What's the difference between a PN junction diode and a normal LED?

How does 0.7V or greater voltage given to an LED makes light emit at the junction but PN junction doesnt emit any light when
Voltage of 0.7V or greater is given?

Please explain clearly.

 

It is all in the materials used to "dope" the silicon. In this context, the verb "to dope" means to introduce impurities into the pure silicon to create an excess or a deficit of valance electrons. These materials are usually from group III or group V of the periodic table. The emission of visible light, or infrared or ultra-violet radiation happens when an excited electron retunrs to a lower energy quantum state. When it does this, it gives up a quantum of energy in the form of a photon. This photon will have a precise and "quantized" energy level. This implies that it will have a precisely define frequency and wavlength. Planck's constant is the constant of proportionality that relates frequency and energy.

 

It is all in the materials used to "dope" the silicon. In this context, the verb "to dope" means to introduce impurities into the pure silicon to create an excess or a deficit of valance electrons. These materials are usually from group III or group V of the periodic table. The emission of visible light, or infrared or ultra-violet radiation happens when an excited electron retunrs to a lower energy quantum state. When it does this, it gives up a quantum of energy in the form of a photon. This photon will have a precise and "quantized" energy level. This implies that it will have a precisely define frequency and wavlength. Planck's constant is the constant of proportionality that relates frequency and energy.

So, you are telling that materials used in PN junction diode are different than those used in LED?

 

So, you are telling that materials used in PN junction diode are different than those used in LED?

Yes, I am. They both use silicon, what is different is the materials used to dope the silicon. See the following article for a list of the different materials.

https://en.wikipedia.org/wiki/Light-emitting_diode

 

The base material for LEDs is not silicon. It is generally gallium phosphide (GaP), gallium arsenide (GaAs), or a mix of the two, gallium arsenide phosphide (GaAsP). This wafer is then doped with other materials to make the pn junction.
http://www.madehow.com/Volume-1/Light-Emitting-Diode-LED.html

 

The base material for LEDs is not silicon. It is generally gallium phosphide (GaP), gallium arsenide (GaAs), or a mix of the two, gallium arsenide phosphide (GaAsP). This wafer is then doped with other materials to make the pn junction.
http://www.madehow.com/Volume-1/Light-Emitting-Diode-LED.html

My mistake -- I thought the GaP or GaAs were the dopants, not the semiconductor itself.

 

UV ("white" LEDs when used with a phosphor, some single color with a 3.3V drop are also likely GaN) are now based on a common material, Gallium Nitride. The GaN is grown by vapor deposition methods on sapphire (most common), silicon (cheap but difficult and thermal issues) or GaN (expensive but very high power density is possible because there is no thermal expansion mismatch - only one company is making these that I know of).

 

The primary difference in the diodes (and the requirement for different substrates and dopants) is the need for a direct band gap semiconductor for efficient photon generation.
http://www.doitpoms.ac.uk/tlplib/semiconductors/compound.php

You can improve the optical characteristics of silicon (like in solar cells or Thermal imaging) with indium doping.

 

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How does 0.7V or greater voltage given to an LED makes light emit at the junction but PN junction doesnt emit any light when Voltage of 0.7V or greater is given? .................

But a silicon junction can emit light, it's just invisible infrared, such as described here.

The frequency (and thus color) of the emitted light is proportional to the bandgap and forward voltage of the diode, since higher light frequencies have higher energy.
Thus the 0.7V forward drop of a silicon diodes produces lower frequency infrared light, and the higher bandgap, and forward voltage drop of GaAs LED diodes produces higher frequency visible light.

You will notice that LEDs that produce the higher frequency colors towards the blue end of the spectrum typically have higher forward drops.

 

But a silicon junction can emit light

Conversely, a silicon junction also exhibits a photovoltaic effect.

 

Conversely, a silicon junction also exhibits a photovoltaic effect.

Ah, yes, I remember the days (1960's) of the poor man's phototransistor. Scrape the paint off an OC71 (germanium PNP, all glass case) and use plasticene to attach it to the inside of a spin dryer (poor man's centrifuge) to move the 'jelly' away from the actual transistor inside.