Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

PN junction diode

Status
Not open for further replies.

skylines

New Member
Hi ,

can any one let me know how current flows in PN junction when it is forward biased and reverse biased?

As of i know when PN junction is connected in forward bias the depletion layer decreases and when it is connected in reverse biase the depletion layer increases. I am able to understand upto this. I am not able to undrstand how current flows?

pls let me know.

thanks in advance
 
skylines,



can any one let me know how current flows in PN junction when it is forward biased and reverse biased?

As of i know when PN junction is connected in forward bias the depletion layer decreases and when it is connected in reverse biase the depletion layer increases. I am able to understand upto this. I am not able to undrstand how current flows?

pls let me know.

First of all, current does not "flow", it instead exists. It is charge that sometimes flows. Current is charge flow, and it doesn't flow twice.

In a PN junction diode, if the N-type material of the emitter is put in contact with a P-type material, the electrons if the N-type material will migrate over into the P-type material material and vice versa. Eventually this will stop happening, because the N-type material will have positive ions along the boundary due to the absence of electrons that departed. The charge from these positive ions will keep the holes of the P-type material from coming over. The same thing happens in the p-type material in reverse. The holes coming from the p-type material leave negative ions whose charge will oppose further migration of the electrons from the n-type material. Eventually an equilibrium is reached resulting in the charge flow being completely blocked by the repulsive forces. These repulsive forces form a region along the NP boundary where the electrons and holes are neutralized, called the depletion layer, defined by a barrier voltage. By applying a forward voltage Vpn, one can effectively make the boundary layer thinner by lowering the barrier voltage and allow the charge carriers to move again. Or an opposite Vbe can increase the barrier voltage and completely block the movement of the charge carriers.

The first paragraph in this link describes this. Depletion region - Wikipedia, the free encyclopedia

Ratch
 
Last edited:
hey ratch, thanks for your reply.

I have following doubts regarding this.

Due to the migration of electrons and holes across the junction +ve ions form in the n region and -ve ions form in the p region. Now my question is why can't the +ve ions in the n region are neutralise by taking electrons from n region it selt? and vice versa?
 
skylines,

Due to the migration of electrons and holes across the junction +ve ions form in the n region and -ve ions form in the p region. Now my question is why can't the +ve ions in the n region are neutralise by taking electrons from n region it selt? and vice versa?

Taking your question to the extreme, the whole diode would become one big depletion region. That does not happen. Evidently, the atoms located away from the PN boundary are strong enough to hold on to their holes and electrons. Only the more poweful diffusion forces are able to strip the holes and electrons from the front line atoms and turn them into permanent ions.

Ratch
 
No one know what exactly happens inside a diode, its just a explanation given.
Skylines, i did have doubts like yours. Practically, there are no Holes, it's just a deficiency of electron - so neutralisation happens between a proton (+) and electron(-).
 
Electroenthusiast,

No one know what exactly happens inside a diode, its just a explanation given.

Semiconductors are a old and mature technology. There is very little that is not known about them, especially how they work. Just because popular explanations are simplified does not mean that the knowledge is lacking.

Practically, there are no Holes, it's just a deficiency of electron - so neutralisation happens between a proton (+) and electron(-).

At the quantum level, a hole has mass and mobility just as an electron has. It is a whole different world when things get really small. Holes and electrons combine and get neutralized, thereby giving off energy. Protons are not affected because they are in the nucleus, and semiconductors do not engage in nuclear reactions.

Ratch
 
A problem that's always confusing is "conventional current" or "hole flow" and the "flow of electrons". These are in opposite directions. Using the symbol of a diode, a diode is considered forward biased when "conventional current" flows in the direction of the arrow. e.g. The point of the triangle is connected to the more negative voltage. Electrons go the other way.
 
KeepItSimpleStupid,

Using the symbol of a diode, a diode is considered forward biased when "conventional current" flows in the direction of the arrow.

As I pointed out to skylines in the second post of this thread, current does not flow. It exists. Charge can flow or remain stationary.

Also all ammeters assume that the charge flow consists of positive charge carriers. That is the way they are marked. That means if you connect the pos post of an ammeter to a positive voltage and the neg post to a more negative voltage, the meter needle will deflect to the right. This happens even though the physical charge carriers (electrons) are moving in the opposite direction.

Ratch
 
In forward bias condition when voltage is applied to the diode the depletion layer decrases and charges flow with in diode. Then how come the current exists in the circuit when chrges flow only in the diode?
 
skylines,

In forward bias condition when voltage is applied to the diode the depletion layer decrases and charges flow with in diode. Then how come the current exists in the circuit when chrges flow only in the diode?

Charge flows throughout the circuit; through the diode, through the wires, and through the voltage supply. Whoever said it did not?

Ratch
 
So, in which direction d0es the electrons flow? From N type (as it has surplus electrons) material to p type and from p to throughout circuit?
 
skylines,

So, in which direction d0es the electrons flow? From N type (as it has surplus electrons) material to p type and from p to throughout circuit?

You should be able to answer that. How do you forward bias the diode? Then, how do the physical electrons flow from a voltage supply, keeping in mind that they have a negative charge?

Ratch
 
Ratch:

Clearly the Coulomb is not a unit of energy. I didn't catch that. Britanica is clearly wrong.

In any event the charge on an electron is the smallest amount of quantized charge that can be transferred. And electrons don't really flow, they basically colllide within the material, so it's not the same electron that started the journey as the one that finishes it. Hole and electron flow have historically been used to describe semiconductors.

Way back when, I learned that electrons are in circular orbits. Later I learned that they are in essentially probablility functions as to the position around the neucleus of an atom.

One day you learn that paper is an insulator and the next your measuring the resistance of a piece of paper.

Another day, you learn that waving a wire around in the air generates a voltage. It's a wire in the earth's magnetic field, so it does generate a voltage based on the fundamental concepts and can be measured.

Flexing a shieleded cable also generates a current and there are special graphite impregnated cables that lessen these effects.

Squishing an insulator also generates a current.

Two different metals at different temperatures, e.g. solder generates an offset voltage.

Most people don't have to take these effects into account and most people probably have never seen an instrument that can measure the phonomena.
 
Clearly the Coulomb is not a unit of energy. I didn't catch that. Britanica is clearly wrong.

Britanica was right. You quoted it wrong.

And electrons don't really flow, they basically colllide within the material, so it's not the same electron that started the journey as the one that finishes it. Hole and electron flow have historically been used to describe semiconductors.

That's why current flow is correct. And why it's been used forever to describe the transport of charge, and will continue to be used.
 
KeepItSimpleStupid,

And electrons don't really flow, they basically colllide within the material, so it's not the same electron that started the journey as the one that finishes it.

Charge carriers do flow in a conductor. It is called the drift velocity, and it is very much slower than the propagation response. It is like the hose filled with marbles analogy. One marble is forced in at one end, and almost immediately another different marble pops out the other end. After many repetitions, the original marble finally makes it to the end.

[One day you learn that paper is an insulator and the next your measuring the resistance of a piece of paper.

Yes, some insulators are better or worse than others.

Another day, you learn that waving a wire around in the air generates a voltage. It's a wire in the earth's magnetic field, so it does generate a voltage based on the fundamental concepts and can be measured.

Yes, a conductor moving at right angles to a magnetic flux field will develop a voltage.

Squishing an insulator also generates a current.

And applying a force to a piezoelectric material like a crystal will likewise will cause a voltage to appear.

Two different metals at different temperatures, e.g. solder generates an offset voltage.

That is called a thermocouple.

Most people don't have to take these effects into account and most people probably have never seen an instrument that can measure the phonomena.

A sensitive voltmeter or ammeter can.

Ratch
 
Last edited:
skylines,

I have following doubts regarding this.

Due to the migration of electrons and holes across the junction +ve ions form in the n region and -ve ions form in the p region. Now my question is why can't the +ve ions in the n region are neutralise by taking electrons from n region it selt? and vice versa?

Sorry to drag this thread back from the crypt, but your question above deserved a better answer that I gave it. You asked why, when a PN junction creates a depletion region, do the ions not take electrons from the N region or holes from the P region to neturalize themselves along the PN boundary? The real answer is they are limited from doing so. Here is why as shown in the attachment. The junctions of the wires attached to the N and P material form a junction voltage called a contact potiential. At equilibrium (no external voltage applied), the barrier voltage (also called built-in voltage) is equal to the two junction voltages. So the width of the depletion region is limited by the two contact voltages. If the region because wider, then the voltages across the diode would not be zero anymore due to the barrier voltage becoming larger than the contact voltages, and a current would form when the diode was shorted. This would self-heat the diode and violate the second law of thermodynamics. As an aside, you cannot directly measure the barrier voltage because the metal microprobes would generate their own contact potiental and skew the results. I hope this answers your question in a more satisfactory way.

Ratch
 

Attachments

  • Forward & Reverse Bias.jpg
    Forward & Reverse Bias.jpg
    1.9 MB · Views: 230
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top