The basic construction of silicon controlled rectifier (SCR) is shown in above fig.
It is made up of silicon and its operation as a rectifier can be controlled, hence the name given as silicon controlled rectifier. The silicon controlled rectifier (SCR) is made up four layers of p and n type of semi conducting material. As a result there are three junction j1, j2 and j3. It has three terminals anode (A), cathode (K), and gate (G). The anode terminal is connected on the outer p-type material layer and cathode terminal is connected on the outer N-type material layer. Similarly, the gate terminal of the silicon controlled rectifier is connected on the inner P-type material layer which is nearest to the cathode type material layer, hence conventional silicon controlled rectifier (SCR) is also called as cathode gate silicon controlled rectifier (SCR). The semiconductor material layers are formed by the diffusion of lightly doped semiconductor material into highly doped semiconductor material.