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Well I have seen diodes with many breakdown voltages. How is this possible ? I mean
when an electron fills a boron hole it a bond is created between the boron atom and the nearby silicon atom.Well in order for the chemical bond to be broken you need energy . This is energy is from the electric field and temperature . But many diodes have many different breakdown voltages. How is this possible? Are they made of different material?

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No they are not made of different materials, it is the difference of doping which causes this effect. Maybe it is hard to believe but yes it is doping.

Well, it is a quantum-science concept. But on increasing doping (and not changing the material, as this is done in Zener diode, which nothing but a simple diode with more doping), the barrier electric field changes like this:

more ions are exposed near the junction, so more electric field is present (simple right!), i.e. electric field is proportional to ion density

the effect of strong field:

For ordinary diode: increasing reverse bias voltage above breakdown voltage, increases energy of electrons so they collide more frequently and the new electrons formed also collide and so on, collisions increase exponentially.. (BONUS: collisions creates heat)

For diode with high doping: the electric field is strong enough that only small increase can force electrons to jump out of the bonds.. (BONUS: no collisions, no heating, therefore zener diode can work good at breakdown voltages) (although it involves a quantum concept called tunneling)

see this video: link and the near by videos for hint

i hope it helped :)

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