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How the breakdown voltage and avalanche effect formed?

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How the breakdown voltage and avalanche effect formed?

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  1. Three things:

    1. doping concentrations

    2. distance from anode to cathode

    3. junction area

    1. Higher doping concentrations will mean higher mobility for electrons and holes, which translates to lower zener voltages.  For zener diodes with breakdowns below about 6 volts, the mechanism becomes avalanche breakdown and in these diodes the dopant concentrations are "huge" (compared with normal diodes).

    2. Distance.  If the junction length is very small, then avalanche can happen much easier, as the electric field (Volts/meter) becomes very large for smaller voltages.  In other words if the meters goes down, the Volts per meter goes up.

    3. Area.  If the area of the junction is large, then lots of current can flow without overheating the junction, but 2nd-order effects start to affect the electric field, so the breakdown might not be uniform across the junction, thus affecting the exact zener.

    So, a real diode designer must actually take all 3 of these into account to get a zener diode that can handle the amount of power required, while at the same time maintaining its zener voltage within tolerances.

    Any reverse biased diode has a small reverse leakage current due to the minority carriers (holes in the n-type material and electrons in the p-type material) being swept across the depletion region.  As the reverse bias is increased the maximum electric filed increases and the minority carriers sweep across the junction with even greater force.  At some point (of increasing the reverse voltage) they have so much kinetic energy that they start colliding with atoms in the lattice with enough force to knock electrons off those atoms.  Once this happens all he|| breaks loose -- more current flows -- and the junction breaks down into avalanche mode (zener).

    .

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