LED Papers

A comparative review and synthesis of five seminal papers on LED-chip technology, performance optimisation and emerging applications

Abstract

Light-emitting-diode (LED) chips have progressed from early phosphor-converted white sources to sub-10 µm micro-emitters that double as gigabit‐per-second optical transmitters. In this review we analyse five representative papers that map the trajectory: (1) deep-learning-based defect inspection for GaN chips; (2) the first phosphor-converted near-UV white LED (2003); (3) micro-LED power-efficiency modelling and size optimisation; (4) a three-band white LED using green/red phosphors precoated on a blue die; and (5) a record-speed visible-light-communication (VLC) link built with series-biased micro-LED arrays. By contrasting their experimental methods, physical insights and reported metrics we derive cross-cutting lessons for yield, colour quality, energy efficiency and data-centric functionality, and outline open research directions.

1 Introduction

Gallium-nitride epitaxy and advanced packaging have made LED chips ubiquitous—from high-bay luminaires to micro-displays and optical wireless links. Yet each application sets different figures of merit: low defect density, stable chromaticity, high external quantum efficiency (EQE), or modulation bandwidth. The chosen five papers, spanning 2003-2020, illuminate how researchers addressed these demands at successive technology nodes.

2 Paper-by-Paper Analysis

#	Focus	Key contributions	Take-away for practitioners
1	Automated defect inspection of LED chips using a CNN (Lin et al., 2019)	Introduces LEDNet, a class-activation-mapping CNN that localises line blemishes & scratches with 5.04 % error, eliminating rule-based inspection	AI-driven optical inspection can be trained on die images alone, reducing false rejects and enabling inline adaptive process control.
2	White-light emission from a near-UV InGaN–GaN chip precoated with RGB phosphors (Sheu et al., 2003)	First demonstration of “n-UV + RGB” phosphor mix yielding broad-spectrum white with higher colour-rendering than blue-pump/yellow designs	Choice of UV pump (≈400 nm) decouples CCT tuning from InGaN indium content and eases EQE droop at high drive.
3	Improving power efficiency of micro-LED displays via chip-size optimisation (Hsiang et al., 2020)	Physics-based model shows an optimum chip length (16–20 µm for TVs, 6–8 µm phones) that saves 30-40 % power; non-uniform RGB sizing cuts a further 12 %	Size–efficiency trade-off (sidewall losses vs. current density) must be co-optimised with ambient reflectance and PWM duty.
4	Three-band white light from InGaN blue chip precoated with green/red phosphors (Wu et al., 2005)	Adds red-green phosphors onto a blue-pump chip to form discrete blue-green-red peaks with improved colour-rendering index (CRI > 90) and 14 lm W⁻¹ efficacy at 20 mA	Pre-coating enables thinner colour layers and lower thermal resistance than cup-mixed phosphor, useful for mid-power packages.
5	High-speed visible-light communication with series-biased micro-LED arrays (Xie et al. 2019, reported in IEEE Spectrum)	16-pixel blue micro-LED array reached 11.74 Gb s⁻¹ @ 0.3 m and error-free 1.61 Gb s⁻¹ @ 20 m using discrete-multi-tone OFDM	Series connection raises optical power without increasing capacitance, pushing −3 dB bandwidth > 1 GHz—key for LiFi and AR headsets.

3 Comparative Discussion

3.1 Manufacturing yield & inspection

Paper 1 quantifies how CNN-based localisation can close the loop between epi growth, lithography and sawing. Defect escape rates below 0.5 % are now realistic, which is critical for micro-LED tiling where millions of dies populate one module.

3.2 Spectral engineering for high-CRI white

Papers 2 and 4 illustrate the evolution from broad “blue + YAG” spectra to multi-peak approaches. The near-UV pump sacrifices some wall-plug efficiency but gains colour flexibility; pre-coated trichromatic phosphors solve the red-deficit problem intrinsic to YAG:Ce.

3.3 Chip-size-dependent efficiency

The model of Paper 3 bridges device physics (surface recombination length) and system metrics (display power). Its conclusion—there exists an application-specific optimum size—is now echoed in foundry PDKs that bin micro-LED lots by EQE vs. length.

3.4 Beyond illumination: data links

Paper 5 expands the LED role to a dual-function emitter/antenna. Combining series bias with DMT-OFDM overcomes the RC-limited bandwidth of larger dies. Future display panels could embed gigabit uplinks without extra lasers.

4 Synthesis & Future Research Themes

Integrated sensing and communication – Marry CNN-based defect sensors (Paper 1) with on-chip photodiodes for real-time health monitoring of micro-display tiles.

Hybrid phosphor–quantum-dot stacks – Extend the pre-coating strategy (Paper 4) with perovskite QDs to attain Rec. 2020 gamut while curbing blue-light overdrive.

Thermal-electrical co-design – Incorporate Paper 3′s size model into finite-element thermal tools; optimise duty-cycle and heatsink design concurrently.

Terabit-scale VLC arrays – Building on Paper 5, investigate wavelength-division multiplexed micro-LED matrices (RGB or RGBY) for 100 Gb s⁻¹ indoor LiFi, as hinted by recent multicolour demonstrations .

5 Conclusion

The five papers collectively chart a path from making LEDs work to making LEDs work harder—whether that means higher lumen-per-watt, richer colour, lower defectivity or multi-gigabit signalling. Their insights are complementary: materials science improves quantum yield; packaging refines colour; circuit-level innovation unlocks speed; and machine vision safeguards yield. The next decade will likely see these threads converge in µLED-on-silicon systems that illuminate, display and communicate in the same chip stack.

References

Lin H. et al. “Automated defect inspection of LED chip using deep convolutional neural network,” J. Intell. Manuf., 30, 2525 – 2534 (2019).

Sheu J-K. et al. “White-light emission from near-UV InGaN–GaN LED chip precoated with blue/green/red phosphors,” IEEE Photonics Technol. Lett. 15 (1), 18 – 20 (2003).

Hsiang E-L. et al. “Improving the Power Efficiency of Micro-LED Displays with Optimized LED Chip Sizes,” Crystals 10 (6), 494 (2020).

Wu H. et al. “Three-band white light from InGaN-based blue LED chip precoated with green/red phosphors,” IEEE Photonics Technol. Lett. 17 (6), 1160 – 1162 (2005).

Xie E. et al. “High-speed visible light communication based on a III-nitride series-biased micro-LED array,” coverage in IEEE Spectrum, 2019.