LEDs & Diode Sources — Abridged Guide

Quick-reference guide to LEDs and diode sources — band gap physics, material systems, spectral width, efficiency, thermal management, SLEDs, and photonics applications. For full derivations and worked examples, see the Comprehensive Guide.

Comprehensive LEDs & Diode Sources Guide →

1.Introduction

LEDs are semiconductor devices that convert electrical energy directly to light via electroluminescence in forward-biased p-n junctions. They offer quasi-monochromatic emission (20–70 nm FWHM), nanosecond switching, and lifetimes exceeding 50,000 hours.

LEDs fill the niche between broadband lamps (too wide, too much heat) and lasers (too narrow, too coherent). Choose LEDs when you need spectral selectivity without coherent speckle.

2.Types & Classification

Wavelength from Band Gap

\lambda \text{ (nm)} = \frac{1240}{E_g \text{ (eV)}}

Surface-emitting LEDs produce Lambertian beams (120° FWHM). Edge-emitting LEDs produce higher-radiance asymmetric beams (~30° × 120°) better suited to fiber coupling. SLEDs provide amplified spontaneous emission with laser-like beam quality.

Type	Beam FWHM	Typical BW	Best For
Surface-emitting	~120°	15–50 MHz	Illumination, displays
Edge-emitting	~30° × 120°	~200 MHz	Fiber coupling, comms
SLED	~10° × 30°	~200 MHz	OCT, fiber sensing, FOG

White LEDs use phosphor conversion (blue LED + yellow phosphor), not a single white-emitting junction. For color-critical applications, check the CRI and spectral power distribution — not just “white.”

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3.Semiconductor Physics

Photon Energy

E = h\nu = \frac{hc}{\lambda}

Only direct band gap semiconductors (GaAs, GaN, InGaN, AlGaInP) emit light efficiently. Indirect materials (Si, Ge) require phonon-assisted recombination and are extremely poor emitters. This is why LEDs use III-V compound semiconductors.

The forward voltage of an LED approximately equals the band gap voltage E_g/e: ~1.8 V for red, ~3.0 V for blue, ~3.5 V for UV. If you know V_f, you can estimate the wavelength.

4.Material Systems

Three material families cover UV through SWIR: III-nitrides (AlGaN/InGaN) for 200–530 nm, phosphides (AlGaInP) for 540–650 nm, and arsenides (GaAs/InGaAs) for 700–1700+ nm. The “green gap” at 530–580 nm is where neither system performs well.

Material	Range (nm)	Color	V_f (V)
AlGaN	200–365	UV	4–6
InGaN	365–530	Violet–Green	2.8–3.5
AlGaInP	540–650	Yellow–Red	1.8–2.2
GaAs/AlGaAs	630–870	Red–NIR	1.4–1.8
InGaAs/InP	920–1700	NIR–SWIR	0.7–1.0

AlGaInP red LEDs are significantly more temperature-sensitive than InGaN blue LEDs. Budget extra thermal headroom for red sources in hot environments.

5.Spectral Characteristics

Theoretical FWHM

\Delta\nu \approx \frac{1.8\, k_B T}{h} \quad;\quad \Delta\lambda = \frac{\lambda^2}{c}\,\Delta\nu

LED FWHM ranges from ~15 nm (UV) to ~100 nm (SWIR). The spectral width broadens with wavelength because Δλ ∝ λ². Temperature increases both red-shift the peak (~0.1 nm/°C for red LEDs) and broaden the spectrum.

For pump or excitation applications, check what fraction of the LED’s broad emission actually overlaps the target absorption band. A 40 nm FWHM LED may put only 5–10% of its power into a narrow (~3 nm) absorption line — a laser diode is far more efficient in that case.

6.Efficiency Metrics

Wall-Plug Efficiency

\eta_{\text{WPE}} = \frac{P_{\text{optical}}}{I_F \times V_F} = \eta_{\text{EQE}} \times \frac{h\nu}{e V_F}

EQE = IQE × extraction efficiency. WPE accounts for the additional loss of voltage efficiency (V_f > E_g/e). State-of-the-art blue LEDs reach 60–70% WPE; red ~30–40%; green ~20–30%.

Type	EQE (%)	WPE (%)	Efficacy (lm/W)
Blue InGaN	60–80	50–70	—
Green InGaN	25–40	20–30	80–120
Red AlGaInP	30–50	25–40	40–70
PC White	—	35–55	150–220

Efficiency droop at high current means two LEDs at half-current produce more light per watt than one LED at full current. For maximum efficiency, underdriving is always beneficial.

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7.Thermal Management & Output Stability

Junction Temperature

T_j = T_a + (I_F V_F)(1 - \eta_{\text{WPE}}) \times R_{\theta(j\text{-}a)}

Output power drops ~5% per 10°C for blue LEDs and ~10–15% per 10°C for red LEDs. Wavelength shifts ~0.1–0.2 nm/°C (red) or ~0.03–0.05 nm/°C (blue). Lifetime halves for every ~10–15°C above rated T_j.

For spectroscopy and fluorescence work, always use a TEC-stabilized LED source with monitor photodiode feedback. Without active stabilization, output can drift 5–10% over a 20°C ambient temperature swing.

8.Packaging & Mounting

Through-hole and TO-cans for low power; SMD/COB for lighting; butterfly (14-pin) for precision photonics. Butterfly packages integrate TEC, thermistor, monitor PD, and fiber pigtail — the gold standard for stable, fiber-coupled output.

Package	Power	Thermal	Stability	Cost
Through-hole	< 100 mW	Poor	None	$
SMD	0.1–5 W	Moderate	None	$
COB	10–200 W	Good	None	$$
TO-can	< 500 mW	Moderate	Optional PD	$$
Butterfly 14-pin	< 100 mW (fiber)	Excellent (TEC)	TEC + PD	$$$

FC/APC connectors on fiber-pigtailed SLEDs are essential — FC/PC connectors produce ~4% back-reflection that degrades SLED performance and can cause parasitic lasing.

9.Superluminescent Diodes

SLEDs generate amplified spontaneous emission — higher power and spatial coherence than LEDs, broader spectrum than laser diodes. Bandwidths: 5–100+ nm. Key applications: OCT (coherence length = resolution), fiber gyroscopes (reduces coherent backscatter error), and component testing.

SLEDs are fragile regarding back-reflections. Always use an optical isolator or FC/APC termination. Even 0.1% feedback can produce spectral ripple.

10.Driving & Modulation

Series Resistor

R = \frac{V_{\text{supply}} - V_f}{I_f}

Always drive LEDs with a constant-current source, not a voltage source. LED dynamic resistance is very low (~1 Ω), so small voltage changes cause large current swings. PWM dimming preserves spectral shape; analog dimming shifts wavelength.

Modulation Bandwidth

f_{\text{3dB}} = \frac{1}{2\pi \tau_c}

For time-resolved fluorescence, LED pulse drivers can produce optical pulses as short as 1–2 ns. Surface-emitting LEDs support ~50 MHz modulation; edge-emitting LEDs reach ~200 MHz.

11.Selection Guide

Wavelength

Match to absorption band, fluorophore, or detector range.

Spectral width

< 5 nm → laser/SLED. > 20 nm → LED is simpler and speckle-free.

Optical power

Estimate required irradiance accounting for coupling losses.

Beam pattern

Lambertian for area illumination; edge-emitting for fiber.

Package

Butterfly for precision; SMD for PCB integration; free-space for microscopy.

Thermal budget

Calculate T_j. Ensure heatsink keeps T_j below rated maximum.

Follow the selection sequence: wavelength → spectral width → power → beam pattern → package → thermal budget. The LED vs. laser vs. lamp choice comes down to spectral width (lamp > LED > laser), coherence (laser > LED ≈ lamp), and switching speed (laser > LED >> lamp).

If the application needs < 5 nm spectral width, choose a laser diode or SLED with a filter. If it tolerates > 20 nm, an LED is simpler, cheaper, and speckle-free.

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The Comprehensive Guide includes 7 worked examples, 6 SVG diagrams, 3 data tables, and 10 references.