If the doping concentration is low, the breakdown in a reverse biassed PN junction diode will most likely occur due to the buildup of the greater charge in the biassed region and the large velocity of the minority charge. This is the key reason for the failure.
When the p-type is connected to the negative terminal of the battery and the n-type is connected to the positive side then the p-n junction is said to be reverse biased. In this case, the built-in electric field and the applied electric field are in the same direction. When the two fields are added, the resultant electric field is in the same direction as the built-in electric field creating a more resistive, thicker depletion region. The depletion region becomes more resistive and thicker if the applied voltage becomes larger.