What is Silicon Controlled Rectifier SCR

biasing of scr

Silicon Controlled Rectifier (SCR) is one of the oldest four-layer solid-state power device. SCR has the highest power handling capacity in comparison with other semiconductor devices. It is a member of SCR family, it was invented in 1957.

  • It is a four-layer device with three terminals that are accessible to the user namely anode, cathode, and gate.
  • SCR is a latching device, we can turn on SCR using the gate terminal but we can’t turn off SCR using the same gate terminal.
  • In the case of the Shockley diode, the user can’t control the break overvoltage. In the case of silicon controlled rectifier(SCR, it is possible to control the breakover voltage.

we can use SCR as a controlled device the construction of SCR is as shown in the figure.

Symbol of Silicon Controlled Rectifier SCR:

Symbol of SCR


The internal structure of SCR:

Silicon Controlled Rectifier SCR

Construction of Silicon Controlled Rectifier:

SCR is a four-layer PNPN device, with three terminals available to the user namely anode, cathode, and gate. The gate terminal is a controlling terminal that can turn on the device whenever required.

The symbol of SCR is shown in the figure. It can be seen that SCR is a thyristor with a third terminal known as gate. The direction of anode current, the voltage across the thyristor, and the direction of the conventional gate current are shown in the figure.

There are three junctions in SCR namely J1, J2, and J3 to turn on SCR the anode must be at the highest positive potential then cathode. This means SCR should be in forward bias mode.

The direction of anode and gate current is conventional current directions. SCR is a unidirectional device and the gate current can only be positive the gate current can flow in one direction, into the gate terminal.

Operation of Silicon Controlled Rectifier SCR:

The operation of SCR can be studied by two conditions first with gate current and second without the gate current.

silicon controlled rectifier biasing

Operation of SCR without gate current:

  • The SCR is biased in a forward direction by applying a positive voltage to the anode with respect to the cathode, the gate terminal is left open so that If = 0.
  • Out of three junctions, the junction J1 and J3 are forward biased and junction J2 is reverse biased. No current flow through the SCR and the entire applied voltage appears across the junction J2.
  • As the anode to cathode voltage is increased the voltage across junction J2 increases.
  • After a certain voltage the junctions J2 will break down and SCR will start conducting this voltage is called a forward breakover voltage Vbo.
  • Thus it is possible to turn on an SCR without any gate current by exceeding its forward voltage beyond Vbo.

Operation with gate current:

biasing of scr

  • The SCR is forward biased and the gate-cathode junction J2 is also forward biased using an external power source.
  • The gate current Ig starts flowing due to forward bias this current can be adjusted by resistance Rg.
  • The value of the gate current will decide the breakover voltage of SCR. As Ig increases, breakover voltage decreases that will turn on SCR at a lower and lower voltage.


Operation in reverse bias mode:

  • If the anode to cathode voltage is negative the SCR is reverse biased. Junction J1 and J3 are reverse biased and a very small leakage current flows through the device due to thermally generated minority carrier electrons and holes.
  • If reverse voltage increases above the reverse breakdown voltage the SCR will turn on due to the avalanche breakdown effect.
  • A large reverse current flows through the device and a large reverse voltage appears across it. The SCR can get damaged due to excessive power dissipation. Hence reverse breakdown should be avoided.

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