V- I Characteristics

The operation of the  diac can be explained by imagining it as two diodes connected  in series. When  applied voltage in either polarity is small ( less than break over voltage) a very  small amount  of current called the  leakage current flows  through the diac. Leakage  current caused due to the drift of  electrons and holes in the depletion region is not  sufficient to cause conduction in the device. The device remains in non conducting mode. How ever when T₁ is positive with  respect to T1 the layers P-N-P-N starts conducting only  when applied  voltage of T1 exceeds break over voltage  V _boi . once the conduction starts the current through the diac becomes very  large  and has to be limited by the external resistance in the circuit.

When T₂ positive  with respect to T_p  the layers P-N-p-N  conduct. This happens when the applied voltage of T₂ exceeds break over voltage  V _Bo2. In both  the cases the current during  blocking regions are small  leakage current . the  behaviour  in both  the directions are  is similar  because doping level   is same in all the  layers in tow directions. The break  over  voltage  for commonly  used  disc is about 30v.

When T₁ is positive  and voltage  is less than V_bo1 only a small  leakage current  flows  through  the device. When voltage  exceeds  V_boi it starts  conducting and  current  becomes  large. As the current increase the voltage  drop  across diac decrease.  Thus  it exhibit negative resistance characteristics. The  characteristics of diac  in reverse direction when  T₂  is positive  lies in  the third quadrant and is  exactly similar to that in the first  quadrant. The break over voltage  V_BOi and V Bo2 are exactly equal in magnitude. In both  the cases the device exhibits negative  resistance behaviours  during  conduction  region. Diac is mainly used for triggering triacs.