Ultraviolet (UV) photodetectors are critical components in applications requiring precise detection of UV radiation, such as flame sensing, environmental monitoring, and space instrumentation. Unlike visible or infrared detectors, UV photodetectors must exhibit high sensitivity to UV wavelengths while suppressing response to longer wavelengths. Wide and ultra-wide bandgap semiconductors like zinc oxide (ZnO), gallium nitride (GaN), and silicon carbide (SiC) are particularly suited for this purpose due to their intrinsic bandgap properties, thermal stability, and radiation hardness. This article examines the materials, device architectures, performance metrics, and key applications of UV photodetectors.

ZnO, GaN, and SiC possess direct or indirect bandgaps that align well with the UV spectrum. ZnO has a direct bandgap of approximately 3.37 eV, making it responsive to UV-A (320–400 nm) and UV-B (280–320 nm) radiation. Its high exciton binding energy (60 meV) enhances radiative efficiency, while its low-cost synthesis routes, such as hydrothermal growth and sputtering, make it attractive for commercial applications. GaN, with a bandgap of 3.4 eV, is widely used for solar-blind UV-C (200–280 nm) detection due to its tunability through aluminum incorporation (AlGaN), which extends the cutoff wavelength. SiC, an indirect bandgap material (3.2 eV for 4H-SiC), is valued for its high thermal conductivity and robustness in harsh environments, including high-temperature and high-radiation conditions.

Two dominant device designs for UV photodetectors are metal-semiconductor-metal (MSM) and Schottky photodiodes. MSM detectors consist of interdigitated metal electrodes on a semiconductor surface, forming back-to-back Schottky contacts. This design offers low capacitance, fast response times, and ease of fabrication. For example, ZnO-based MSM detectors with gold electrodes demonstrate responsivities exceeding 1 A/W at 365 nm, with dark currents below 1 nA at 5 V bias. GaN MSM detectors with AlGaN active layers achieve solar-blind operation, showing peak responsivity at 280 nm and rejection ratios greater than 10^4 for visible light. Schottky photodiodes, on the other hand, utilize a single rectifying contact to separate photogenerated carriers under reverse bias. SiC Schottky detectors exhibit low dark currents (pA range) and high breakdown voltages, making them suitable for high-power applications.

Key performance metrics for UV photodetectors include responsivity, quantum efficiency, dark current, response time, and spectral selectivity. Responsivity, measured in A/W, quantifies the photocurrent generated per unit of incident optical power. High-quality GaN detectors achieve responsivities of 0.1–0.2 A/W in the solar-blind region, while ZnO detectors can exceed 10 A/W under bias due to internal gain mechanisms. External quantum efficiency (EQE), the ratio of collected electrons to incident photons, often exceeds 80% for optimized GaN devices. Dark current, a critical parameter for low-light applications, is minimized through defect reduction and proper passivation. For instance, SiC Schottky detectors maintain dark currents below 10^-12 A/cm^2 at room temperature. Response times, dictated by carrier transit and recombination, range from nanoseconds in MSM devices to microseconds in photovoltaic-mode detectors. Spectral selectivity is enhanced by bandgap engineering and optical filtering, ensuring minimal response outside the target UV range.

Flame detection is a prominent application of UV photodetectors, as fires emit characteristic UV radiation below 300 nm. Solar-blind GaN detectors are ideal for this purpose, as they avoid interference from solar background radiation. These detectors are integrated into industrial and residential fire alarms, offering rapid response times (<1 ms) and high reliability. In space instrumentation, UV photodetectors monitor solar activity, ozone depletion, and cosmic phenomena. SiC-based detectors are favored for satellite payloads due to their resistance to cosmic rays and thermal cycling. For example, NASA missions employ SiC photodiodes for UV spectrometers, where stability under prolonged radiation exposure is critical.

Challenges in UV photodetector development include defect-mediated dark current, long-term stability, and scalability. ZnO detectors often suffer from oxygen vacancy-related traps, which increase noise and degrade responsivity over time. GaN devices require expensive substrates (e.g., sapphire or SiC) and complex epitaxial growth to minimize threading dislocations. SiC detectors face limitations in achieving high quantum efficiency due to indirect bandgap transitions. Advances in material engineering, such as doping (Mg in ZnO, Si in GaN) and heterostructure design (AlGaN/GaN superlattices), aim to address these issues.

Future directions include the integration of nanostructured materials (ZnO nanowires, GaN quantum dots) to enhance light absorption and reduce defect densities. Hybrid designs combining 2D materials (graphene electrodes) with conventional semiconductors may improve charge collection efficiency. Additionally, AI-driven optimization of growth parameters could yield higher-performance devices at lower costs.

In summary, UV photodetectors based on ZnO, GaN, and SiC offer tailored solutions for demanding applications in flame detection and space instrumentation. Their performance is closely tied to material properties, device architecture, and fabrication quality. Continued research into defect mitigation and novel device geometries will further expand their utility in industrial, environmental, and scientific fields.