What is SCR ?

SCR Stands for silicon controlled rectifier. Almost all type of semiconductor devices are made up of P-Type & N-type semiconductor materials, in this post we will see basic structure of “SCR” a thyristor.

The word “Thyristor” derived from the capital letter of two words THYRATON and TRANSISTOR.

Hence a thyristor has a combine feature of thyratron and transistor. the thyristor  basically consists of PNPN type semiconductor layers in which switching action depends upon PNPN regenerative feedback.

The SCR is the most widely used and important among all the semiconductor type devices.

The SCR (silicon controlled rectifier) is universally referred as a thyristor.


The SCR is a three junction four layer & three terminal type PNPN switching device it consist of three electrode – anode (A) cathode (K) & control electrode is called gate (G). it allows current to flow in only one direction so it is called as unidirectional device




The fabrication of the particular SCR depends upon service expected from that type. Basically the SCR consists of a multilayered “pallet” of alternate P & N type semiconductor material.

The semiconductor material is almost always silicon in which proper impurities are added. The junction are either Planer diffused or Alloy diffused.





When the andoe (A) is made positive wtr to cathode (K)  & gate is open, two junction J1 & J2  are forward biased but the middle junction J2 became riverside biased. In this condition, because of depletion layer at the junction J2, any current does not flow through the device. Only small amount of leakage current flow due to drift of the mobile chargers. As the SCR dose not conduct in forward biased condition, this is called as forward blocking or off state of the device.

When the cathode is made positive wtr to anode and gate is open two outer junction J1 & J3 became reverse biased wherever middle junction J2 became forward biased. Due to reverse biased of the junction J1 & J3 any current does not allow to flow through the device. only small amount of leakage current flow through the device which is not sufficient to turn on the device. This is known as reverse blocking state of the device.

In the forward blocking state, if anode to cathode voltage increases. the width of the depletion layer at the junction J2 decrease. if the anode to cathode voltage progressively increase, at certain level, depletion layer width became zero and middle junction J2 breakdown. It is known as avalanche breakdown because of J1 and J3 already forward biased, large amount of current flow through the device from cathode (K). this is known as conducting or ON state of the device

Another way of conduction SCR is by using gate control. in this way when gate is positive wtr cathode junction J2 is reversed biased while junction J3 id forward biased. Now the electron from N-type material moves towards junction J3 while holes from P-type material moves towards cathode hence, gate current starts to flow.

Due to gate current, anode increase, so more no. of electrons available at junction J2. In a very small time junction J2 breakdown and SCR conduct heavily resulting gate loses all the controls, in spite of removing gate signal, SCR became turn OFF.




1. Power Control.

2. Switching.

3. Zero Voltage Switching.

4. Over-Voltage Protection.

5. Pulse Circuits.

6. Battery charging Circuits.




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