TRIAC:

The diode ac switch and the triode ac switch are extremely similar to the thyristors, as they both are latching multi-layer device structures. Both are meant to be utilized in ac-powered systems and, thus, respond similarly to positive and negative applied voltages. The circuit symbols & layer structures are illustrates for both devices in Figure The diode ac switch also referred to as DIAC contain a gate-less npnp structure connected in parallel to a gateless pnpn structure . Therefore this device acts like an open circuit until the threshold voltage is reached - either positive or negative after which the device behaves as a short. To achieve this function one starts with a pnp structure. An n region is added to the front and the back to yield the DIAC structure eliminate the n₃ layer and gate terminal connections for DIAC.) The triode ac switch (TRIAC) also contains the same vertical structure as a DIAC. Additionally a contact is made to the p-type gate of the npnp structure as well as the n-type gate of the pnpn structure. This additional gate contact permit lowering the threshold for latching for both positive & negative applied voltages applied among main terminal M₁ and main terminal M₂.

                   (a)                  (b)                   (c)                           (d)

Figure: (a) DIAC Structure; (b) DIAC Symbol; (c) TRIAC Structure; and (d) TRIAC Symbol