What is a Silicon Controlled Rectifier (SCR)? Understanding its Working Principle, Characteristics, and Applications

Thyristor(SCR) is a general name given to a family of power semiconductor switching devices, all of which are characterized by a bistable switching action depending upon the PNPN regenerative feedback. The thyristor has four or more layers and three or more junctions. The SCR is the most widely used and important member of the thyristor family. This device has revolutionized the art of solid-state power control. The SCR is almost universally referred to as the thyristor.

From the construction point of view, the thyristor (PNPN structure) can be best visualized as consisting of two transistors (a PNP and an NPN interconnected-to-form a regenerative feedback pair). The name thyristor is derived by a combination of the capital letters from thyratron and transistor.Thus, a thyristor is a solid-state device like a transistor and has characteristics similar to that of a thyratron tube.

The structure and symbol of the thyristor (SCR) are shown in Fig. 1. It is a four layered PNPN switching device, having three junctions J1, J2 and J3. It has three external terminals, namely, the anode (A), cathode (K) and gate (G). The anode and cathode are connected to the main power circuit. The gate terminal
carries a low-level gate current in the direction gate to cathode. Normally, the gate terminal is provided at the P layer near the cathode. This is known as cathode gate.

When the end P layer is made positive with respect to the end N layer , the two outer junctions, J1 and J3 are forward biased but the middle junction J2 becomes reverse biased. Thus, the junction J2 because of the presence of depletion layer, does not allow any current to flow through the device. Only leakage current,negligibly small in magnitude, flows through the device due to the drift of the mobile charges.

This current is insufficient to make the device conduct. The depletion layer, mostly of immobile charges do not constitute any flow of current. In other words, the SCR under the forward biased condition does not conduct.This is called as the forward blocking state or off-state of the device.

When the end n layer is made positive with respect to end p layer, the middle junction J2 becomes forward biased, whereas the two outer junctions, J1 and J3 become reverse biased. The junctions J1 and J3 do not allow any current to flow through the device.

Only a very small amount of leakage current may flow because of the drift of the charges. The leakage current is again insufficient to make the device conduct. This is known as the reverse blocking state or off-state of the device.

The width of the depletion layer at the junction J2 decreases with the increase in anode to cathode voltage (since the width is inversely proportional to voltage).If the voltage between the anode and cathode is kept on increasing, a stage comes (corresponding to forward break-over voltage) when the depletion layer at J2 vanishes. The reverse biased junction J2 will breakdown due to the large voltage gradient across its depletion layer.

This phenomenon is known as the Avalanche breakdown. Since the other junctions, J1 and J3 are already forward biased, there will be a free carrier movement across all the three junctions resulting in a large amount of current flowing through the device from anode to cathode.Due to the flow of this forward current, the device starts conducting and it is then said to be in the conducting state or on state.

An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 2. Here, the anode and cathode are connected to the main source through a load. The gate and cathode are fed from another source `E_g`.The static V-I characteristic of an SCR is shown in Fig. 3. Here, `V_a` is the anode-cathode voltage and `I_a` is the anode current. The thyristor V-I characteristics is divided into three regions of operation. These three regions of operation are described below.

When the cathode is made positive with respect to anode with the switch s open (Fig. 2), the thyristor becomes reverse biased.In Fig. 3, OP is the reverse blocking region. In this region, the thyristor exhibits a blocking characteristic similar to that of a diode. In this reverse biased condition,the outer junction `J_1` and `J_3` are reverse biased and the middle junction `J_2` is forward biased.

Therefore, only a small leakage current (in mA) flows. If the reverse voltage is increased, then at a critical breakdown level called reverse breakdown voltage VBR, an avalanche will occur at `J_1` and `J_3` increasing the current sharply. If this current is not limited to a safe value, power dissipation will increase to a dangerous level that may destroy the device. Region PQ is the reverse avalanche region. If the reverse voltage applied across the device is below this critical value, the device will behave as a high-impedance device (i.e., essentially open) in the reverse direction.

The inner two regions of the SCR are lightly doped compared to the outer layers. Hence, the thickness of the `J_2` depletion layer during the forward biased conditions will be greater than the total thickness of the two depletion layers at `J_1` and `J_3` when the device is reverse biased. Therefore, the forward breakover voltage `V_{BO}` is generally higher than the reverse breakover voltage `V_{BR}`.

In this region, the anode is made positive with respect to the cathode and therefore, junctions `J_1` and `J_3` are forward biased while the junction `J_2` remains reverse biased. Hence, the anode current is a small forward leakage current. The region OM of the V-I characteristic is known as the forward blocking region when the device does not conduct.

When the anode to cathode forward voltage is increased with the gate circuit kept open, avalanche breakdown occurs at the junction `J_2` at a critical forward break-over voltage (VBO), and the SCR switches into a low impedance condition (high conduction mode). In Fig. 3, the forward breakover voltage is corresponding to the point M, when the device latches on to the conducting state.

The region MN of the characteristic shows that as soon as the device latches on to its ON state, the voltage across the device drops from say, several hundred Volts to 1-2 Volt, depending on the rating of the SCR, and suddenly a very large amount of current starts flowing through the device. The part NK of the characteristic is called as the forward conduction state. In this high conduction mode, the anode current is determined essentially by the external load impedance. Therefore, when the thyristor conducts forward current, it can be regarded as a closed switch.

When a gate-signal is applied, the thyristor turns-on before `V_{BO}` is reached.The forward voltage at which the device switches to ON state depends upon the magnitude of gate current; higher the gate current, lower is the forward breakover voltage. Figure 3 shows that for gate current `I_G` = 0, the forward breakover voltage is `V_{BO}`. For `I_{G1}`, the forward breakover voltage is less than `V_{BO}` and for `I_{G2}` > `I_{G1}`, it is still further reduced. In practice, the magnitude of gate-current is more than the minimum gate current required to turn-on the SCR. The typical gate current magnitudes are of the order of 20 to 200 mA.

Once the SCR is conducting a forward current that is greater than the minimum value, called the latching current, the gate signal is no longer required to maintain the device in its ON state. Removal of the gate current does not affect the conduction of the anode current. The SCR will return to its original forward blocking state if the anode current falls below a low level, called the holding current (`I_h`).

For most industrial applications, this holding current (typically 10mA) can be regarded as being essentially zero. Note that latching current is associated with turn-on process and holding current with turn-off process. The holding current is usually lower than, but very close to the latching current.Hence, from the above discussion it becomes clear that the more convenient,reliable and efficient method of turning on the device employs the gate drive.

Conclusion