Introduction

Semiconductor gas sensors based on metal oxide materials are widely used in environmental monitoring, industrial safety, automotive systems, and air quality control because of their high sensitivity and simple fabrication process. These sensors operate by detecting changes in electrical conductivity caused by interactions between the sensor surface and surrounding gas molecules. The sensing performance of metal oxide semiconductors strongly depends on their structural, electrical, and surface properties, including grain size distribution, grain boundaries, doping concentration, surface defects, and electronic surface states.

Conventional metal oxide gas sensors generally require elevated operating temperatures, typically between 175 °C and 425 °C, to achieve sufficient sensitivity and fast response characteristics. However, high-temperature operation increases power consumption and limits their application in portable, flexible, or temperature-sensitive environments. To overcome these limitations, ultraviolet (UV) radiation has been explored as an alternative activation source for enhancing gas sensing performance at low or room temperature conditions.

Previous studies demonstrated that UV illumination can significantly improve the sensing properties of tin oxide (SnO₂) thin films and other metal oxide semiconductors. Experimental investigations showed that thermally treated SnO₂ films exhibit rapid gas detection capability even at room temperature when exposed to UV light. Other reports confirmed that UV radiation enhances the conductivity of SnO₂ films and improves their sensitivity toward oxygen and reducing gases such as carbon monoxide.

Researchers have also observed that UV-assisted operation decreases both response time and recovery time of metal oxide gas sensors while minimizing poisoning effects caused by certain gases such as nitrogen dioxide. Additional studies examined the influence of light irradiation on the photoconductivity and electron transport properties of SnO₂ coatings prepared through sol–gel and related fabrication techniques.

Although many experimental investigations have analyzed the effect of UV illumination on gas sensing characteristics, limited attention has been given to the theoretical understanding of the sensing mechanism under UV radiation. Therefore, this work focuses on developing a theoretical model describing the behavior of polycrystalline metal oxide thin films exposed to ultraviolet light during gas sensing operation.

Theoretical Description of UV-Assisted Gas Sensing

The proposed model considers a polycrystalline metal oxide film composed of interconnected grains. These grains are electrically connected through grain boundaries and narrow conductive neck regions. In such structures, the overall electrical resistance of the sensor mainly originates from two components:

  • Grain boundary resistance
  • Neck resistance between adjacent grains

When UV radiation illuminates the metal oxide surface, photons with sufficient energy generate electron–hole pairs inside the depletion region surrounding the grains. The generation of these charge carriers modifies the surface potential and increases the conductivity within the grains.

Photoexcitation reduces the energy barrier existing between neighboring grains, which allows a larger number of free charge carriers to move through the material. As a result, electron transport across grain boundaries becomes easier, leading to a significant reduction in electrical resistance.

The model also assumes that electrical conduction inside the sensor occurs mainly through thermionic emission mechanisms across grain boundaries. Under UV illumination, the density of photo-generated carriers increases, reducing the depletion layer width and lowering the inter-grain potential barrier. Consequently, carrier mobility and current flow improve throughout the sensing layer.

In polycrystalline metal oxide films, the neck regions connecting adjacent grains play a dominant role in determining sensor resistance. The conductivity of these neck regions depends on several factors, including:

  • Grain size
  • Depletion width
  • Carrier concentration
  • Electron mobility
  • UV radiation intensity

The total resistance of the sensor is therefore influenced by both grain boundary resistance and neck resistance. Additionally, adsorption of oxygen species and reducing gases on the metal oxide surface modifies the depletion region thickness, which directly affects sensor conductivity and sensitivity.

Gas sensitivity is generally defined as the ratio between sensor resistance in air and sensor resistance in the presence of target gas molecules. Adsorbed oxygen species trap electrons from the semiconductor surface, creating a depletion region and increasing resistance. When reducing gases react with these oxygen species, trapped electrons are released back into the material, decreasing resistance and producing the sensing signal.

Results and Discussion

Theoretical calculations show that gas sensitivity strongly depends on grain size and UV radiation intensity. Smaller grain sizes provide higher sensitivity because a larger fraction of the grain volume becomes influenced by surface depletion effects. As grain size increases, sensitivity decreases due to reduced surface-to-volume interaction.

The simulations also indicate that UV illumination dramatically enhances gas sensing performance at room temperature. Without UV radiation, chemisorbed oxygen species remain thermally stable, resulting in weak sensor response under low-temperature conditions. Under UV exposure, however, additional charge carriers are generated, significantly improving sensitivity and conductivity.

Increasing UV radiation intensity further enhances the sensing response. Higher photon flux produces more electron–hole pairs, increasing carrier density and reducing sensor resistance. This behavior explains the improved detection capability observed experimentally under UV-assisted operation.

The model additionally demonstrates that sensor resistance decreases as the concentration of adsorbed reducing gas increases. This reduction becomes much more pronounced when UV radiation is present. Comparisons between neck resistance and grain boundary resistance reveal that neck resistance contributes most significantly to the total resistance of the sensor.

Temperature-dependent analysis shows that resistance decreases as temperature rises, especially at lower temperature ranges. However, under UV illumination, significant conductivity enhancement can already be achieved at room temperature, reducing the need for conventional high-temperature activation.

Theoretical results therefore confirm that UV radiation effectively activates metal oxide gas sensors by enhancing carrier generation, lowering grain boundary barriers, and improving electron transport throughout the sensing film.

Conclusion

This theoretical study demonstrates that UV radiation can significantly improve the sensing performance of metal oxide semiconductor gas sensors. UV illumination enhances electrical conductivity by generating electron hole pairs and reducing inter-grain potential barriers, allowing efficient gas detection even at room temperature.

The analysis shows that sensor sensitivity increases with increasing UV radiation intensity and decreases with larger grain size. In addition, UV-assisted operation greatly reduces sensor resistance compared to operation without illumination.

These findings suggest that UV activation provides an effective strategy for lowering the operating temperature of metal oxide gas sensors while maintaining high sensitivity and fast response characteristics. This improvement expands the applicability of semiconductor gas sensors in low-power, portable, and temperature-sensitive sensing systems used in environmental monitoring, industrial safety, biomedical diagnostics, and smart sensor technologies.