Thyristor is, a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N.
The conventional thyristor is also known by the name SCR (Silicon Controlled Rectifier).
 They are operated as bistable switches, operating from non conducting state to conducting state.
Thyristors can be assumed as ideal switches for many application, but the practical thyristors exhibit certain characteristics and limitations. 
 

Material File