Zener diode

Zener diode:

• A Zener diode is a particular type of diode that, unlike a normal one, allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the so-called "Zener voltage" is reached.
•  Zener diodes have a highly doped P-N junction.                                                                               
  

PN junction diode

PN junction diode:

• PN junction diode is the most fundamental & simplest electronic device.
• When one side of the intrinsic semiconductor is doped with acceptor i.e. one side is made p-type by doping with n-type material, a pn junction is formed.
• It is a two terminal device.