlled switching and regulation of high voltage and high current in power electronic circuits.

It operates as a bistable device that remains in the non-conducting (OFF) state under forward bias until a suitable gate triggering signal is applied, after which it switches to the conducting (ON) state and continues to conduct even after the removal of the gate signal until the anode current falls below the holding current.

The three terminals of an SCR are:

• Anode (A): Main current-carrying terminal connected to the positive supply
• Cathode (K): Main current-carrying terminal connected to the negative supply
• Gate (G): Control terminal used to initiate conduction

The four semiconductor layers form three junctions, designated as J1, J2, and J3, which determine the switching characteristics of the device.

Construction of SCR

The SCR is fabricated using high-purity silicon and consists of four alternate layers arranged as P1–N1–P2–N2. The junctions formed between these layers are:

• J1: Between P1 and N1
• J2: Between N1 and P2
• J3: Between P2 and N2

The anode is connected to the outer P-layer (P1), the cathode is connected to the outer N-layer (N2), and the gate terminal is connected to the inner P-layer (P2).

The device structure results in a tightly coupled regenerative feedback mechanism between two internal transistors: a PNP transistor and an NPN transistor. This internal transistor model explains the latching and switching behavior of the SCR.

Modes of Operation of SCR

The operation of a Silicon Controlled Rectifier depends on the polarity of the applied voltage across the anode and cathode and on the gate signal. An SCR operates in three distinct modes, namely:

• Reverse Blocking Mode
• Forward Blocking Mode
• Forward Conduction Mode

Each mode corresponds to a particular biasing condition of the internal junctions J1, J2, and J3.

Reverse Blocking Mode

In this mode, the cathode is made positive with respect to the anode, and the gate terminal is kept open.

• Junctions J1 and J3 are reverse biased
• Junction J2 is forward biased

Since two of the three junctions are reverse biased, the SCR offers high reverse resistance and blocks the flow of current. Only a small reverse leakage current flows through the device.

If the reverse voltage exceeds the rated reverse breakdown voltage, avalanche breakdown may occur, leading to permanent damage. Hence, the SCR is normally operated below this limit in reverse blocking mode.

Forward Blocking Mode

In this mode, the anode is made positive with respect to the cathode, but no gate signal is applied.

• Junctions J1 and J3 are forward biased
• Junction J2 is reverse biased

Due to the reverse-biased central junction J2, the SCR remains in the OFF state and blocks the forward current. Only a small forward leakage current flows. In this condition, the SCR behaves like a forward-biased diode with high resistance.

If the anode-to-cathode voltage is increased beyond a certain value called the forward breakover voltage (V_BO), junction J2 breaks down and the SCR turns ON even without gate triggering. However, this method is not preferred due to the possibility of excessive current and device damage.

Forward Conduction Mode

When the SCR is forward biased and a positive gate current is applied with respect to the cathode, the device enters the forward conduction mode.

• Carriers are injected into the inner P-layer through the gate
• The reverse bias at junction J2 is reduced
• Regenerative feedback between internal transistor pairs is initiated

As a result, all three junctions become forward biased, and the SCR switches to the ON state. The device now offers low forward resistance and conducts heavily from anode to cathode.

Once the SCR is turned ON, it remains ON even if the gate signal is removed. This phenomenon is known as latching. The SCR will turn OFF only when the anode current falls below a minimum value called the holding current.

V-I Characteristics of Silicon Controlled Rectifier (SCR):

The V–I characteristics of a Silicon Controlled Rectifier describe the relationship between the anode current (Iₐ) and the anode-to-cathode voltage (Vₐ) under different operating conditions. The characteristic curve is divided into three main regions:

• Reverse Blocking Region
• Forward Blocking Region
• Forward Conduction Region

These regions explain the blocking, triggering, and conduction behavior of the SCR.

V–I Characteristic Regions of SCR

Region	Biasing Condition	Junction Status	SCR State	Key Characteristics
Reverse Blocking Region	Cathode positive with respect to anode	J1, J3 reverse biased J2 forward biased	OFF	Only reverse leakage current flows; breakdown at VBR
Forward Blocking Region	Anode positive, gate open	J1, J3 forward biased J2 reverse biased	OFF	Forward leakage current flows; breakdown at VBO
Forward Conduction Region	Anode positive and gate triggered	All junctions forward biased	ON	Low forward voltage drop (≈ 1–2 V); heavy anode current flows

Important Parameters in V–I Characteristics

Forward Breakover Voltage

It is the minimum anode-to-cathode forward voltage at which the SCR turns ON without gate triggering. Gate current reduces the value of V_BO.

Reverse Breakdown Voltage

It is the maximum reverse voltage that the SCR can withstand without entering breakdown. Beyond this voltage, the reverse current increases sharply.

Latching Current

It is the minimum anode current required to keep the SCR in the ON state immediately after triggering when the gate signal is removed. If the anode current is less than I_L, the SCR turns OFF.

Holding Current

It is the minimum anode current below which the SCR turns OFF during conduction. For proper operation, the anode current must always remain greater than the holding current.

Note:$$I_L > I_H$$

Turn on methods of SCR

The turning on Process of the SCR is known as Triggering. In other words, turning the SCR from Forward-Blocking state to Forward-Conduction state is known as Triggering.

The various methods of SCR triggering are discussed here.

• Forward Voltage Triggering
• Thermal or Temperature Triggering
• Radiation or Light triggering
• dv/dt Triggering
• Gate Triggering

Two Transistor Analogy of SCR

The PNPN structure of the SCR can be represented by two transistors:

• PNP transistor (T₁)
• NPN transistor (T₂)

The collector of each transistor is connected to the base of the other, forming a positive feedback loop. When a gate current is applied, conduction in one transistor increases the base current of the other, resulting in regenerative action. The SCR turns ON when:$$\alpha_1 + \alpha_2 \geq 1$$

This model explains the latching and switching behavior of the SCR.

Let,

• $I_A$​ = Anode current
• $I_C1$ = Collector current of PNP transistor (T₁)
• $I_C2$ = Collector current of NPN transistor (T₂)
• $I_G$​ = Gate current
• $\alpha_1$​ = Common-base current gain of T₁
• $\alpha_2$ = Common-base current gain of T₂

From transistor theory:$$I_{C1} = \alpha_1 I_A + I_{CBO1}$$ $$I_{C2} = \alpha_2 I_K + I_{CBO2}$$

where
$I_{CBO1}, I_{CBO2}$ = leakage currents of transistors

Since cathode current $I_K = I_A + I_G$

The anode current is the sum of collector currents:$$I_A = I_{C1} + I_{C2}$$

Substituting:$$I_A = (\alpha_1 I_A + I_{CBO1}) + (\alpha_2 (I_A + I_G) + I_{CBO2})$$

Expanding:$$I_A = \alpha_1 I_A + \alpha_2 I_A + \alpha_2 I_G + I_{CBO1} + I_{CBO2}$$$$I_A = (\alpha_1 + \alpha_2) I_A + \alpha_2 I_G + I_{CBO}$$

where$$I_{CBO} = I_{CBO1} + I_{CBO2}$$

Rearranging the Equation

$$I_A – (\alpha_1 + \alpha_2) I_A = \alpha_2 I_G + I_{CBO}$$ $$I_A (1 – \alpha_1 – \alpha_2) = \alpha_2 I_G + I_{CBO}$$

Final Expression for Anode Current

$$\boxed{ I_A = \frac{\alpha_2 I_G + I_{CBO}}{1 – (\alpha_1 + \alpha_2)} }$$

This is the fundamental SCR anode current equation derived from the two-transistor model.

Advantages and Limitations of Silicon Controlled Rectifier (SCR)

Advantages of SCR	Limitations of SCR
Capable of handling very high voltage and current	Unidirectional conduction only
Low forward voltage drop resulting in low conduction loss	Cannot be turned OFF by gate control
High efficiency in power conversion applications	Requires external commutation circuit for turn OFF in DC circuits
Simple and low-power gate triggering requirement	Slow switching speed compared to MOSFETs and IGBTs
Latching property simplifies control in many applications	Not suitable for high-frequency applications
High surge current capability and rugged construction	Sensitive to dv/dt and di/dt effects
High reliability and long operational life	Requires snubber and protection circuits
Economical for medium and high power ratings	Produces harmonics in AC power control circuits
Suitable for both AC and DC power control	Poor performance at low current levels
Widely available in large current and voltage ratings	Gate loses control once the device is latched ON

Applications of Silicon Controlled Rectifier (SCR)

• Controlled rectifiers
• DC motor speed control
• AC voltage regulators
• Light dimmers
• Battery chargers
• Inverters
• Cycloconverters
• HVDC transmission
• Soft starters
• Industrial heating
• Welding machines

Conclusion

The Silicon Controlled Rectifier is a fundamental power semiconductor device widely used in controlled power conversion and industrial applications. Its ability to handle high voltage and current with simple gate control makes it highly suitable for power regulation, motor control, and energy conversion systems. Despite the availability of faster devices, SCR continues to dominate medium and high-power applications due to its reliability, efficiency, and rugged construction.

Short FAQ – Silicon Controlled Rectifier (SCR)