Zener diode works like a normal diode except that normal diode only conducts in one direction while zener can conduct current in reverse bias condition after the reverse voltage reaches a threshold called zener voltage. It has high doping level that causes the phenomenon of avalanche breakdown. Zener in reverse bias do not conduct till applied voltage is less than zener voltage but start conducting (like if it is forward biased) after applied voltage exceeds zener voltage..that is why zener is useful in chopping voltage from a desired level….