# Zero bias, why does J=0?

Discussion in 'Physics' started by amilton542, Aug 15, 2014.

1. ### amilton542 Thread Starter Active Member

Nov 13, 2010
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64
At thermal equilibrium in a p-n junction why do the drift and diffusion current components sum to zero to give a net current density J of zero?

The drift-diffusion equation basically states that for every electron (or conversely hole) that diffuses an electron (or conversely hole) must drift back in return?

2. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Are you asking about an open circuit PN junction?

In this case the concentration differences of holes and electrons in the P and N material cause rapid diffusion which, in turn sets up a 'space charge' in the neighbourhood of the junction.

This neighbourhood is known as the depletion zone or barrier zone.

There is separation of charge within this zone which leads to an electric potential and field being set up that opposes the initial diffusion. By opposes we mean it drives the holes and electrons back the other way.

This voltage is known as the contact potential.

After a short time equilibrium is set up and everywhere in the depletion zone the diffusion force is balanced by the electric force.

So the net current is zero.

This is similar to the mechanism of osmosis where diffusion of material from high conccentration to low concentration sets up an opposing pressure (QV voltage) until the two effects balance.

Last edited: Aug 15, 2014
3. ### amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
Yes, that pretty much sums up the Wiki' diagram I've been looking at. So is it safe to say, drift-diffusion components only sum to zero after equilibrium has been established, whereby the potential barrier cannot be overcome for zero bias?

4. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
No, not exactly.

The drift/diffusion components sum to zero before the PN junction is created.
ie there is equilibrium.

The creation of the junction causes disequilibrium and a temporary flow of current.

Yes, when equilibrium is re-established your statement is true and the currents again sum to zero.

As as result of re-establishing equilibrium there is a permanent electric field created.

I am not quite sure what you mean by the above statement about bias. Are you referring to the electric field of the contact potential?

If so what do you mean by overcome it?

The separation of charge creates the 'depletion zone' which is devoid of carriers because they are separated to the ends of this zone. This separation of charge is the source of the field.

So there are no carriers available to overcome anything.
But yes providing sufficient external field or bias will drive in new carriers, causing disequilibrium and a current crossing the junction.