Electrical Revolution: Working and Characteristic of the Diac

Structure and Symbol

The DIAC is a bidirectional, two terminal semiconductor device which can be turned on by either ( Positive or Negative ) polarity of alternating supply voltage.
Figure A shows symbol and structure of the Diac.
The two terminals of the DIAC is labeled as MT1 and MT2.
The DIAC is obtained form Diode works on AC.
The DIAC is also called as triggering diode.

Static ( V – I ) Characteristic

The DIAC consists of two PNPN structures which are connected in anti parallel to each other.
The DIAC conducts only when the voltage applied to its terminal is equal to break over voltage.
When the terminal MT1 is made positive with respect to MT2 and applied voltage less than the break over voltage, only leakage current flows through the device.
This leakage current is due to the drift of electrons and holes at the depletion region which are not sufficient to drive the device into conduction state.
The region OA represents blocking or non conducting state as shown in the Figure B.
When the voltage across its terminal reaches to break over voltage, the device starts to conduct.
As the current through the device increases, the voltage drop across device decreases.
The device exhibits negative resistance characteristic and region AB is known as conducting state.
When the MT2 is made positive with respect to MT1, the device has similar characteristics as that of in the first quadrant. This is due to fact that the doping level is same at the two junctions of the device.
Once the device starts conducting, the current increases sharply which is limited by external characteristics.
The value of break over voltage for DIAC is usually 30 voltage.

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