Diac is a device which has di electrode and as its name suggests it works on AC.
The cross sectional view of a diac is shown if the four layers are PNPN and PNPN . the two terminal of diac are terminal T1 and T2 the symbol of diac is shown in figure diac is bidirectional device. It can be switched on either the positive cycle of AC voltage. It is also known as gateless triac.
Figure basic structure symbol