Zener breakdown occurs\

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Professor PC Thomas Classes · Answer

When the forward bias increases beyond a certain value the diode burns out. When the reverse bias increases beyond a certain value junction breaks down. ie the covalent bonds break and electrons are made available in the conduction band There are two kinds of junction break down\ 1)Avlanche breakdown 2) zener breakdown Avlanche breakdown takes place in lightly dropped diodes. It is due to collision of an electron with covalent electron. If the colliding electron has sufficient energy, it will knock out the covalent electron and it will be available in the conduction band. This builds up slowly and a large number of electrons are made available which cause a large current.\ Zener breakdown takes place in heavily dopped diodes. When the width of the junction is very small, the electric field $E=\frac{V}{d}$ will be very large since $d$ is very small. This large electric field exerts large force on the covalent electrons and the rupture of covalent bond takes place at many places simultaneously. This makes a large number of electrons available for conduction and the current shoots up.\