In short
- An LED is a p-n junction that turns current into light. Electrons and holes recombine at the junction and release photons; this is electroluminescence.
- The semiconductor’s bandgap sets the colour: E = hν = hc/λ, so λ [nm] ≈ 1240 / E [eV]. Doping controls conductivity and helps tune the emission.
- Drive LEDs with a set current, not a set voltage. Visible LEDs need about 1.8–3.3 V, and the current rises exponentially with voltage: a few millivolts change the current noticeably.
- Temperature matters: heat narrows the bandgap, shifts the colour, lowers efficiency and shortens life. A bare LED emits a Lambertian (cosine) beam that optics can reshape.
Introduction
Light-emitting diodes (LEDs) have become a leading lighting technology, valued for their efficiency and long life. They are used everywhere, from home lighting to advanced medical instruments, and their performance comes straight from semiconductor physics. This article covers the semiconductor technology behind LEDs, how they work, their key electrical, optical and thermal properties, and where they are used.
Semiconductors: the foundation of LEDs
Semiconductors conduct electricity at a level between that of conductors and insulators. They are the core of LED technology, and their properties determine how an LED performs.
Band theory and periodic potentials
Semiconductors have a crystal structure: their atoms are arranged in a repeating lattice. Because of this periodic arrangement, the electrons move in a periodic potential. Bloch’s theorem states that an electron in a periodic potential behaves much like a free electron, but with a modified effective mass, and that its energy is periodic in reciprocal-lattice space. This is what gives a semiconductor its band structure: a valence band and a conduction band separated by an energy gap, the bandgap [1].
Doping and carrier concentration
Doping adds small amounts of impurities to the semiconductor to create free carriers: extra electrons (n-type) or holes (p-type). It changes the intrinsic electrical properties of the material, which allows control of its conductivity and, together with the choice of alloy composition, of the wavelength of the emitted light.
Electroluminescence: how LEDs work
LEDs emit light by electroluminescence: a material emits photons in response to an electric current passing through it.
Electron–hole recombination
At the heart of an LED is a p-n junction, where p-type and n-type material meet. Under forward bias, electrons from the n side and holes from the p side meet at the junction and recombine, releasing their energy as light. The photon energy is close to the bandgap of the semiconductor:
E = hν = hc / λ ⇒ λ [nm] ≈ 1240 / E [eV]
where h is Planck’s constant, ν is the frequency of the emitted light and c is the speed of light [2]. A bandgap of 2.23 eV gives green light at 555 nm; 1.32 eV gives near-infrared light at 940 nm.
Watch: how an LED works
Properties of LEDs
LEDs have distinct electrical, optical and thermal properties that are central to how they work and how they are used.
Electrical properties
Visible LEDs typically need about 1.8 to 3.3 V to start emitting light: red and amber at the low end, blue, green and white at the high end. Infrared LEDs need less, UV LEDs more. The forward voltage is roughly the photon energy divided by the electron charge, plus resistive losses.
Light output rises nearly in proportion to the forward current over the normal operating range. The current itself follows the diode law, I = I₀ (eV/(nVT) − 1), where VT = kT/q is the thermal voltage (about 25.7 mV at 25 °C) and n is the ideality factor [3]. Because the current grows exponentially with voltage, LEDs are driven at a set current.
Optical properties
The colour of an LED depends on the bandgap of its semiconductor: a wider bandgap gives shorter, bluer light. White LEDs usually combine a blue chip with a phosphor.
Efficiency is measured in lumens per watt: the visible light produced per watt of electrical power. For IR and UV LEDs, where lumens mean little, it is given as optical watts per electrical watt.
Temperature effects
Temperature has a strong effect on an LED’s efficiency, colour and lifetime:
As the temperature rises, the bandgap shrinks. The emitted colour shifts to longer wavelengths and the efficiency falls.
Temperature also changes the carrier density and mobility, which changes the voltage–current behaviour: at a fixed current the forward voltage drops as the junction heats up [3]. That drop is a practical way to estimate the junction temperature.
LED photon energy and drive calculator
Enter the LED’s peak wavelength, its forward voltage and current from the datasheet, your supply voltage and the junction temperature. The tool gives the photon energy and frequency, the series resistor for a simple resistor drive, where the power goes, and how sensitive the current is to voltage.
Model: E = hc/λ, ν = c/λ; R = (Vsupply − Vf)/I; PR = (Vsupply − Vf)·I; PLED = Vf·I; current rise from the ideal diode law, e0.01 V/(nVT) − 1 with VT = kT/q. Real LEDs also have series resistance, which softens the rise; n is typically 1–2 for IR and red LEDs and higher for blue and white ones. Choose the nearest standard resistor above the computed value and check its power rating.
Beam geometry and spectral distribution
The emission pattern and the spectrum of an LED are critical for technical and industrial applications.
Beam geometryLambertian, I(θ) = I₀ cos θ
A bare LED typically has a Lambertian (cosine-law) emission pattern: the intensity halves at 60° from the axis, a 120° full beam angle. Lenses and reflectors can reshape this pattern for specific applications, as explained in Precision and Power: Engineering LED Systems.
Spectral distributionintensity versus wavelength
The spectrum shows how the intensity varies with wavelength. A coloured LED has a single peak, typically 15–35 nm wide; a white LED adds a broad phosphor band. The spectrum is critical wherever specific colour characteristics are needed, and it is what converts the LED’s optical watts into lumens, as shown in The Basics of Light Measurement.
Applications of LEDs
LEDs suit a wide range of sectors. General lighting is the largest application, and LEDs are also used in automotive lighting, signalling, displays and indicators, sensing and medical technology, among others. For help choosing an LED for a product, see 10 Bright Ideas for LED Choice; for how LED function is tested in production, see What Is LED Functionality, and How Can It Be Tested?
Conclusion
LEDs are a practical example of advanced semiconductor physics in everyday technology. Continued development and new applications will keep improving their efficiency and versatility.
Whatever the application, the LEDs in a product must be checked: colour, intensity and position on the production line with the IZAK LED tester, and total flux and spectrum in an integrating sphere, as described in LED Inspection Part 1: Integrating Sphere.
References
- Kittel, C. (2005). Introduction to Solid State Physics. John Wiley & Sons.
- Holonyak, N., & Bevacqua, S. F. (1962). Coherent (visible) light emission from Ga(As1−xPx) junctions. Applied Physics Letters.
- Pankove, J. I. (1971). Optical Processes in Semiconductors. Dover Publications.
Frequently asked questions
QHow does an LED produce light?
QWhat forward voltage does an LED need?
QWhy does an LED change colour and dim when it gets hot?
Tzachi Sabati
CEO, IZAK Scientific
Physicist specializing in photonics and quantum technologies, with deep expertise in quantum sensors and advanced optical systems. Leads the Advanced Quantum Lab course at the Technion, bridging academic excellence with industry innovation. At IZAK Scientific, provides cutting-edge photonics-based solutions, developing customized inspection and sensing systems for R&D and production. Passionate about advancing quantum sensing applications and integrating novel technologies to meet industry needs.
