# Diffusion and Drift Current under no biasing of a PN junction Diode

In a PN diode, I get the point that total current is zero due to drift and diffusion current contribution cancels their effect when depletion region is formed.

My question is even though just for a moment suppose statistically electron in large amount diffused to p side and due to phenomena of recombination after some distance it will recombine. Then how does one say they constitute a current if they just diffuse by any means to p side, i.e. how does one say or calculate current or current flows if so they recombine with the holes.

If my question doesn't make you understand what i asked, so say in Forward biasing as diffusion will now easily takes place of majority charge carrier and we say a diffusion current is formed. But the majority carrier just simply recombines every time so what constitutes the diffusion current and how do we measure this current flowing in diode.

Also what is the role of battery in analyzing pn diode. Does it also provide extra electrons and they are the reason for current contribution or what. Because in some text some says holes and electron are attracted by a battery and a current flows, which according to me is absurd.

Then how does one say they constitute a current if they just diffuse by any means to p side

Movement of charge is electric current. This is moving charge, therefore it is current.

ANY movement of charge is current. If you put a bunch of electrons on a baseball and threw it across the field, that's an electric current. Its not a very constant current, but it is a current.

Unlike my baseball example, diffusion current in semiconductors is a very predictable process leading to a steady state current.

i.e. how does one say or calculate current or current flows if so they recombine with the holes

Just because the charges recombine after moving doesn't mean they weren't moving. They were moving and therefore were participating in current prior to recombination.

You can calculate diffusion current from the electron and hole gradient and the diffusivity of electrons and holes.

But the majority carrier just simply recombines every time so what constitutes the diffusion current and how do we measure this current flowing in diode.

Those carriers that diffuse over to the other side of the junction, become minority carriers, and recombine have to pair up with a majority charge carrier to do so. This majority carrier participating in the recombination has to be replaced in order to maintain charge neutrality in the semiconductor. This means while the diffusion current of the minority carriers decreases as they recombine, the total current remains constant because majority carriers begin to take over.

You can't measure it directly in a diode, you can only measure the net current. But the model is quite accurate with those measurements that we can do.

There are diffusion current measurements that can be done if you try hard enough. The Haynes-Shockley experiment is such an example

https://en.wikipedia.org/wiki/Haynes%E2%80%93Shockley_experiment

Using a laser to generate a carrier gradient without the presence of an electric field allowed them to measure diffusion current.

Also what is the role of battery in analyzing pn diode.

Don't think about how you get the external voltage, it is irrelevant. Just assume you can apply voltage to the p and n regions. It doesn't need to be a battery, in real life it probably is very rarely a battery anyway.

Does it also provide extra electrons and they are the reason for current contribution or what.

Your external voltage supply will provide a source of electrons to the n side of the junction to counteract those that leave from the p side.

Because in some text some says holes and electron are attracted by a battery and a current flows, which according to me is absurd.

Why is this absurd? Applying an electric field to charges applies a force to those charges and makes them want to move around. One of the most important properties of charge carriers (electrons and holes) is that they can move around, and so they do. This is exactly what happens in a resistor or even a wire.

• Thank you for a wonderful elaboration. My questions is 1) If movement of charge as we study in high school causes current, my question is then as diffusion of majority carrier is not possible but drift of minority is readily possible, then @Matt why we get total current equal to zero. My logic is that there are few minority carrier and few by statistical process some diffusion carrier always cancels their effect. Commented Jun 19, 2021 at 0:58
• 2) Suppose we have a device which notes down the value of current by diffusion, you are saying before they recombine we will have a current, at time t' they recombine. So at what time you will measure this current i.e. for t<t' or t>t'. Its is so to tell you that if they recombine and after that i measure current. Will i measure something or not Commented Jun 19, 2021 at 1:03
• 3) My very important question where i am stuck is that if pn junction now simply diffuses because electric field is reduced now, the holes and electron just will try to maintain charge neutrality by recombining, and on their flow constitute a current. So the current we measure in a diode is due to diode or by external voltage we have applied. Or do we as a measure ment in multimeter get diffusion current or what? Or as explains by you the source voltage provides free electron in pn diode and due to these free electron flowing in circuit constitutes a current. Commented Jun 19, 2021 at 1:14
• 4) You said a comment for my absurd part that applying Electric makes them want to flow them freely, my question is then as free electron and hole recombines then they make pn diode neutral."" So diffusion current is what me measure across diode or the surplus amount of free electron provided by the battery constituting free electron in diode move freely as a time may come that no more free electron and hole will be left all of them will get diffused and recombine"". Commented Jun 19, 2021 at 1:20
• @Anshul You dont measure diffusion or drift current. You measure total current through the diode. The particular type of current changes as you move through the diode, but the total is a constant (to satisfy KCL). You cant run out of electrons and holes in a semiconductor. It looks like you have some other questions here that would be better as a new post.
– Matt
Commented Jun 19, 2021 at 2:59