What Explains the Lack of Current in a PN Junction at Equilibrium?

AI Thread Summary
In a PN junction at equilibrium, the formation of a depletion region leads to an electric field that balances the movement of charge carriers. Electrons from the negatively charged P side and holes from the N side create opposing currents, resulting in zero net current. The currents involved include drift currents, driven by the electric field, and diffusion currents, caused by random thermal motion. These currents cancel each other out, maintaining equilibrium in the junction. Understanding these interactions is crucial for comprehending semiconductor behavior.
unified
Messages
43
Reaction score
0
TL;DR Summary
I am considering a PN junction at equilibrium, and have a couple of questions.
Consider a PN junction doped with say phosphorous on the N side, and Boron on the P side. Initially, there is an opportunity for the electrons just below the N conduction band to drop to the lower available energy states just above the P valence band. This leaves the N side positively charged and the P side negatively charged, forming a depletion region. This means that there will be an electric field pointing in the direction from N to P. Eventually, there will be an equilibrium that is reached, in which case there is no current in the depletion region.

I do not know which of the following explains why there is no current in the depletion region under equilibrium.
1. The electric field causes electrons from the negatively charged P side to flow back to the N type, and in equilibrium this cancels the flow of electrons from N to P. Also, the electric field causes holes to flow from N to P, and this cancels any holes flowing from P to N. The result of these processes is zero net current.
2. There is no current flowing either from N to P or P to N in equilibrium. In equilibrium, the depletion region is essentially an insulator, and while we have an electric field present in the region, the charge is not free to flow.
 
Engineering news on Phys.org
(2) is clearly wrong. The holes and electrons are free to move. (1) is closer, but isn't very clearly stated. Exactly which currents cancel? Try drawing a sketch to show the various currents.
 
phyzguy,

A sketch would include four currents, two from the field (electrons move against the field, holes with the field) and two due to random thermal motions (electrons from P to N, holes from N to P). In equilibrium these effects cancel.
 
unified said:
phyzguy,

A sketch would include four currents, two from the field (electrons move against the field, holes with the field) and two due to random thermal motions (electrons from P to N, holes from N to P). In equilibrium these effects cancel.
Exactly. This is much more clearly stated than your original (1). In semiconductor nomenclature, the currents driven by the electric field are typically called "drift currents" and the currents due to random thermal motions are typically called "diffusion currents". The drift currents obey Ohm's law (J = σE), and the diffusion currents are driven by concentration gradients and obey Fick's law (J=-D∇n).
 
  • Like
  • Informative
Likes unified, berkeman and Baluncore
I have recently moved into a new (rather ancient) house and had a few trips of my Residual Current breaker. I dug out my old Socket tester which tell me the three pins are correct. But then the Red warning light tells me my socket(s) fail the loop test. I never had this before but my last house had an overhead supply with no Earth from the company. The tester said "get this checked" and the man said the (high but not ridiculous) earth resistance was acceptable. I stuck a new copper earth...
Thread 'Electromagnet magnetic field issue'
Hi Guys We are a bunch a mechanical engineers trying to build a simple electromagnet. Our design is based on a very similar magnet. However, our version is about 10 times less magnetic and we are wondering why. Our coil has exactly same length, same number of layers and turns. What is possibly wrong? PIN and bracket are made of iron and are in electrical contact, exactly like the reference design. Any help will be appreciated. Thanks. edit: even same wire diameter and coil was wounded by a...
Thread 'Beauty of old electrical and measuring things, etc.'
Even as a kid, I saw beauty in old devices. That made me want to understand how they worked. I had lots of old things that I keep and now reviving. Old things need to work to see the beauty. Here's what I've done so far. Two views of the gadgets shelves and my small work space: Here's a close up look at the meters, gauges and other measuring things: This is what I think of as surface-mount electrical components and wiring. The components are very old and shows how...
Back
Top