Explain about the Intrinsic breakdown:

It's because of the excitation of electrons into the conduction band across  the  energy  gap  under  the  condition  of  excessive  voltage.  Excited electrons  can  excite  more  electrons  in  turns,  resulting  in  an  avalanche  of conducting electrons. Impurities in the dielectric can create additional energy levels which lie in the energy gap and can help in excitation of electrons into the conduction band.