Avalanche Breakdown is a type of breakdown usually seen in PN junctions which are connected in reverse bias. When the reverse bias voltage exceeds the peak inverse voltage of the PN junction, the junction will break.
When a PN junction is connected in reverse bias i.e. the P material to the cathode and the N material to the anode, the size of the depletion region becomes maximum and the current becomes very low. The electron-hole pairs which cross over to the depletion layer are drawn to opposite sides. The acceleration of these charge carriers depends on the electric potential.
At high potentials, the electron can collide against atoms and consequently cause other electrons to get knocked out. These electrons in turn will collide with other atoms and knock out more electrons. The process continues until more and more electrons are knocked out. This ultimately leads to the failure of the PN junction.
This is a common mechanism of failure of diodes connected in reverse bias. Hence, the Peak inverse voltage of diodes should be checked against the maximum voltage they would have to withstand.