what is the meaning of punch through in reversied biased diode?

Answer 1

See:http://www.semikron.com/internet/webcms/objects/applica_help/e/1_3_1_4-1_3_3_3.pdf
High-voltage diodes use a lightly doped layer on one side of the junction. To minimize series resistance, the lightly doped region is backed by a heavily doped region of the same type. Punch through is when the space-charge layer thickness exceeds the lightly doped region and impinges on the heavily doped region, causing breakdown.

This is slightly different than normal avalanche breakdown, where the field is simply so large that impact ionization results in a cascade multiplication of carriers.

Answer 2

A reverse biased diode will not conduct electricity if the applied voltage is moderate. If the voltage is increased, eventually the junction will break down and current will flow. This is punch-through.