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A4.2.1

A Novel Theoretical Model for Semiconductor Oxide Gas Sensor Satyajit Shukla and Sudipta Seal* University of Central Florida (UCF) Mechanical Materials Aerospace Engineering (MMAE) Department and Advanced Materials Processing and Analysis Center (AMPAC) Engineering # 381 4000 Central Florida Blvd. Orlando, FL 32816 Phone: (407) 823-5227 Fax: (407) 823-0208 E-mail(s): [email protected], [email protected] *

To whom the correspondence should be addressed.

ABSTRACT A new constitutive equation for the gas sensitivity of n-type semiconductor oxide thin film gas sensor has been proposed here based on a single-crystal model. The derived constitutive equation shows the dependence of the gas sensitivity on various critical parameters such as nanocrystallite size, space-charge-layer thickness, reducing gas concentration, bulk charge-carrier-concentration, surface-density of states, oxygen-ion-vacancy concentration, operating temperature, and film thickness. The present theoretical model is applicable to all n-type semiconductor oxides gas sensors. INTRODUCTION When a polycrystalline n-type semiconductor oxide thin film, such as tin oxide (SnO2) and titania (TiO2), is exposed to air, the physisorbed oxygen molecules pick-up electrons from the conduction band of these oxides and change to O2-ads or O-ads species [1]. Consequently, positive and negative space-charge-layers form just below and above the surface of oxide particles respectively, which create a potential barrier between the particles increasing the electrical resistance of the thin film. However, when a reducing gas comes in contact with the thin film, it gets oxidized via reaction with the O2-ads or O-ads species, and subsequently, electrons are reintroduced into the electron depletion layer, leading to decrease in the potential barrier, and hence, its electrical resistance. The sensitivity of the thin film is usually determined by the ratio Rair/Rgas or Ggas/Gair, where Rair and Rgas are the resistances and Gair and Ggas are the conductances of the gas sensor in air without and with the reducing gas respectively. Various experimental characteristics of n-type semiconductor oxide gas sensor are well known [2]. Typically, they exhibit enhanced gas sensitivity below a critical nanocrystallite size of 10 nm. The maximum gas sensitivity is observed at higher operating temperatures (250-300 oC); above and below this range the gas sensitivity decreases. Moreover, with increasing film thickness, the gas sensitivity is observed to decrease and with increasing gas concentration, the gas sensitivity of n-type semiconductor oxide thin film sensor is known to increase; however, it gets saturated in the higher concentration range.

A4.2.2

In the literature, various theoretical models [3-14] have been proposed to explain the observed characteristics of the n-type semiconductor oxide gas sensor. However, none of the existing theoretical model can satisfactorily explain the effect of all critical variables on the gas sensitivity of n-type semiconductor o