Led operating principle - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: October 02, 2020, 04:12:39 pm
content: From what I understand, an LED works on the principle of 'recomibination'That is when I power the LED with a voltage. This voltage provides sufficient energy to the semiconductor inside the LED which causes the electrons in it to cross the "band gap".The electrons are generally moving in the "valence band" but the external energy provided is sufficient to help them to cross over to the "conduction band" buy jump the "band gap"This jump leaves a hole in the valence band, the electron then reportedly falls back to the "valence band" and recombines with the hole, the excess energy that it had is than released as 'photons"My question isa. Why does the electron want to fall back to the "valence band" since we are continuously pumping voltage into the LED which is some sort of external energy source. i.e once the electron has crossed the band gap and moved to the "conduction band" after receiving the external energy why would it need to come back to the "Valence band" and what causes it to fall back?b. Will increasing temperature without providing external voltage also cause the LED to light up. If I think about it it should light up since heat is also a source of energy and would help the electrons to cross over. My question is how much would I have to heat the led for it to light up.TIA

author: atmfjstc
author_rank: Regular Contributor
timestamp: October 02, 2020, 04:56:37 pm
content: Let's try a more physical metaphor. Imagine a field full of pits, stretching to infinity. It's pitch black and impossible to navigate. You, as the electron, start at the bottom of one of these pits. You can't get out by yourself - gravity keeps you in your place. Then comes our friend Voltage and pulls you out of the pit. You can now move freely, but, given that the field is infinite and you can't see, sooner or later you will fall into a pit again. There's no magical force or reason why this is the case. The way gravity works, it's just easier to fall into a pit and remain there, than it is to get out. And our friend Voltage is still pulling people out of pits, yes, but it cannot be in all places at once - you'll have to wait until it stops by your pit again.

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2020, 05:14:08 pm
content: Answer aWho said the electron returns?When you apply a voltage, a current of electrons flow due to the electrical potential. Electrons move around the circuit.There is no guranttee that the same electron returns back to where it started.For semi conductors, the electrical potential has to over come the depletion zone formed by the union of n and p doped semi conductor. The union of n and p material sucks the electrons/protons from each side, creating this zone where there is a lack of free electrons or protons - hence it is called the depletion zone. The electrical potential pushes free electrons into the depletion zone until enough free electrons are available to flow.Answer BI think you are confusing browning motion of particals and electron orbit in a partical.Heating a material is known to increase the resistance.I think heating semi conductor has different effect based on what the device is and made of.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: October 02, 2020, 10:01:57 pm
content: Quote from: atmfjstc on October 02, 2020, 04:56:37 pmIsn't voltage like a pump that is sucking the electrons out of these pits, and as long as the pump is on the electrons should have been floating out of their pits. Why do they fall back even when the pump is on?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: October 02, 2020, 10:07:46 pm
content: Quote from: MosherIV on October 02, 2020, 05:14:08 pmThere is some sort of theory called "band theory".A. Conductors have overlaping valcence and conduction bands.B. insulators have a large gap between the valence and conduction bands (> 3ev)C. Semiconductors are in between the above two.My question on LED was related to the band theory.

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2020, 11:11:07 pm
content: OK, I have just reminded myself on how LEDs workYour quetion was about LEDs - hence the question is about semi-conductors.Velance and conduction band are about the atomic level of materials.As you pointed out some materials are conductors because electrons are free to move from atom to atom.Other materials are insualtors because the electrons are very difficult to move from atom to atom.When electricity flow, electrons flow through the circuit and through the material(s).How the electrons move from atom to atom depends on the properties of the materials, how many valence bands the atoms has.Some materials have multiple bands that allow electrons to leave the atom.In an LED, the electrons jump from atom to atom, pushed along by the voltgae (electro motive force).As they jump into an atom, they may jump to an outer band but always want to drop down to the band below if I remeber correctly.I am not sure if this jump between bands is random or must alway happen.This is the band gap jump that you are refereing to, that releases the engergy as a photon.The process is allowd to continue due to a) electrons on outer most valence bands are free to move from atom to atomb) electrons always want to be on the lowest valence band in an atomI think this happens in all PN junctions but the amount of electrons is normally quite low. In an LED the effect is exagerated by the materials and the PN manufacturing process.

author: Jwillis
author_rank: Super Contributor
timestamp: October 02, 2020, 11:36:34 pm
content: The Valence bands of an atom are never unoccupied . When a free electron enters a valence band the occupying electron is expelled instantly . The expelled electron becomes the new free electron.When ever an electron is kicked or freed a photon is emitted but energy of the photo depends on the angular momentum of the electron or spin . Electrons spin give the photons spin but lose angular momentum when emitting a photon . The energy of a photon is proportional to it's electromagnetic frequency and inversely proportional to it's wave length . Free electrons in a circuit with common conductors have relatively low angular momentum . The phosphorus doping in LEDs give off photons with a much higher angular momentum or the visible light .When you heat a material the atoms absorb heat and the electrons excite and move away from the nucleus to the next highest state or orbit . This can release electrons to a freed state from the valence band there by emitting a photon but it requires an enormous amount of heat to emit enough photons with enough energy to be visible . Near melting point of the element being heated . Like a heated piece of copper gives off photons near it's melting point of 1085 C . Phosphorous would give off photons near it's melting point of around 280 C .This of course is a simplified idea of what goes on because theirs more going on at the quantum level but I hope it helps.

author: Cerebus
author_rank: Super Contributor
timestamp: October 03, 2020, 01:12:59 am
content: Wow, talk about the blind leading the blind. Angular momentum? Melting points? Free protons? Now, as semiconductor physics goes I qualify as partially sighted, which isn't great, but is better than what's here so far. Much has been said that is either plain wrong or highly misleading. I'm not going to quibble with, or correct what has already been said but I'd suggest that one ignores it.Let's start again.This is all about the energy levels of electrons. Energy in electrons is quantized, the quantization levels are discrete. The amount of energy an electron has dictates how far it can wander away from it's associated nucleus that exhibits a powerful attractive force on it, the more energetic an electron the further 'out' it can wander from its current parent atom.In semiconductors there are two collections of possible energy levels that are important for our purposes, the valance band, and the conduction band. Note, these are just names for collections of energy levels, don't attach the word 'band' in the sense of a band of cloth in your mind.The outermost few electrons of any atom are in the valance band. The number of valence electrons, and the levels of energies they may occupy are characteristic of the atom's element. A missing valence electron leaves a 'hole' that the atom 'wants' to be filled. Being in the valance band provides electrons with just enough energy to partially 'escape the orbit' of the atom, provided that there is another atom nearby that can pull them 'off' the atom. So an electron in the valence band can leave one atom and join another that is adjacent. This leaves the atom it left with a 'hole' where the electron was, and a higher positive charge, a charge that is just enough to 'pull' a valence electron off an adjacent atom to fill the hole. Voila! The valence band electrons, or simply 'valence electrons', get shared by adjacent atoms, leading to the chemistry principle of valency and co-valent bonding.You can put more energy into an electron by accelerating it through an electric field. This is exactly what happens when we apply a voltage across a semiconductor. If an electron gains enough energy that it can truly 'escape orbit' from an atom with enough 'speed' that it won't immediately be captured by an adjacent atom then it has entered the conduction band. Once in the conduction band it is free to wander through the semiconductor away from its atoms and become part of an electric current.Valance elections only have enough energy to wander between nearby atoms, conduction electrons have more energy and can wander freely.Materials are very sparse. The gaps between electrons and nuclei are huge in comparison to the size of those component parts. The gaps between atoms are similarly huge. So a 'free' electron can go quite a way before it will bump into another particle but eventually it will. If this other particle is an electron (most likely, there are a lot scudding about) the two electrons will scatter each other and a series of collisions like this will result in an electron following a random walk, electrons are flying in every direction. This mass of electrons flying around at very high speed are sometimes referred to as an 'electron cloud'. Remember this is all happening in an electric field, which exerts an accelerating force on all the elections; the result of this is the cloud of electrons is gently wafted wafted in the direction of the electric field - this is the electric current, the drift of the cloud.Now, what happens if our fast flying electron, instead of bumping into another electron bumps into an atom that has lost an electron? That atom has a hole, which also means that it has a positive charge that attracts the electron. If the electron is not too energetic it will 'stick' in the hole, and the atom will have a full compliment of electrons again. Remember that we said that you neededmoreenergy to be in the conduction (free flying) band, and less to be in the valence band? Well that freshly captured, orrecombined(with the hole in the atom), electron has conductance band energy and it's now a valance electron again and the rules of the quantum world says it has to give up that energy to stay. So it does, in the form of a photon that is characteristic of the difference between the two energy levels.Remember too that in LEDs we have multiple materials, materials sufficiently different that the levels of energy appropriate to valance bands and conduction bands will be different for the different materials. In LEDs it is all worked so that the difference in energy of the conduction band for one material, and the valence band for another, nicely coincides with the characteristic energy (and therefore wavelength) of the photons that LED is designed to emit.Note also that a photon hitting an electron can give it more energy. If that's enough energy to move into the conductance band then the photon has set up the conditions for an electric current to form. If it's even more then it might kick the electron out of the material entirely. This is the photoelectric effect, it's what earned Einstein a Nobel prize, and it's what drives photodiodes and solar cells. Note that 'photon' is very broad, it can refer to a very long wave infrared photon (i.e. radiated heat) or a very energetic, short wave photon (i.e. a gamma wave, which is how we get silicon gamma detectors). Note that the temperature of a semiconductor material also has an effect on the precise energy levels that electrons are occupying, partially explaining why semiconductors have temperature dependant characteristics.Providing the energy for the whole 'LED process' through simple heat wouldn't work, because you'd be heating both the electron donor materials and the electron acceptor materials. The necessary finely tuned energy differences to get the LEDs characteristic wavelength wouldn't be there.As I say, I qualify as 'partially sighted' in this particular area, so take what I say with a pinch of salt. I'm sure that the knowledgable will be along shortly to correct me, and the other lot (you all knowexactlywho I mean) to quibble with what I've said for no good reason other than they like a good quibble