Gallium nitride (GaN) is a wide bandgap semiconductor with exceptional properties for ultraviolet (UV) photodetection. Its direct bandgap of approximately 3.4 eV enables intrinsic sensitivity to UV radiation while remaining blind to visible and infrared light, reducing the need for optical filters. The material’s high breakdown field, thermal stability, and radiation hardness further make it suitable for demanding applications, including flame detection, environmental UV monitoring, and space-based instrumentation.
The spectral response of GaN-based photodetectors is primarily determined by the material’s bandgap and defect states. Devices typically exhibit peak responsivity in the UVA (320–400 nm) and UVB (280–320 nm) ranges, with cutoff wavelengths near 365 nm for pure GaN. By incorporating aluminum to form AlGaN alloys, the cutoff can be shifted deeper into the UV spectrum, extending sensitivity to UVC (200–280 nm) and even vacuum UV (VUV) regions. The spectral rejection ratio, defined as the responsivity at peak UV wavelength divided by responsivity at 400 nm, often exceeds four orders of magnitude, ensuring high selectivity against visible light.
Responsivity is a key performance metric, typically measured in amperes per watt (A/W). GaN photodetectors achieve responsivities ranging from 0.05 to 0.2 A/W under zero bias (photovoltaic mode), while biased operation (photoconductive mode) can enhance this to several A/W due to internal gain mechanisms. However, gain often comes at the expense of increased noise and slower response times. Quantum efficiencies for well-optimized devices reach 40–60% without antireflection coatings, with improvements possible through surface passivation and nanostructuring. Dark currents are critical for low-light applications; high-quality GaN detectors maintain dark currents below 1 pA at room temperature for active areas around 1 mm².
Flame detection leverages GaN’s ability to sense UV emissions from hydrocarbon combustion, which produces strong spectral lines near 310 nm. Unlike silicon-based detectors, GaN devices avoid false triggers from visible light sources such as incandescent bulbs or sunlight. Flame sensors based on GaN exhibit nanosecond-scale response times, enabling rapid fire detection in industrial settings, aircraft engines, and power plants. Integration with readout electronics allows for compact systems with low power consumption, suitable for battery-operated safety monitors.
UV monitoring applications include environmental sensing of solar UV radiation, which is critical for assessing skin cancer risks and ecosystem health. GaN detectors provide stable, long-term performance under continuous solar exposure, with minimal degradation compared to organic or silicon-based alternatives. Meteorological stations and wearable UV dosimeters utilize these sensors to measure erythemal UV dose, with calibration traceable to international standards. The absence of moving parts or filters enhances reliability in field deployments.
Space instrumentation benefits from GaN’s radiation tolerance and low outgassing, making it ideal for satellite-based UV astronomy and solar monitoring. GaN photodetectors have been employed in missions to study stellar UV spectra, planetary atmospheres, and solar flares. Their immunity to displacement damage from high-energy protons and electrons ensures prolonged operation in harsh orbital environments. Additionally, the material’s tolerance to temperature extremes allows functionality in deep-space missions where thermal cycling is severe.
Device architectures for GaN UV detectors include photodiodes, metal-semiconductor-metal (MSM) structures, and avalanche photodiodes (APDs). Photodiodes offer linear response and low noise, while MSM devices provide simplicity and high-speed operation. APDs, though more complex, deliver internal gain for weak signal detection. Recent advances in epitaxial growth have reduced dislocation densities below 10⁶ cm⁻², minimizing leakage paths and improving breakdown characteristics.
Challenges remain in extending the cutoff wavelength below 250 nm without compromising crystal quality, as higher aluminum content in AlGaN introduces strain and cracking. Passivation techniques using silicon nitride or atomic layer deposition (ALD) oxides have improved surface stability, reducing dark current drift over time. Further progress in doping control and contact engineering will enhance responsivity uniformity across large-area arrays.
In summary, GaN-based photodetectors and UV sensors excel in applications requiring solar-blind operation, fast response, and environmental robustness. Their adoption in flame detection, UV monitoring, and space systems underscores the material’s versatility. Ongoing research aims to push detectivity limits, improve manufacturability, and expand functionality into extreme UV wavelengths for emerging scientific and industrial needs.