Silicon controlled rectifier (scr/thyristor)

Diac & triacdiacA diac is a two-terminal semiconductor device that can conduct current in either direction when activated

diacDiac functions basically like two parallel 4 layer diodes turned in opposite directions

SCR working ( Case 1: Gate open )

Diac characteristic curve

TriacA triac is like a diac with a gate terminal

triacTriac basically functions like two parallel SCRs turned in opposite directions with a common gate terminal

Triac characteristic curve

referenceElectronic Devices & Circuits by David A Bell