SCRs are also called thyristors, which first became available in the early 1960s. According to the website Radio-Electronics.com, the theory of their operation was first described in the 1950s, when they were called "bipolar transistors with a p-n hook-collector."
Unlike a mechanical switch, a thyristor has no moving parts. Instead, it has three terminals consisting of a gate, an anode and a cathode. When a certain threshold of current flows into the gate, it initiates the thyristor. Electricity then flows between the anode and cathode as long as the voltage is maintained. If not, the thyristor switches off.
Switching a thyristor on and off only requires millionths of a second, according to the Electrical Engineering Training Series. The thyristor is commonly used in devices such as computer logic circuits, remote switching units and circuit overload protectors.