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.
LED Diodes – Why LEDs Must Be Diodes
diodesledlight
Related Solutions
You get more lumens per watt from phosphor converted LEDs. You can tune the color temperature with phosphor composition. Additionally, blue diode used in these diode is very stable with temperature variations so when the temperature of the die increases you do not loose much lumens.
RGB mixed white LEDs do not have high efficacy (lm/W). The color point is not stable because the red diode used in the chip is very temperature dependent. When the temperature of the die rises, the color point of the whole LED will shift towards cyan and the overall efficacy drops.
These two structures will have different specra and this it the cause for different efficacy figures. Eye has its peak sensitivity around 555nm. Take a look at V(λ) function:
and compare it to the spectrum of phosphor converted LED:
and RGB LED:
You can clearly see the PC white LED has much bigger overlap.
One more advantage of phosphor converter LED over RGB ones is color mixing. RGB diode will have three separate dies for generating red, green and blue light. If you use this type of LEDs in a lamp the overall light will be a mix of three colors originating from different places. This may give a color shadow effect which is sometimes undesired.
On automobiles, the same LEDs are used for both taillights and brakelights. When braking, they're on full — no flickering — but when they're just taillights, they're dimmed, so they flicker. Very annoying.
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Best Answer
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.