An LED and a photodiode are two semiconductor devices built around a p-n junction, but they use light and electricity in opposite ways. An LED converts electrical energy into light when forward bias pushes electrons and holes together. A photodiode converts light into electrical current when incoming photons create electron-hole pairs.
These devices matter because they are found in displays, fiber-optic links, sensors, remote controls, solar measurement tools, and many modern instruments.
Inside the junction, the band gap sets the energy needed for an electron to move between energy bands. In an LED, electron-hole recombination releases energy as a photon with E = hf, so larger band gaps usually produce higher frequency, shorter wavelength light. In a photodiode, a photon with enough energy can lift an electron across the band gap, creating a charge pair that the junction field separates into current.
This reverse relationship makes the LED and photodiode a powerful pair for understanding how semiconductors link circuits to light.
Understanding Physics: The LED and Photodiode
The material inside a light emitting diode is chosen very carefully. Good light emitters usually use direct band gap semiconductors, such as gallium arsenide compounds or gallium nitride. In these materials, an electron can lose energy as light without needing a large change in momentum.
Silicon has an indirect band gap. Electrons in silicon usually transfer much of their energy to vibrations in the crystal instead of photons.
That is why silicon is excellent for chips and light sensors but poor for visible LEDs. Engineers adjust the material composition to make red, green, blue, or infrared LEDs.
An LED needs a current limiting component in a circuit. Once current begins to flow strongly through a forward biased LED, a small rise in voltage can cause a large rise in current. Too much current heats the junction and can permanently damage it.
A series resistor is common in simple circuits. Its value is selected from the supply voltage, the LED operating voltage, and the intended current.
In brighter lamps, electronic driver circuits control the current more accurately. LED brightness is mainly linked to current, though the relation is not perfectly linear because heating changes the device behavior.
A photodiode is often operated with reverse bias, but it can work without an external voltage in photovoltaic mode. Reverse bias creates a stronger electric field across the depletion region. This field sweeps newly created charges in opposite directions before they recombine.
The result is a faster signal. Speed matters in barcode scanners, optical communication receivers, and pulse sensors. A photodiode current is often very small, so a circuit may use a transimpedance amplifier.
This converts the tiny current into a measurable voltage. The output depends on light intensity, detector area, wavelength, temperature, and the electronics connected to the device.
Real detectors are never perfectly silent. Even in darkness, heat can create charge carriers and produce dark current. Reverse bias can increase this unwanted current.
Random arrival of photons creates another limit called shot noise. These effects make it harder to measure weak light. Students should distinguish light intensity from photon energy.
Brighter light means more photons arriving each second. A different color changes the energy carried by each photon. A photodiode only responds when photon energy is large enough for its semiconductor.
When studying circuit diagrams, pay close attention to the diode symbol direction, the bias arrangement, and whether the task asks about emitted light, detected light, current, or voltage. Those details determine the correct physical explanation.
Key Facts
- An LED emits light when it is forward biased and electrons recombine with holes in the p-n junction.
- A photodiode produces current when light with enough photon energy creates electron-hole pairs in or near the depletion region.
- Photon energy is E = hf = hc/λ.
- The approximate emitted wavelength of an LED is λ = hc/Eg, where Eg is the semiconductor band gap.
- Forward bias lowers the junction barrier, while reverse bias widens the depletion region and helps collect photo-generated charges.
- Silicon photodiodes respond well to visible and near-infrared light, but silicon is not efficient as a visible LED because much recombination does not emit light.
Vocabulary
- p-n junction
- A boundary between p-type and n-type semiconductor regions where charge separation creates an internal electric field.
- LED
- A light-emitting diode that converts electrical energy into photons through electron-hole recombination.
- Photodiode
- A semiconductor diode that converts incoming light into an electrical current by separating photo-generated charges.
- Band gap
- The energy difference between the valence band and conduction band that controls photon absorption and emission in a semiconductor.
- Electron-hole pair
- A mobile electron in the conduction band and the empty state it leaves behind in the valence band.
Common Mistakes to Avoid
- Thinking any LED color can be made by changing only the voltage, which is wrong because the semiconductor band gap mainly determines the photon energy and color.
- Using E = hf but forgetting to convert electron volts to joules, which gives incorrect frequencies or wavelengths when using SI constants.
- Assuming a photodiode always needs forward bias, which is wrong because photodiodes are often operated in reverse bias to widen the depletion region and improve charge collection.
- Treating LEDs and photodiodes as perfectly reversible devices, which is wrong because material choice, efficiency, packaging, and wavelength range affect how well each process works.
Practice Questions
- 1 A red LED emits light with wavelength 650 nm. Calculate the photon energy in joules and in electron volts using E = hc/λ.
- 2 A semiconductor has a band gap of 2.25 eV. Estimate the wavelength of light emitted by an ideal LED made from this material using hc = 1240 eV nm.
- 3 Explain why an LED is usually forward biased while a photodiode used as a light sensor is often reverse biased.