semiconductor basic 2

[timkuc]

My question is about drift and diffusion current.In a semiconductor diode no "bias" condition diffusion is present and due to transfer of charges due to diffusion depletion region created and then this region has ions which produce electric field which results in drift current due to force the minority charges from both the sides.So the diffusion current and after that drift current comes into picture .In forward bias there will be potential difference across the diode so there must be electric field in the diode.So in forward bias current should be due to the battery voltage .What I don't understand is ,is there any phenomenon of diffusion after applying the battery (because battery produces electric field in material,that means only drift current should be there as the drift current definition says "Drift current is the movement of electrons in electric field) ? Some says that drift current only depends on temperature (Why ?). I think don't getting the drift current what it really is . I am in really in trouble.

 

[cl10greg]

ions which produce electric field

Ions don't produce an electric field, they move in the presence of an electric field.

I think don't getting the drift current what it really is

You have the basics down. The diffusion current goes from the P to the N region while the drift current opposes this with equal magnitude. The drift current is independent of biasing but is thermally dependent because minority carriers can be generated thermally which would throw the diffusion out of balance. The electric field causes the charge carriers to move at a certain drift velocity. The velocity is based on mobility (resistance of median and other variables) which gives you the charge carries flow and the amount of current in your semiconductor.

 

[timkuc]

Ions don't produce an electric field, they move in the presence of an electric field.


You have the basics down. The diffusion current goes from the P to the N region while the drift current opposes this with equal magnitude. .

This is when there is no biasing on diode ,right? Yeah I am agree that the charge carriers move with a drift velocity and mobility concept comes into picture .But they are charge carriers I mean more voltage produce more electric field so more current. Am I wrong here?

 

[cl10greg]

Well we can keep with the diode theme here. The diode has no drift current until there is a certain voltage (electric field) that causes the charge carriers to move. Before then, there is just diffusion to equalize the holes and electrons. This movement causes the depletion region to form. Your bias comes in for how the depletion region interacts. A reverse bias causes the region to widen and causes the drift to increase and the diffusion to decrease, this results in a very small current density. A forward bias causes the region to narrow and decrease the drift and increase the diffusion, this results in a very high current density. This holds true until either the current demanded is too much and the electrons collide and heat the part until failure or the reverse bias is too high in which the depletion region is shorted and the device flows when it isn't supposed to. If the forward voltage is too right, the current density is too much and causes heating and failure. That's why you have a forward rating, a reverse breakdown, and a current rating.