zener diode and working principle

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What do you mean by zener diode, explain its working principle with neat and clean diagram? 

Zener diode is a special type of diode designed to reliably allow current to flow "backward" (that is Reverse Biased) when a certain set reverse voltage known as a zener voltage is reached. zener diode are manufactured with a great variety of Zener voltage and some even variable. 

Working principle:-
When a PN junction diode is highly doped ,  then concentration of impurity atoms will be high in the Crystal. The higher concentration of impurity atoms cause the highest concentration of ions in the depletion layer. Hence for some applied reverse Bias Voltage the width of depletion layer , becomes than that  in a normally doped diode due to thinner depletion layer, voltage gradient for electric field strength across the depletion layer is quite high. if the reverse voltage is continued to increase, after a certain applied voltage , the electron from the covalent bonds  within the depletion region conductive this breakdown is called Zener breakdown. 

             Circuit symbol of zener diode

            V-I characteristic of zener diode

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