compare PN junction diode and zener diode
Answers
Zener diode
Special purpose diode
Diode especially designed and doped to work in reverse bias(i.e. with P being-ve & N being +ve)
works like a normal diode in forward bias(i.e. with N being-ve & P being +ve)
But, it conducts in reverse bias only when sufficient reverse biasing voltage is reached called breakdown voltage, Vz.
So it has 3 regions of operation:
When applied voltage,V > cutin voltage (0.7V for Si & 0.3V for Ge), it conducts just like a normal Si or Ge diode. Here, diode is replaced by a battery with cutin voltage.
When applied voltage V is between 0V to -Vz, it doesn't conduct, becomes open circuit
When applied voltage V goes beyond Vz(becomes more -ve), it again conducts. Here, P terminal is -ve whereas N terminal is +ve and diode is replaced by a battery with breakdown voltage(Vz).
Usage: voltage regulation, waveform generation, etc.
When output voltage is greater than Vz, the zener diode that is connected in shunt with output circuit goes ON, keeps output voltage constant at Vz volts. This principle makes it a popular device for volatge regulation