Avalanche Diode vs Zener Diode

Zener Diodes allows current flow in one direction and also in reverse direction if voltage is higher than breakdown voltage called breakdown voltage or zener knee voltage. Zener Breakdown happens in heavily doped pn-junctions

But its working is due to Zener Breakdown. In case of Avalanche diode there is Avalanche breakdown

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Avalanche diode is also made of silicon. Its working is similar to zener diode. Avalanche breakdown happens in lightly doped pn-junctions. Avalanche diodes are available with breakdown voltages of 4000v much higher than zener diodes which is limited to few tens of volts.

In Simpler terms, Avalanche breakdown and Zener breakdown refer to the breakdown of diode in reverse bias