Zener breakdown:

Zener Breakdown takes place in very thin junction i.e. when both sides of junction layer is narrow. When a small reverse bias voltage is applied a Very strong electric field is set up across the thin depletion layer. This field is enough to break or capture the covalent bonds. Now extremely large number of electrons and holes are produced which constituted the reverse saturation current. Zener current is independent of the applied voltage and depends only on the external resistance. The temperature coefficient is negative. At reverse voltages less than 6V, zener breakdown predominates.