Reason to explain why n−p region of zener diode is heavily doped
Calculation of current through zener diode n and p regions of zener diode are heavily doped so that depletion region formed is very thin and electric field at the junction is extremely high even for a small reverse bias voltage.
Current in the circuit is :
I=‌

V

R

=‌

5

250

=‌

1

50

=0.02A
Current through resistor of 1kΩ is:
I=‌

15

1000

=0.015A
As zener diode and 1kΩ resistor are in parallel, current through the zener diode is :
I=0.02−0.015=0.005A