# Saturation current on photoelectric effect

tayliangcai
From the post
and
http://www.thephysicsforum.com/quantum-physics/3921-photoelectric-current-dependence-potential-difference.html [Broken]
I have some idea on why does a higher potential different will not increase the saturation current.

What I understand is that there are a "upper limit of photoelectron produced", although the photoelectrons moved faster towards anode under higher potential difference, but the number of photoelectrons arrived anode PER SECOND is still a constant. Am I wrong with this idea?
If not, then, consider the case:
I have a sensitive, ideal ammeter, that can record current in the circuit every single unit of time. Then, I fixed the frequency and intensity of the illuminating radiation on cathode(that always higher then threshold frequency), then slowly increase the potential different even after reached the saturation current.
Say, we obtained the saturation current, 1A once the potential difference is 5V.
Will I get the reading like, example, at 500V, the current is 100A in a instant then continuously decreased for a while, where giving me the mean current in 1s = 1A ?

Because I am still thinking that the drift velocity of the photoelectron will affect the current.
Sorry if I cannot state my problem clearly due to poor command on english.

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Staff Emeritus
From the post
and
http://www.thephysicsforum.com/quantum-physics/3921-photoelectric-current-dependence-potential-difference.html [Broken]
I have some idea on why does a higher potential different will not increase the saturation current.

What I understand is that there are a "upper limit of photoelectron produced", although the photoelectrons moved faster towards anode under higher potential difference, but the number of photoelectrons arrived anode PER SECOND is still a constant. Am I wrong with this idea?

No you're not. This is how we understand the photoemission phenomenon.

In fact, when I design a setup to measure the quantum efficiency of a photocathode, I first scan the applied potential difference until I reach a plateau of the photocurrent. Only then am I assured of a potential difference that I should use that collects all the emitted photoelectrons.

Zz.

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tayliangcai
Thanks, ZapperZ.
But what about the concept of "drift velocity of the photoelectron will affect the current" idea?
And another small question which is out of topic:
My friend asked me whether it is possible to reach a saturation current even when a reverse voltage applied or at 0 voltage? Then I answered him that “ I don't think so because no matter how fast the energy the photoelectron had, or how many they are, lots of them doesn't travel towards the anode, so when reverse voltage applied or at zero external voltage, no saturation current is achieved, due to not ALL photoelectron will reach the anode."
My friend seems not so satisfy with my answer, so am I wrong in any manner ?

Staff Emeritus
Thanks, ZapperZ.
But what about the concept of "drift velocity of the photoelectron will affect the current" idea?

Define "drift velocity of the photoelectron".

This is not in a solid and there is no "drude-like" description here. Most photoelectric effect experiment emits very small amount of current, and thus, space-charge effects are negligible. So no electron-electron interaction via coulombic repulsion. And at saturation, ALL the photoelectrons emitted per second reaches the anode. So what's left here?

And another small question which is out of topic:
My friend asked me whether it is possible to reach a saturation current even when a reverse voltage applied or at 0 voltage? Then I answered him that “ I don't think so because no matter how fast the energy the photoelectron had, or how many they are, lots of them doesn't travel towards the anode, so when reverse voltage applied or at zero external voltage, no saturation current is achieved, due to not ALL photoelectron will reach the anode."
My friend seems not so satisfy with my answer, so am I wrong in any manner ?

This is confusing. If you apply no bias, only electrons that happen to be emitted in the direction of the anode will register in your current measurement. This will typically be very small especially if your anode is either far away or has a small surface area. If you reverse bias, then at some point, you see zero current.

So what is defined as "saturation current" here? "Saturation" means that as you increase some quantity, the value of another quantity doesn't change anymore. It is when you've reached the maximum of that second quantity. I don't see how this applies here.

Zz.

tayliangcai
Sorry for the late reply, I was busy with the exam.
Sorry again because my question is not clear, so basically my question is:
Usually the graph of photoelectric current against voltage will be looks like this ( Picture 1)
Is it possible that we can get a graph looks like this ? (picture 2)

Staff Emeritus
Sorry for the late reply, I was busy with the exam.
Sorry again because my question is not clear, so basically my question is:
Usually the graph of photoelectric current against voltage will be looks like this ( Picture 1)
Is it possible that we can get a graph looks like this ?

Not that I know of. That is way to abrupt, and in physics, we only see such abrupt change when there is something like a phase transition. No such thing is happening here. You are only collecting more and more 'stray' electrons. Nothing exotic here.

It still doesn't change the fact that there is a saturation current. Its existence doesn't require such an abrupt plateau to occur.

Zz.

Gold Member
Picture 2 is what we have seen in experiments with weak plasma's, where the charged particles created in the weak plasma are a function of the potential difference over the plasma. The zero-current is then at V=0 and the transition to the steady saturation current is in some cases extremely sharp (but still smooth). In our experiments, a stronger current causes (linearly) more charged particles to be captured and at saturation we capture all charged particles. The total number of charged particles created stays the same, independent of potential difference.

So a saturation current like figure 2 is possible, but maybe (probably?) not in your situation.

Staff Emeritus