Semiconductors have practical reverse-voltage limitations. The reverse breakdown voltage (BVR) of silicon is determined primarily by three factors: the resistivity of the silicon (zener), the depletion region width (punch-through or reach-through) and the surface stress at the junction/passivation (avalanche) interface.

Reverse breakdown occurs when the voltage stress across one of these three areas exceeds the withstanding threshold and triggers a high reverse current across the entire junction area, as shown in Figure 3 below. This condition is non-destructive in properly designed devices, provided that the reverse current is limited to a level that minimizes thermal dissipation. This phenomenon should be seen as "water flowing over a spillway" and not as a failure of dielectric material, which is a catastrophic condition.

Zener diodes routinely operate in this mode and are designed for breakdown to be initiated by resistivity limitations rather than punch-through or avalanche. It should be noted, however, that high voltage zener diodes are less practical because the higher-resistivity silicon and deeper-diffusion depths required to achieve the higher voltage ratings make it difficult to predict the voltage at which breakdown occurs.

In the past, the term controlled avalanche diodes referred to those diodes that had "sharp" breakdown characteristics and which would survive over-voltage with controlled reverse current. The term non-controlled avalanche diodes typically referred to those diodes that had very high reverse currents at considerably lower voltages than the breakdown voltages. This high reverse current leads to overheating and very round breakdown curves.

Rectifiers are generally subjected to a peak inverse voltage (PIV) test to identify their breakdown characteristics. This test is performed by applying 60-hertz half-wave reverse voltage of sufficient amplitude to initiate breakdown. During the test, the reverse current is usually limited to 50ľA. The resulting waveform is observed on an oscilloscope to determine the sharpness of the "knee" at the point of breakdown. Both planar and deep-diffusion processes yield controlled avalanches under PIV test conditions.