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LED vs Laser Diode: Light Emission, Beam Properties and Uses Compared

LEDs and semiconductor lasers both pass forward current through a PN junction and emit light when electrons and holes recombine. An LED's light comes from spontaneous emission, so it has a wide range of wavelengths and spreads in many directions. A semiconductor laser has a cavity that bounces light back and forth and amplifies it by stimulated emission, producing an intense, straight beam with matched wavelength and phase. LEDs are used for lighting and displays; lasers for optical communication, optical discs and sensing.

AspectLEDLaser diode
Emission mechanismSpontaneous emission: electrons and holes recombine at random times and emit lightStimulated emission: light generates more light of the same wavelength, phase and direction, and is amplified
Optical cavityNoneYes. Light bounces between cleaved facets (edge-emitting) or multilayer mirrors (VCSEL)
Wavelength range (spectral width)Wide (around tens of nm)Narrow (can be 1 nm or less; DFB lasers emit at a single wavelength)
Beam spreadSpreads over a wide angleNarrow; can be focused tightly or sent over long distances
Phase alignment (coherence)LowHigh
Current vs light outputLights up as soon as current flows, with brightness roughly proportional to currentLasing starts above the threshold current and light output rises sharply; below threshold there is only weak spontaneous emission
Modulation speedRelatively slowFast; some lasers for optical communication are directly modulated by current to send signals at tens of Gb/s
Effect of temperatureEmission efficiency drops as temperature risesThreshold current rises and wavelength shifts as temperature rises, so temperature control is sometimes used
Ease of useSimple drive circuit and long lifetimeOften needs control to keep light output constant. Intense light is hazardous to the eyes, so laser safety standards (classes) apply
Typical usesLighting, display backlights, indicator lights, remote controlsFiber-optic communication, optical discs, laser printers, LiDAR, face-recognition sensors, laser machining

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

LED
When you want to light a wide area or use it as an indicator. It is cheap, long-lived and simple to drive.
Laser diode
When you want to focus light tightly and send it far, modulate it quickly to carry information, or need light with matched wavelength. Used for optical communication, distance measurement and machining.
What they share
In both, the color (wavelength) of the light is set mainly by the band gap of the semiconductor in the emitting layer. Direct-bandgap compound semiconductors with high emission efficiency, such as GaN-based, GaAs-based and InP-based materials, are used; silicon, an indirect-bandgap material, is not suited to light emitters.

FAQ

How is laser light different from LED light?

Laser light has matched wavelength and phase and travels straight with little spread, while LED light covers a wide range of wavelengths and spreads in many directions. This is because an LED emits by spontaneous emission, while a laser emits by stimulated emission inside a cavity.

What is the threshold current of a laser diode?

It is the current at which lasing begins. Lasing starts when amplification of light by stimulated emission in the cavity exceeds the losses; above that current, light output rises sharply with current. Below threshold, only weak, LED-like spontaneous emission comes out.

What determines the color of an LED?

Mainly the band gap of the semiconductor in the emitting layer, following the relation wavelength λ(nm) ≈ 1240 / Eg(eV). InGaN is used for blue and green, and AlGaInP and similar materials for red. Most white LEDs convert part of the light from a blue LED to yellow and other colors with a phosphor and mix them.

Why is it hard to make LEDs or lasers from silicon?

Silicon is an indirect-bandgap semiconductor, so for an electron and hole to recombine and emit light, a phonon (lattice vibration) must also be involved, making emission efficiency very low. Silicon photonics therefore combines silicon with lasers made of compound semiconductors such as InP.

Check your understanding

Which correctly describes the difference between a semiconductor laser and an LED?

  1. AAn LED has a narrower wavelength spread and better monochromaticity
  2. BA semiconductor laser uses stimulated emission and an optical resonator to emit highly directional light with uniform wavelength and phase
  3. CA semiconductor laser emits light without current
Show answer and explanation

Answer: B. A semiconductor laser uses stimulated emission and an optical resonator to emit highly directional light with uniform wavelength and phase

LEDs emit by spontaneous emission, with spectral widths of tens of nm, spreading in many directions. A semiconductor laser drives current above threshold in an optical cavity formed by cleaved facets or similar, producing stimulated emission: coherent, highly directional light with a narrow linewidth (even below 1 nm). Lasers serve optical communication, optical discs, laser printers, LiDAR and materials processing.

What mainly determines the emission color (wavelength) of an LED?

  1. AThe shape of the package
  2. BThe direction of the drive current
  3. CThe band gap of the semiconductor used in the light-emitting layer
Show answer and explanation

Answer: C. The band gap of the semiconductor used in the light-emitting layer

An LED emits light with energy near the band gap when electrons and holes recombine; wavelength λ(nm) ≈ 1240/Eg(eV). InGaN at about 2.7 eV emits blue at about 460 nm, and AlGaInP at about 1.9 eV emits red at about 650 nm. Adjusting the active-layer composition tunes the band gap—more indium in InGaN shifts it to green. White LEDs combine a blue LED with a yellow phosphor.

How does an LED emit light?

  1. AElectrons and holes injected under forward bias emit light when they recombine
  2. BIt is heated by reverse breakdown current and glows by thermal radiation
  3. CCurrent flows when light falls on it
Show answer and explanation

Answer: A. Electrons and holes injected under forward bias emit light when they recombine

When an LED is forward-biased, electrons from the n side and holes from the p side are injected into the junction and emit light with energy near the band gap when they recombine. The color is set by the material: AlGaInP for red, InGaN for blue and green. White LEDs combine a blue LED with a yellow phosphor. Efficient emission requires a direct-gap material, so practical LEDs cannot be made from silicon.

  1. Which semiconductor laser has a built-in diffraction grating so that it oscillates stably at a single wavelength for optical communication?
  2. In an indirect-gap semiconductor such as silicon, why is the probability of electron-hole recombination with light emission low?
  3. Which value is closest to the energy of a single photon of light with a wavelength of 620 nm? (E [eV] ≈ 1240 / λ [nm])

Practice Image sensors and optical devices questions

Key terms explained

Forward bias
Applying a positive voltage to the p side and a negative voltage to the n side. The barrier drops and current flows readily. The opposite is reverse bias, under which almost no current flows.
Direct band gap
A band structure in which the bottom of the conduction band and the top of the valence band are at the same wave number. Electron-hole recombination emits light efficiently. Example: GaAs.
Band gap
The energy difference between the top of the valence band and the bottom of the conduction band. About 1.12 eV for silicon at room temperature. A larger band gap means better tolerance to high temperatures and voltages.
Quantum well
A structure in which a thin layer, a few nanometers thick, with a small band gap is sandwiched between layers with a larger band gap. It confines carriers to promote recombination and is used as the active layer of LEDs and lasers.
VCSEL (vertical-cavity surface-emitting laser)
A semiconductor laser that emits light perpendicular to the substrate. It can be tested on the wafer and easily arranged in 2D arrays, so it is used in data links and as the light source for face recognition.

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