Construction of SCR

A silicon-controlled rectifier (SCR) is a three-terminal device mainly used at power electronics circuits.

It is a four-layered electronic component, which is made by sandwiching the four-layer of P and N-type silicon-based semiconductor material.

Symbol of SCR

Symbol of SCR
Symbol of SCR

The symbolic representation of SCR or silicon-controlled rectifier is similar to the rectifier diode, the only difference is the SCR symbol had a gate terminal in-between anode and cathode terminal.

Construction diagram of silicon controlled rectifier (SCR)

Construction diagram of silicon controlled rectifier (SCR)
Construction diagram of silicon controlled rectifier (SCR)

This is the constructional diagram of SCR, it is an actual combination of two PN junction diodes.

The two n-type and p-type silicon materials form a three junction-based semiconductor device.

Terminal details of SCR  

The silicon-controlled rectifier (SCR) has three terminals namely anode, cathode, and gate, each of these terminals has different functions to do in an SCR device.

Terminal anode (A) is known as the entry point of current flow towards the SCR device

Terminal cathode (K) is known as the exit point of current flow on the SCR.

And last, the intermediate gate (G) is the terminal that controls the flow at SCR, it is known as the trigger terminal of the SCR.

Polarity of SCR

The polarity of SCR is mainly based on the flow of current at SCR, so the current entry point anode is positive and the current leaving point is negative.

Three junctions of the silicon controlled rectifier (SCR)

We already mention that SCR has three junctions due to the combination of four semiconductor layers.

Junction J1 is formed between the first P-type and N-type layers, second junction J2 is formed between N-type and second p-type layer and last junction J3 is formed between P-type and N-type layer.

Each of these semiconductor layers has a different doping level.

Doping proportions of layers  

The doping percentage of each SCR layer is different, the outermost P and N-type layers are heavily doped, and where middle P and N-type layers are partially doped.

Three construction methods of silicon controlled rectifier (SCR) 

  • Planar type
  • Mesa type
  • Press pack type

The planer SCR construction method is used for low-power SCRs and the junctions in an SCR are diffused at this type.

The Mesa SCR construction method is used for the junction is formed by diffusion and also the outer layer is alloyed to it.

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