# Archived The drift and diffusion currents under equilibrium

1. Jun 8, 2013

### anhnha

Can you help me explain this part from my book?

In equilibrium, the pn junction has no external connections, i.e., the terminals are open and no voltage is applied across the device.
If so, I wonder why there is drift current in the equation above. In equilibrium, the total current is zero but I don't understand how can they get the equation.
Is it right that in equilibrium, the total current is zero but each current (drift and diff currents) are not necessary to be zero?

PS: I have just seen the image. It is relatively clear now. In equilibrium, the net current is zero.
From that I can get the equation above. However, I am confused about these currents. I think they are only exist in a short of time, right?
After that time:
The diffusion is zero when the concentration of electrons and holes are equal in P and N sandwiches.
The dift currents are zero when all minority charge carriers are depleted.
If possible, can you explain about the next part?

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Last edited: Jun 8, 2013
2. Feb 7, 2016

### CrazyNinja

@anhnha... I see you are still active now. Can you explain to me what the next part in the following sentence is?

3. Feb 7, 2016

### anhnha

Thank you!
Could you explain about unrealistic phenomenon here?
Why is there a charge accumulation?
I don't see how is each side of equation 2.61 can be zero.
For the right hand side part, the total current always flows from p to n and in the left hand side the total current always flows from p to n.
I don't see how possible it is to assume that each side of the equation is zero.

With the next part, I meant to say that because there is a limited number of electrons on each side of the pn junction so after the diffusion all electrons and holes are spread equally so how is it possible for the current to continue here?
I think I can answer it now. Because of the drift current, the concentration will never be equal so there is always diffusion current.

4. Feb 8, 2016

### CrazyNinja

OK. The problem with my understanding of this phenomenon is that it does not contain all these terms. Can I know how old you are? Because I have just learnt this topic myself (I am 17), and I could help you understand it the way I understood it, because my way makes a little more sense physically. But that is only if you want me to.

PS: I hope you are not female. Last time I checked, asking females their age was not chivalrous.