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Forward-bias
Forward-bias occurs when the P-type block is connected to the positive terminal
of a battery and the N-type block is connected to the negative terminal, as
shown below.
A silicon p-n junction in Forward-bias.With this set-up, the 'holes' in the
P-type region and the electrons in the N-type region are pushed towards the
junction. This reduces the width of the depletion zone. The positive charge
applied to the P-type block repels the holes, while the negative charge applied
to the N-type block repels the electrons. As electrons and holes are pushed
towards the junction, the distance between them decreases. This lowers the
barrier in potential. With increasing bias voltage, eventually the nonconducting
depletion zone becomes so thin that the charge carriers can tunnel across the
barrier, and the electrical resistance falls to a low value. The electrons which
pass the junction barrier enter the P-type region (moving leftwards from one
hole to the next, with reference to the above diagram).
This makes an electric current possible. An electron starts flowing around from
the negative terminal to the positive terminal of the battery. It starts at the
negative terminal, moving towards the N-type block. Having reached the N-type
region it enters the block and makes its way towards the p-n junction. The
junction barrier can no longer keep the electron in the N-type region due to the
forward-bias effect (in other words, the thin depletion zone produces very
little electrical resistance against the flow of electrons). The electron will
therefore cross the junction and move ahead into the P-type block. Once inside
the P-type region, the electron, being thermally free (from bonding)—or
mobile—will move through the rest of the crystal, making its way to the positive
terminal of the power supply. Please note that the electron does not jump from
one hole to the next in the p-region. This actually qualifies as electron-hole
recombination which immobilises both hole and electron. The electron can move
freely through the crystal without needing to jump into holes which is what
happens when electrons do cross the depletion layer. This process will be
repeated over and over again, producing a complete circuit path through the
junction.
The Shockley diode equation models the operation of a p-n junction outside the
avalanche region.
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