Sensors and Actuators B 97 (2004) 387–390 Detection mechanism of metal oxide gas sensor under UV radiation Sunita Mishra a,∗ , C. Ghanshyam a , N. Ram a , R.P. Bajpai a , R.K. Bedi b a Microelectronics Instrumentation Division, Central Scientific Instruments Organisation, Sector-30, Chandigarh 160020, India b Department of Physics, Material Science Laboratory, GND University, Amritsar, India Received 2 June 2003; received in revised form 25 July 2003; accepted 12 September 2003 Abstract The effect of ultraviolet radiation on the sensing mechanism of polycrystalline metal oxide gas sensor has been studied analytically. The model used to describe the sensing mechanism is based on the combination of the neck mechanism and grain boundary mechanism. We found that increasing the UV radiation flux density increases the conductivity of the film by decreasing the resistance. It has been shown theoretically that due to incident UV radiation, it is possible to detect the gas even at room temperature. The effect of radiation on the sensitivity is discussed as a function of grain size and chemisorbed gas concentration. © 2003 Elsevier B.V. All rights reserved. Keywords: UV radiation; Gas sensor; Metal oxide; Grain size; Sensitivity; Grain boundary 1. Introduction Semiconductor gas sensors are widely used for sev- eral applications in gas sensing. The device responds to the change in the electrical conductivity occurring in the surface–surrounding atmosphere. It is well known that the performance of these semiconductor gas sensors, which are generally metal oxides, is related to their structural and electronic properties such as grain size distribution, local doping, grain boundaries and surface states. These oxidic semiconductors show their sensing behaviour around the temperature range of 175–425 ◦ C. J. Saura [1] studied the gas sensing properties of SnO 2 films subjected to UV ra- diation and found that thermally-treated SnO 2 films are capable of fast detection of gaseous compounds even at room temperature. It has been seen that thermally-treated pyrolytic SnO 2 films develop strong conductivity changes under irradiation with band gap light [2]. Many authors have shown that SnO 2 films are sensitive to oxygen and other reducing gases like CO under UV light illumination at room temperature [3,4]. Comini et al. [5] have shown that exposure of UV radiation results in decrease in the response and recovery time of tin oxide gas sensor at low temperature with no poisoning effect when NO 2 come in contact. Messias et al. [6] have studied the electron scat- tering and effect of light sources on photoconductivity of SnO 2 coatings prepared by sol–gel method. Several exper- ∗ Corresponding author. E-mail address: s mishra8@rediffmail.com (S. Mishra). imental studies are available on the effect of UV radiation on the metal oxide sensing properties but no work has been done on the theoretical modeling of the sensing mechanism in the presence of UV radiation. In the present paper, we have developed the theory for the detection mechanism of metal oxide thin films under UV illumination. It has been assumed that the metal oxide films are polycrystalline in nature and that the metal oxide grains are connected to each other either by grain bound- aries or necks. When UV radiation falls on the metal oxide films, electron–hole pairs are generated and it increases the intra-grain conductivity by modifying the surface potential. The theory developed is presented below. 2. Theory Fig. 1 shows the polycrystalline structure of the metal oxide made up of grains. The figure includes the effects of grain boundaries as well as neck. It is assumed that the resistance is mainly due to the neck resistance R n and grain boundary resistance R gb . When UV radiation falls on the metal oxide polycrys- talline film, electron–hole pairs are generated in the grain depletion region. Photo excitation decreases the inter-grain barrier height, thereby increasing the density of free carriers throughout the material. Under the depletion approximation, the Poisson’s equation is given by d 2 V(x) dx 2 =- q ε (N d - n) (1) 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.09.017