Question

What causes the setting up of high electric field even for small reverse bias voltage across the diode?

Answer 1

Because of heavy doping, a very thin (<10⁻⁶ m) depletion region is formed between the p and n sides, and hence, the electric field of the junction is extremely high (~5 × 10⁶ V/m) even for a small reverse bias voltage of about 5V.