What is Silicon Controlled Rectifier (SCR)? Working, Circuit Diagram, Construction, Symbol, Applications & VI Characteristics

A silicon-controlled rectifier is a four-layer PNPN device with a gate control terminal. Basically, it is a semiconductor device made of silicon material.

Construction & Working of Silicon Controlled Rectifier (SCR)

The basic structure of SCR is illustrated in figure 2.

Figure 2: Silicon Controlled Rectifier (SCR) Structure.

It consists of four layers (i.e., p, n^–, p and n⁺), three terminals A(Anode), C(Cathode) and G(Gate) and three junctions J₁ J₂ and J₃.

The anode and cathode terminals are connected to the main power circuit whereas, the gate terminal is connected to the control circuit. When never the p layer near the anode is made more positive, when compared to n layer near the cathode, junctions J₁ and J₂ are forward biased and the middle junction J₂ is reverse biased. and it acts as a capacitor in this mode. Due to the depletion layer formed at this junction, no current flows through the device, but due to the drift carriers, a small amount of leakage current flows through the circuit.

As the leakage current is negligibly small, the device operates in OFF state and blocks forward voltage. This state is known as forward blocking state. Whenever the n layer near the cathode is more positive, when compared to p layer near the anode, junctions J₁ and J₃ are reverse biased and middle junction J₂ is forward biased. As the junctions J₁ and J₂ are in reverse biased condition, they does not allow the current to flow through the device. This state is known as reverse blocking state or OFF state. The width of the depletion layer at the junction J₂ reduces with increase in voltage across cathode and anode. At certain voltage, J₂ disappears this is due to the breakdown, of the junctions by large voltage gradient also called as avalanche breakdown. As junctions J₁ and J₂ are already in the forward-biased condition, the carriers moves from anode to cathode. In this case, the device is said to be in the conducting state or ON state. From the above analysis, it is clear that the SCR conducts only when all three junctions i.e., J₁ J_2, and J₃ are in forward biased condition.

V-I Characteristics of Silicon Controlled Rectifier (SCR)

Figure 3: V-I Characteristics of Silicon Controlled Rectifier (SCR),

The voltage-current characteristics of SCR are plotted as shown in figure (3). From figure (3), it can be observed 1 that the characteristics of SCR are same as normal semiconductor diode characteristics, but in this the horizontal offshoot exist when the device is in OFF state. At I_G = 0, i.e., if the voltage across gate terminal is zero, the high value of break over voltage, V_BO can be observed. It indicates the device does not conduct at this point. If the voltage, V_G increases at input the current, I_G increases, in this case the device staffs conducting for a voltage less than V_BO If the current, I_G reaches certain value, the characteristics of SCR appears same as that of semiconductor diode. Further, increase in gate current, I_G causes the damage to device. In forward and reverse blocking regions, the device operates in OFF state and blocks the flow of current. The avalanche breakdown occurs, when the voltage in reverse bias condition reaches breakdown voltage.

Applications of Silicon Controlled Rectifier (SCR)

1. SCR can be operated as a static switch.
2. It is used as zero point triggering circuit.
3. These are commonly used in phase control and temperature control systems.
4. It is used in charging devices.
5. In emergency lighting systems SCRs are used.