How does a zener diode work below the breakdown voltage without getting damaged??

[SanjKrish]

I noticed the graph of zener diode and the normal diode is almost the same..
Now, while a normal diode when operated below the breakdown voltage gets permanently damaged...
how does the zener diode survive even after being operated below the breakdown voltage??

All I know is tat zener is heavily doped.. and when a strong reverse bias is applied.. causes mobile holes in the P region and the mobile electrons in the N region to sucked by the battery terminals leaving behind a depletion region made of immobile negative and positive ions in the P and N regions respectively..

Having said till this, how does it make a difference in working of both the diodes below the breakdown voltage..



Also I'm confused with the difference between zener breakdown and avalanche breakdown..
I read zener breakdown is cause by rupture of covalent bond due to heavy revese bias so the electrons make themselves free of the covalent bond and start conducting..
and avalanche breakdown is caused by the minority careers hitting the covalent bonds at high speed which creates the avalanche(like a glacier falling)..

both seems to be one at the same.. caused by rupture of covanlent bonds and subsequently conduction..

 

[steve.hunch]

About first question. Non of the diodes will be damaged if you will not go over their maximum power specification. And even for Zener diode, if you go beyond breakdown voltage and go over Pmax, you will permanently damage it.

 

[Schwarz-Brot]

Inside a diode with applied voltage exists a strong electrical field wich accelerates moving electrons. These fast electrons can kick out several other electrons out of their bonding. These are accelerated, too and lead to the avalanche effect if the voltage is high enough to start this effect.

Zener Breakdown is a completely different mechanism based on the electronic band structure. Forced by a special structure of high doped semiconductors and a very thin depletion layer (hope this is the word for "Sperrschicht") the valence band is pushed to the level of the conduction band. This allows electrons in the area of the depletion layer tunneling to the other side. The higher the applied voltage the higher the possibillity they do so.

Both effects lead to current flow, which one is actually happening is not that important.
Basically the zener effect works until voltages of about 5.6V while above this voltage avalanche effect dominates. It may be important to recognize those effects are inverse temperature dependent.

Hope this helped you.
Sorry for my english - technical subjects are hard to translate for me.

Jan

PS
The zener diode is more likely not destroyed as this effect is forced and wanted while with normal diodes an operation in "wrong" direction is not what you want. You can do so but the area of the reverse breakdown is not well defined leading to destruction in most cases.

 

[klystron]

The power dissipation in the reverse bias zener diode is limited and controlled by the series resistor

 

[SanjKrish]

Thanks Steve & klystron ...

Thanks Schwarz, for ur detailed explanation
Now I understand that the heavy doping causes a strong reverse electric potential.. which forces
the immobile n type ions in the P region move from their valence band to the conduction band of the N region.. thus enabling current flow..
and this process is called tunneling...

 

[Syncopator]

A voltage below (a value less than) the breakdown voltage of a zener does nothing.

A zener doesn't work "below" the breakdown voltage, it works at the breakdown voltage.

A series resistor limits the current through it to a safe value.

 

[Schwarz-Brot]

Depending on what you are going to do you should take a look at the current below breakdown voltage. There is always a small current which in some cases can lead to problems! In most cases this doesn't matter anyways, so you are not wrong.