The DIAC is a diode that conducts electrical current only after its breakover voltage, VBO, has been reached momentarily. The term is an acronym of "diode for alternating current".
When breakdown occurs, the diode enters a region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains in conduction until the current through it drops below a value characteristic for the device, called the holding current, IH. Below this value, the diode switches back to its high-resistance, non-conducting state. This behavior is bidirectional, meaning typically the same for both directions of current.
Most DIACs have a three-layer structure with breakover voltage of approximately 30 V. Their behavior is similar to that of a neon lamp, but it can be more precisely controlled and takes place at a lower voltage.
DIACs have no gate electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs. Some TRIACs, like Quadrac, contain a built-in DIAC in series with the TRIAC's gate terminal for this purpose.
DIACs are also called "symmetrical trigger diodes" due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or main terminal MT1 and MT2.