A Semiconductor Controlled Rectifier or an SCR or a Silicon Controlled Rectifier is a 4 layered solid-state current controlling electrical equipment. This technique of 4 P-N-P-N switchings was established in 1956 by Tanenbaum, Holonyak, and Goldey. In January 1958, Dr. Ian M. Mackintosh of Bell Laboratories presented the practical explanation of SCR. The name Silicon Controlled Rectifier is the trade name for General Electric for a kind of thyristor. The Silicon Controlled Rectifier was made by a group of power engineers which was led by Gordon Hall, furthermore, marketed by Frank W. "Bill" Gutzwiller in 1957.

A few sources characterize thyristors and Silicon Controlled Rectifiers as almost the same thing but some say that they are an appropriate subset of the arrangement of thyristors and it comes with at least 4 layers of alternating P and N-type materials. Bill Gutzwiller says that the word “Controlled Rectifier” and “SCR” were used earlier and the word “thyristor” came into existence when the equipment started to get spread internationally.

SCRs are devices that can make the current flow in only one direction (i.e. unidirectional). It is completely different from TRIACs (Triode for Alternating Current) where current can flow in both the directions (i.e. bidirectional). SCRs can be activated regularly just by the flow of current into the gate rather than TRIACs, which can be activated ordinarily by either a negative or a positive current applied to its gate electrode.

Modes of Operation

Depending on the biasing given to an SCR, it can be classified into 3 modes.

Forward Blocking Mode

In this category, the cathode is provided with a negative potential and the anode gets a positive potential and the gate is kept disconnected i.e. at Zero potential.

As you can see here, the junction J2 is reverse biased and J1 and J3 are biased forward. Thus, permitting the flow of only small amount of leakage current from another to the cathode. An avalanche breakdown is initiated when the applied potential gets to the break-over value for J2. J2 starts to conduct at this break over but below that the current used to face high resistance from it and the SCR is known to be in off state.

Forward Conduction Mode

An SCR can be converted to Conduction mode from Blocking mode in 2 different ways.

• At the gate offer a positive pulse.
• Uplifting the potential difference between the cathode and the anode over the break-over voltage. As soon as the SCR starts conducting, to maintain it in the ON state no gate voltage is needed.

Reverse Blocking Mode

The SCR is said to be in reverse blocking mode when a positive potential is provided to the cathode and a negative potential at the anode. Thus, J2 is biased forward and J1 and J3 are reverse-biased. The equipment acts as 2 reverse-biased diodes associated in series. A small amount of leakage current flows through it.

So this was all we have about SCRs. Hope you enjoyed reading it. Brands that make SCRs are Littelfuse, STMicroelectronics, and NTE. Refer to them through the links as provided.

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