Characteristic nonlinear responses for gas species on the surface of different semiconductor gas sensors

A novel gas-sensing system based on the multi-dimensional information contained in dynamic nonlinear responses is reported. A sinusoidal temperature change was applied to a semiconductor gas sensor, and the resulting conductance of the sensor was analyzed by fast Fourier transformation (FFT). The higher harmonics of the FFT characterized the nonlinear properties of the response at the sensor surface. The amplitudes of the higher harmonics exhibited characteristic changes which depended on the concentration and the kinetics of gas species on the sensor surface, and on the composition of the semiconductor. The combination of two different gas sensors makes it possible not only to differentiate among gas species but also to quantitatively discriminate between gaseous mixtures based on nonlinear dynamic responses. Nonlinear responses are discussed in relation to the kinetics of the gas species at the sensor surface and the surface barrier potential, which depends on the thermodynamic properties of the semiconductor.