Reverse breakdown effect

When a p-n junction diode is reverse biased, only a tiny reverse saturation current (due to minority carriers) flows. If the reverse voltage is increased beyond a certain value, the current suddenly rises sharply. This effect is called reverse breakdown, and the voltage at which it occurs is the breakdown voltage. If the current is not limited by a resistor, the heat can destroy the diode; if it is limited, the junction is not harmed.

There are two mechanisms:

1. Avalanche breakdown (usually above about 5-6 V, in lightly doped junctions). The wide depletion region lets minority carriers gain high kinetic energy in the strong field. They collide with atoms and release new electron-hole pairs, which are accelerated and collide again, so the current multiplies like an avalanche.
2. Zener breakdown (below about 5 V, in heavily doped junctions). The depletion layer is very thin, so even a small reverse voltage creates a very strong field ($>10^7$ V/m) that pulls electrons directly out of the covalent bonds (field emission), giving a large current.

                     I (forward)
                     |        /
                     |       /  forward
                     |      /   conduction
  -V ------+---------+-----'------------> +V
         -Vbr        |
           |  tiny reverse saturation current
           |
           |  breakdown: current rises sharply
           v  while the voltage stays near Vbr

The nearly constant voltage during breakdown is used in Zener diodes for voltage regulation.