1 - In a forward biased pn junction, will barrier voltage increase or decrease?

Question

1 - In a forward biased pn junction, will barrier voltage increase or decrease? Will diffusion current increase or decrease? Which direction does diffusion current flow? What about drift current? Which current will dominant?

2 - In a reverse biased pn junction (before breakdown), will barrier voltage increase or decrease? Will diffusion current increase or decrease? Which direction does diffusion current flow? What about drift current? Which current will dominant?

3 - What is reverse breakdown of pn junction? What triggers reverse breakdown?

Explanation / Answer

1. If the external voltage applied is greater than the barrier potential,then only the diode conducts. So when diode is forward biased, barrier potential appears to be decreasing.

Diffusion current increases exponentially with a slight increase in applied voltage( this is because of the increase in the number of charge carriers that have enough energy to cross the junction). The electrons flow from n to p. As the direction of current is opposite to the flow of electrons, diffusion current is in p to n direction

Drift current is not affected by applied voltage. So it remains the same.

As diffusion current increases exponentially,it dominates the drift current( which remains the same)

2. Barrier potential increases with increasing applied reverse bias voltage ( till breakdown)

Diffusion current becomes negligible and almost Zero ( as the width of depletion region increases with applied reverse bias voltage). The direction of diffusion current is from n to p direction.

Drift current remains unaffected. As diffusion current becomes negligible, drift current becomes dominant

3. When the reverse bias voltage being applied to a pn junction reaches a maximum limit,there is a rapid increase in current. This is called breakdown.

Parameters that trigger the breakdown are avalanche multiplication and quantum mechanical tunnelling of charge carriers through the band gap

Avalanche breakdown is caused by ionization of electron hole pairs

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