A diac (diode for alternating current) is a two-terminal thyristor that is equivalent to back-to-back inverse four-layer diodes that can conduct current in either direction when activated. The right side of the stack can be regarded as a pnpn structure with the same characteristics as a four-layer diode, while the left side is an inverted four-layer diode having an npnp structure. The two terminals are both anodes (\(A_1\) and \(A_2\)).
Conduction occurs in a diac when the breakover voltage \(V_\textrm{BR}\) is reached with either polarity across the two terminals. The device turns off when the current drops below the holding value \(I_H\).