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I know LED stands for Light Emitting Diode; but why does this component need to be a diode to emit light?

My question assumes that the "leds" we see everywhere (for lighting, screens, etc) are actually diodes -- this assumption might be wrong.

noamtm
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    Welcome to EE.SE! Find out everything you need to get started by taking a 2-minute tour (electronics.stackexchange.com/tour). – F.Ahmed Oct 12 '18 at 21:02
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    There were times the light sources were never LEDs. To your question: What is called LED is LED. If someone is calling LED something that is not a LED, they are wrong. – Eugene Sh. Oct 12 '18 at 21:03
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    It doesn't have to be an LED to emit light, however it does have to be a diode to be a light emitting diode. – Robert Fay Oct 12 '18 at 21:04
  • @EugeneSh. of course, there are normal lightbulbs, CFLs, etc -- I mean the type of light people call LED. – noamtm Oct 12 '18 at 21:07
  • As I said, "people" can call things wrong names. If you see a product claiming it is LED, which it is actually not - you probably can sue them for misleading. – Eugene Sh. Oct 12 '18 at 21:08
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    But of course, the LED lightbulb that is commonly sold for household lighting is not a single LED, but a bunch of LEDs and some circuitry (if it is transparent you can see the actual small SMD LEDs inside). – Eugene Sh. Oct 12 '18 at 21:10
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    your question is a bit unclear .... it looks like you are asking something else .... you should simply ask `Why does an LED have to be a diode?` – jsotola Oct 12 '18 at 21:16
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    @jsotola Looks like the opposite. The OP is asking is it really a LEDs what people call LEDs in everyday life. – Eugene Sh. Oct 12 '18 at 21:19
  • the first sentence contains the phrase `why does this component need to be a diode to emit light` ..... `this component` is referring to an LED as explained in the first part of the sentence – jsotola Oct 12 '18 at 21:27
  • @noamtm, please clarify if you are referring to LED that is an electronic component that would be soldered onto circuit boards – jsotola Oct 12 '18 at 21:32
  • Yes, I mean why does it have to be a diode (regardless of the name, that is). – noamtm Oct 12 '18 at 21:36
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    There are also light emitting capacitors (commonly called EL-wire) and light emitting resistors (commonly called incandescent light bulbs) and light-emitting vacuum tubes (commonly called CRTs) and light-emitting gas-discharge tubes (commonly called fluorescent lights) and... – Hearth Oct 12 '18 at 21:48
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    Do you want to know if a two-wire component has to be a rectifying diode to emit light (it doesn't -- see comments above), or are you trying to understand the physics of LEDs (which are fascinating -- an almost better question to ask is "why don't *all* semiconductor diodes emit light?"), or are you looking for something else yet again? – TimWescott Oct 12 '18 at 23:14
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    i do not wish to sound rude ... the more i read your question, the less sense it makes ... maybe your question needs to be re-worded ... right now, your question is something like this one `i know that birds lay eggs, but do they really have to lay eggs?` ... maybe ask about the `light emission from a PN junction` instead of asking if `"what we know as LED" has to be a diode` ... bottom line, it was discovered that a PN junction emits light, and has desirable properties while emitting light ... that discovery has been developed to the level that we see today ... it has to be a diode (PN jnc) – jsotola Oct 13 '18 at 02:10
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    I think OP is onto something. I was once shopping for Christmas lights an asked "are those LEDs?" and the seller said "oh, no, those are just regular diodes, LEDs would be way more expensive". – Agent_L Oct 13 '18 at 09:08
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    @Felthry There's even light emitting wood without the need of electricity ... – Hagen von Eitzen Oct 13 '18 at 10:54
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    @Agent_L LOL, so at least he had an excuse if the device should fail to emit light – Hagen von Eitzen Oct 13 '18 at 10:55
  • And there are light emitting fish and insects that are know to contain electrons. – richard1941 Oct 22 '18 at 18:08

4 Answers4

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The existing answers miss the core of the question.

An LED needs to be a diode, specifically because the way the charge carriers recombine in the forward-biased diode junction releases the correct amount of energy to create photons in the visible range. Passing a current through a chunk of semiconductor with no diode junction in it would simply produce heat.

It's also important for efficiency that the semiconductor be a direct band gap material, so that energy is not lost to phonons (crystal vibrations — heat) rather than photons.

Regular silicon diodes emit light, too, but because the band gap is too low, the photons emitted are in the infrared range, and invisible to the eye. Also, silicon is an indirect band gap material, which greatly reduces its efficiency at producing photons at all.

Dave Tweed
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  • Great techno answer. –  Oct 13 '18 at 00:30
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    Silicon diodes are vastly less efficient light emitters than what we usually call LEDs because silicon is an indirect band bandgap material. And there are also numerous LED types that emit infrared in the same way visible LEDs do, but they aren't made from silicon. – The Photon Oct 13 '18 at 01:09
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    So you need a junction to emit light. Could a device with two (or more) semi-conductor junctions emit light? Is a light-emitting transistor possible? – Jetpack Oct 13 '18 at 01:24
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    @Jetpack: Sure, it's conceptually possible. In fact, [some preliminary work](https://en.wikipedia.org/wiki/Organic_light-emitting_transistor) has been done in that area. – Dave Tweed Oct 13 '18 at 02:18
  • +1. Phonon is a new word to me. You live and learn. –  Oct 13 '18 at 10:59
  • Well, you can get visible light from a semiconductor if you pass through enough current...but yes, it will be black body radiation from the temperature, not from the bandgap, and it will be no more efficient than an incandescent light bulb. – alex.forencich Oct 15 '18 at 02:57
  • What does " the band gap is too low" mean? – Misunderstood Oct 15 '18 at 22:05
  • @Misunderstood: Did you look at the link I provided? – Dave Tweed Oct 16 '18 at 04:29
  • Yes I read the linked article. My question more specifically is: How is a bandgap "too low"? Lesser than what? I am familiar with the term small bandgap. I have never heard the term too low or too small bandgap. – Misunderstood Oct 16 '18 at 15:37
  • Are you sure a regular silicon diode emits IR? I am aware silicon carbide diode emitting visible light, typically 470 nm blue. A Schottky diode is known to emit visible light, but that's also a metal–semiconductor junction. I am not aware of any research indicating that a regular Si p-n diode can emit a photon of any wavelength. I do not know of any semiconductor other than GaAs that can emit IR. Si is often used as a nitride LED dopant to improve crystal quality. Wouldn't "Passing a current through a chunk of semiconductor" emit light if the semiconductor is silicon carbide? – Misunderstood Oct 16 '18 at 16:15
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why does this component need to be a diode to emit light?

By conservation of energy, light emission implies power input. It is normal to deliver electrical power through two wires, so the simplest electric powered light emitter has two wiring terminals, i.e. is a diode.

Two-terminal semiconductors replaced two-terminal tubes (vacuum or gas-filled) having two electrodes, which were called 'diode', and the name has stuck. Electroluminescent panels of yesteryear were also semiconductors that gave off light, but weren't produced in the kinds of high-tech assembly lines that electronic diodes are made in. So, those weren't called diodes.

White "LED" devices around you are not simple semiconductor diodes, but are structures with diodes and phosphors that give off useful amounts of white light, having a blue-emitting diode and red/orange/yellow/green phosphors that convert the blue light. Lenses and other features for effective light emission are common; LEDs do not resemble other practical diodes, except that they have two wires or connecting terminals.

Whit3rd
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    Light emitting electroluminescent solid state devices were, ALWAYS called diodes when it was noticed the first experimental diodes emitted light in 1907. LEDs today still have all the characteristics of a common diode. – Misunderstood Oct 13 '18 at 00:00
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    @misunderstood Yeah it is the voltage vs current characteristics that make it a diode, it has been like that since they were glasses with mercury and whatnot in the early days. The single pn junction is responsible for the characteristics that makes it a diode. – Stian Oct 13 '18 at 07:36
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    The simplest electric powered light emitter is a resistor, a.k.a. incandescent light bulb. It is not a diode. – user71659 Oct 14 '18 at 20:21
  • @user71659 Because an incandescent lamp has two terminals, it can be powered; the terminals are wired (through the filament) together, so it only has one electrode (and wouldn't get the 'diode' name attached). – Whit3rd Oct 14 '18 at 20:51
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    @Whit3rd But user71659 is right though. The simplest electric powered light emitter is not a diode - it is a resistor (incandescent light bulb). Just because it has two terminals does not mean it needs to be called a diode: `having two electrodes, which were called 'diode', and the name has stuck` - that makes zero sense otherwise capacitors, resistors and coils should also be called diodes – slebetman Oct 15 '18 at 02:57
  • @slebetman The wires in vacuum tubes were used for heaters, for support, and for active cathode/grid/plate etc. Only the electrodes count; a 'diode' might have two or three heater terminals as well as multiple wires to the cathode or anode. Only two electrodes (the cathode and anode) were counted in declaring the tube to be a 'diode'. Ballast tubes (similar to incandescent bulbs) were not called diodes. – Whit3rd Oct 15 '18 at 08:51
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LED stands for Light Emitting Diode; but why does this component need to be a diode to emit light?

Because LEDs are a diode which posses the same characteristics a common solid state diode.

My question assumes that the "leds" we see everywhere (for lighting, screens, etc) are actually diodes -- this assumption might be wrong.

Your assumption is correct.

A diode is an electronic component that has low resistance in one direction. It is a dual electrode (anode and cathode) device where electron flow from cathode to anode is low conductance and the primary electron flow is the high conductance flow from anode to cathode.

The most common diodes are made of crystallized semiconductor materials (e.g. silicone, germanium and gallium arsenide, indium phosphide, sapphire, and quartz) which are doped with p and n type impurities which are separated by the simplest semiconductor building block, the p-n junction.

There are many types of diodes with various characteristics. It is the properties of the p and n dopants and their affect on the voltage-current characteristics of the p-n junction that separates one type of diode from another.

The above applies to all diodes including LEDs.

In LEDs the dopants have electroluminescence properties. When the electrons are crossing the p-n junction, many of the electrons are transformed into sub-atomic particles called photons.

Light Emitting Diodes are called diodes because the are indeed semiconductor diodes that also emit photons in the form of UV, visible light, and IR.

Misunderstood
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A diode is the simplest semiconductor device. And, the simplest semiconductor device can also be made to emit light. Voila! Light emitting diode! One could potentially call it a light emitting semiconductor device(LES), but that would just sound like a real name... sooo not cool.

[edited] From the comments below, semiconductor devices without diodes exist. However, they seem to be sensors and do not emit light. The peltier module does transfer heat, but does not create it. So, it is conceivable that a light emitting semiconductor device may be created in future which is not a diode. However, it would likely have a new name (not a diode or LED).

So, thus far, in a TV screen and everywhere else, if it is called an LED, the light (visible or invisible part of the spectrum) actually originates from the LED itself. It is the cheapest efficient light source to produce. There can off course be "fake LED"... neon bulbs or mini incandescent bulbs that look like LED. I have seen those in christmas light decorations, and they are a bit cheaper than the real thing.

Indraneel
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    Is LES a typo? If it is then I don't see how can something which is not a diode be called a diode. – Eugene Sh. Oct 12 '18 at 21:20
  • It is always a diode. A diode is a semiconductor. So a light emitting diode is also a light emitting semiconductor. And for all practical purposes, a light emitting semiconductor is also a light emitting diode (unless you know of a non diode semiconductor that produces light). – Indraneel Oct 12 '18 at 21:26
  • OK, so LES is Light Emitting Semiconductor. That what I wanted to clarify. – Eugene Sh. Oct 12 '18 at 21:28
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    @EugeneSh. to clarify, "LES" is not a real (recognized) term. It is just something I came up with. To only me, it is an acronym for Light Emitting Semiconductor. There is no such thing sold in the market by that name. – Indraneel Oct 12 '18 at 21:29
  • A diode is a semiconductor device, not a semiconductor. – Hearth Oct 12 '18 at 21:50
  • And there are semiconductor devices without diodes in them; photocells come to mind, as well as NTC thermistors and Peltier modules. – Hearth Oct 12 '18 at 21:51
  • A plain, undoped Si wafer is a semiconductor, but doesn't have any diodes. – Shamtam Oct 12 '18 at 21:56
  • Don't forget organic LED's (OLED) and a more recent Quantum LED (QLED). QLED maybe just a Samsung marketing term for a better LED. The goal over time is to migrate QLED quality to the level of a Laser LED, for ultra pure colors. –  Oct 13 '18 at 00:28
  • You drifted off track a little bit, and our answers need to be technically precise as possible, or the OP make pick you apart due to confusion. It would be wise not to create your own acronyms that add confusion and take up space for a better answer. –  Oct 13 '18 at 00:34
  • @Indraneel ""LES" is not a real (recognized) term" - confusingly, LES *is* a real acronym referring to light bulbs - it stands for "Lilliput Edison Screw," the smallest size of Edison Screw fitting (about 5mm diameter) - often used for the bulbs in torches, etc. – alephzero Oct 13 '18 at 16:02
  • @alephzero I'll have to go check my granpa's 5 D cell hunting rifle torch (all brass) to see if you're right about the 5mm..... if I can find where it is. It's the only non LED torch in the house... – Indraneel Oct 13 '18 at 18:46
  • A peltier module definitely produces heat. The heat on its hot side is the sum of the heat moved and the electrical energy used to move that heat. – Dan D. Oct 14 '18 at 10:59
  • @Indraneel "A diode is a semiconductor" is practically true, but there are diodes that are no semiconductors. Vacuum tube diodes - which are no longer in practical use. – Volker Siegel Jul 31 '19 at 00:16