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### Course: Class 12 Physics (India)>Unit 14

Lesson 4: PN junction biasing

# Forward and reverse current mechanism

What's the mechanism of charge flow in forward and reverse bias? In this video, let's go deeper and explore the mechanism of the current in both forward and reverse bias. We will see that's way different than what happens inside a conductor.  Created by Mahesh Shenoy.

## Want to join the conversation?

• What's wrong with the following statement :
"If the depletion layer widens on increasing voltage,the thermal generation should also increase as the area is increased and hence the current should also increase"
• I am thinking the ambient (ambient = surrounding the depletion region) thermal energy will produce more holes/electrons for conduction when the depletion region becomes larger because there will be more surface area for the ambient thermal energy to interact with. In addition, since thermal energy is a form of kinetic energy (moving particles), the larger depletion region offers more total particles and hence greater chance of collisions in the depletion region when the reverse current is traversing the depletion region. This begins to cause more electrons/holes being generated for conduction in the depletion region from this thermal energy. Eventually the reverse current will reach a point where the collisions are so great in number that an avalanche occurs, and a very large quantity of charge carriers are created in the depletion region...the diode has many broken bonds in the depletion region in this scenario and the diode is said to have reached the breakdown voltage level and is now actually conducting a substantial current in reverse.
• Again and again u are using word thermal generation what does it mean
• It's just a fancy way of saying that an electron is excited from the valence band to the conduction band. This excitation is usually due to some sort of thermal agitation.
• Ok, so in forward bias the current flow is due to diffusion, owing to concentration differences which in turn is maintained due to recombination. But then, what what is the reason that we have constant "generation of holes" at the far left (respectively, "generation of electrons" at the far right)? From the presentation, it seems that it can't be the holes (electrons) coming in from the wire at the left (right), because these are equal to the few ones that make it to the other end. Differently put, what is the reason that we don't reach an equilibrium, after some time, where concentrations become almost equal (any holes get created at the left get also recombined at the right) and current flow stops?
• The system won't settle down and reset into its original state where all Si's electrons are back on the valency band and donors reclaim (P) and release (B) electrons. Because of how Si are doped and how the doped parts arranged the leftmost side of P will have the most vacant holes and the rightmost side of N will have the least (equally, left leftmost side of P will have the lease amount of electrons and the rightmost side of N will have the most). Thus there is always a gradient and thus there is always a diffusion current, however small it might be in the presence of external voltage.
• Doesn't recombination process decrease the amount of free carriers in the semiconductor ? How can it continue working after adequate time ?
• There is an equilibrium between regeneration and recombination of electron-hole pairs.
• Why is recombination/concentration gradient needed for current to flow? If the voltage difference is greater than .7 won't current flow simply due to the net voltage difference across the diode, as it would across any circuit element with a potential difference (e.g. resistor)?
• Because P simply does not have (enough) electrons to for the current and it needs N to donate them which happens because of diffusion. Battery only assists in the process.
• In the reverse bias video, u have clearly said that current is no of holes per second. Its true that no of holes dont change. Bht more accelerated motion means lesser time, so current, being inversely proportional to time, should increase with voltage increment (as voltage increment means acceleration increment?
• I think the drift current does increase, but since it's so small (like 5 holes per generation), it stays roughly constant.
(1 vote)
• At you stated that a hole is "made" at the left end of the diode thus releasing an electron into the external circuit. But where are these electrons coming from in the first place and how are they being replaced? And what happens to the electrons which diffuse over from n to p? Or do they not diffuse over?
(1 vote)
• Do not think of electrons as balls coming from the - terminal of the battery through the wire and diode all the way to to the + terminal. This is a wrong, wrong way to think of current.

If you consider an individual electron it may still move back and forth or get stuck (e.g. on the valence band). It will just have an overall tendency (i.e. if you consider its behavior over a period of time) to move in direction from - to +. Since all electrons of a conductor will behave like that, their cumulative tendency to move from - to + is called current.