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