Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
| Bias needed to emit light | LED: forward bias only | Photodiode: reverse bias for operation |
| Energy conversion | LED: electrical energy to light | Ordinary diode: energy mostly to heat |
| Material | LED: GaAs, GaAsP, GaP | Ordinary diode: Si, Ge |
| Turn-on voltage | LED: about 1.8 to 3 V | Silicon diode: about 0.7 V |
No. An LED emits light only in forward bias. In forward bias, electrons and holes are pushed toward the junction, they recombine, and release energy as light. In reverse bias almost no current flows, so there is no recombination and no light. Also, LEDs have a very low reverse breakdown voltage (around 5 V), so reverse bias can even damage them.
Both are p-n junctions and both have recombination. The difference is the material. Ordinary silicon and germanium diodes release the recombination energy mostly as heat (their band structure favours heat). LEDs are made from special compound semiconductors like GaAs, GaAsP and GaP, which release the energy as photons of visible or infrared light. So the material band structure, not the diode idea, decides whether you get light.
The colour depends on the energy gap (Eg) of the semiconductor. The photon energy of the emitted light is roughly equal to Eg, and photon energy is E = hc/lambda. A larger Eg gives shorter wavelength (toward blue/violet); a smaller Eg gives longer wavelength (toward red/infrared). So by choosing a material with the right Eg, we choose the colour.
To emit visible light the photon energy (and so Eg) must be about 1.8 eV or more. Since the turn-on voltage roughly matches this energy, an LED typically needs about 1.8 V to 3 V, higher than the 0.7 V of silicon. A resistor is always put in series with an LED to limit the current, otherwise the current becomes too large and burns the LED.
Electroluminescence means giving out light because of an electric current, not because of heat. In an LED, forward current drives electrons and holes to the junction where they recombine. Each recombination releases energy equal to the band gap as a photon of light. This direct conversion of electrical energy into light is electroluminescence.
An LED is constructed from a p-n junction diode using GaAsP. The energy gap is 1.9 eV. The wavelength of the light emitted will be equal to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An LED is a light emitting diode: a p-n junction that turns electrical energy directly into light when it is forward biased. The light comes from electrons and holes recombining at the junction.
Only in forward bias. Forward current causes recombination that releases photons. In reverse bias there is no current and no light, and reverse bias can damage the LED.
Compound semiconductors such as gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP). Their energy gap is chosen to get the desired colour.
The emitted photon energy is about equal to the energy gap Eg. Using E = hc/lambda, a larger Eg gives shorter wavelength (blue side) and a smaller Eg gives longer wavelength (red or infrared).
To limit the current. Once the LED turns on, its resistance is very low, so without a series resistor the current would rise too high and destroy the LED.
