Modelling the Impact of Grain Size on Carrier Concentration & Sensitivity for Gas Sensing Applications

This work presents investigations on a numerical model for cylindrical and spherical shaped grains on n-type sensing film. It has been shown that modulating grain size impacts the surface potential, effective carrier concentration, and sensitivity of a metal-oxide-based gas sensor. The modeled expression presents surface potential function and effective carrier concentration in relation to surface state density for varied grain sizes of radius 2.5nm-40nm. It is observed that for smaller grain size (R< 2L) all charge particles moves from the center of grain to the surface of sensing film when excessive oxygen adsorbents interact with the sensing film. Finally, the situation of full depletion is reached and grains looks depleted of charge particles and result of it the surface potential of the sensing film rises. Further it is validated by mathematical simulations. The movement of charge particles from conduction band of film to surface state energy level (Ess) will continue till the two Fermi energy levels aligned with each other and this situation is named critical surface state density. Critical surface state density is presented for both spherical and cylindrical shape of grain sizes. The results are compared to spherical shape grains with cylindrical in terms of surface charge density, effective carrier concentration and sensitivity. For a particle size of 2.5 nm and a constant volume, the estimated value of Nt,crit for cylindrical grains is 6.5*10⁹ cm⁻², whereas for spherical grains it is 8.1*10⁹ cm⁻². The value of Nt can be calculated using several values of the height of the cylindrical grain. Cylindrical grain shape shows better sensitivity in comparison with spherical grain shape.