What is DIAC?

It is a bidirectional avalanche diode that can be turned ON by either positive or negative alternating of AC input voltage.

A DIAC (Diode for Alternating Current) is a member of the thyristor family and is essentially a two-terminal semiconductor device. It is designed to conduct electric current when the voltage across its terminals reaches a certain break over voltage, and it can be turned off when the current drops below a certain level.


It is a two-electrode device and can be conducted in either direction. So, it is an ac switch. It has four layers โ€“ PNPN and PNPNโ€™. Two terminals are MT1 and MT2. The doping level is the same in all the layers.

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Working Principle

If MT1 is positive with respect to MT2 layer then PNPN conducts. This happens when the voltage of MT1 exceeds the breakover voltage VBO1 and a large current flows through the DIAC.

When terminal MT2 is positive concerning MT1, then the PNPNโ€™ layer starts conducting. This conduction starts when the voltage at MT2 exceeds the breakover voltage VBO2. The current in this case flows in the opposite direction.

V-I Characteristics

The VI characteristics of a DIAC (Diode for Alternating Current) represent the relationship between the voltage (V) applied across its terminals and the current (I) flowing through the device. The VI characteristics of a DIAC are unique and distinct from regular diodes or other thyristors due to its negative resistance behavior.

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