Theoretical exploration has been conducted to formulate the receptor function and response of a semiconductor gas sensor using thin plate-like crystals. The receptor function can be formulated over a wide range of depletion from conventional one (regional depletion) to new type one (volume depletion) by focusing attention on how the surface density of adsorbed oxygen ions (O(-)) is related with chemical moiety outside the crystals and physical moiety inside. The responses to gases thus derived are inversely proportional to the thickness of the plates used in the range of volume depletion. Electron-traps, if dispersed on the crystals, are shown to exert drastic effects in promoting sensitivity to gases. (C) 2009 Published by Elsevier B.V.