PN junction of zener diode is heavily doped than ordinary diode. For the occurrence of Zener effect the zener diode is heavily doped. If the zener diode is not heavily doped then Zener effect will not occur. When the zener diode is heavily doped depletion layer become very narrow. If the diode is heavily doped The breakdown voltage is sharp. If the diode is lightly doped the diode will have higher breakdown voltage.