# Why all the applied Voltage appears across the Depletion Region ?

1. ## Why all the applied Voltage appears across the Depletion Region ?

when we apply forward voltage across the PN junction, all of the voltage appears across the junction. why ?
I am reading Robert Pierret book and it says that as long as we have a low level injection case i.e. when Vapplied is less than Vbuilt in, this is a case of low level injection -how ? and therefore no voltage drop across the quasi neutral regions - how ?

Thank you.

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2. ## Re: Why all the applied Voltage appears across the Depletion Region ?

How? Because they define it so. That is, they say there's
negligible current so I*R in the ohmic regions is to be neglected.

Now outside academia there are many cases where ohmic resistance
and voltage drops still matter. But professors don't care, they are
not on the hook for circuit performance because they are not
creators but educators. They impart the rules, and leave it to
industry to let you "learn by doing" the exceptions in a practice-
oriented environment, populated by people with that sort of
experience (you hope).

3. ## Re: Why all the applied Voltage appears across the Depletion Region ?

Simplistically, you can think of a diode being a series connection of three elements("resistors") - one resistor for depletion region, and two resistors for quasi-neutral regions (in n- and p-type doped semiconductor).
Resistance in quasineutral region is much lower than the depletion region resistance at low applied voltages, that's why voltage drop on it, IR, is much smaller than a voltage drop on depletion region.
However, depletion region resistance is exponentially decreasing with applied voltage (simplistically, for diode I~exp(V/Vt) --> R=V/I ~ exp(-V/Vt), where Vt is a thermal voltage).
At high applied voltage (this corresponds to high injection level - i.e. when injected minority carrier concentration is higher than the majority carrier concentration), resistance of depletion region becomes lower than the series resistance of quasi-neutral regions, and so there is a significant voltage drop on quasineutral regions at high voltage / high injection level.

There are many good semiconductor device physics books that explain this, in particular - the "bible" by S.Sze, "Physics of semiconductor devices".

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4. ## Re: Why all the applied Voltage appears across the Depletion Region ?

Resistance in quasineutral region is much lower than the depletion region resistance at low applied voltages, that's why voltage drop on it, IR, is much smaller than a voltage drop on depletion region.

I followed everything but why resistance in Quasi neutral regions under low level injection is low ?

5. ## Re: Why all the applied Voltage appears across the Depletion Region ?

Low and high are relative terms.
It's low as compared to the diode intrinsic (depletion layer) resistance.
It's low because of some decent level of doping (1e16-1e18 cm-3), to have series resistance of a diode low.
If series resistance is high - higher than the diode resistance - why do you need diode then?

I did not say that series resistance of quasineutral regions at low injection level is low.
It is low at any level of injection, my point was that it is lower (by many orders of magnitude) than the diode (depletion region) resistance, the latter being very high at low injection level (i.e. low applied forward voltage).

6. ## Re: Why all the applied Voltage appears across the Depletion Region ?

The N and P regions will conductivity-modulate at
high injection (high being relative to the doping,
lightly doped materials modulate a lot, usually a
good thing unless you don't like storage time).
This brings those regions' resistance -down-.
This is why Schottkies have poor high current
Vforward despite having a lower datasheet Vf.

Like I said low level injection is by definition low
current and thus low I*R relative to the barrier
voltage and there you are.

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7. ## Re: Why all the applied Voltage appears across the Depletion Region ?

Low level injection definition is - when concentration of minority carriers is lower than that of majority carriers (or doping).

One can create a diode with very large series resistance of the quasineutral regions, so the voltage drop IR on quasineutral regions can be higher than the barrier voltage (close to bandgap), but the injection level may still be very low.

In other words, in general, voltage drop on quasineutral region being higher or lower than the barrier voltage (or bandgap) is not an equivalent of a high injection level.

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