Resistive gas sensors are typically based on granular semiconductor materials. Electrical conduction is limited by potential barriers associated with grain boundaries. Potential barrier height is controlled by the grain boundary charge which is modulated by chemical reactions. Barrier limited conduction provides sensitive monitoring of gas reactions.
Time-dependent phenomena in granular materials cover a very broad time scale from hours to nanoseconds. In this paper we employ impedance spectroscopy in investigation of the low frequency regime, where chemical reactions together with electronic trapping may occur.
The samples are commercial resistive metal-oxide microhotplate gas sensors MOS1 (material WO3) and MOS3 (SnO2) from Environics Oy. The admittance spectra in sub-hertz region were measured with a special scheme: A Labview program varies the voltage of a Keithley 236 unit which also measures the current. The applied voltage signal consists of a constant DC bias and a uniformly distributed pseudorandom component. The admittance spectra were calculated from the recorded current and voltage signals by using FFT.
The results are represented by an equivalent circuit model consisting of a resistor representing the gas sensitive conductivity, and a DC-bias dependent series RC branch with negative admittance in parallel. In SnO2 this type of negative capacitance has been observed by Varghese et al. [1]. They explained the phenomenon by the movement of adsorbed hydrogen ions at high humidity levels.
We interpret the phenomenon in terms of the barrier limited conduction and chemical reaction models. Introduction of current injects free carriers at barrier top. This leads to increase of grain boundary charge by trapping and chemical reactions. This has twofold consequences: (i) Owing to increased charge, potential barrier will increase leading to decrease of current. This effect can be represented by a parallel RC circuit having negative admittance. (ii) Modulation of grain boundary charge needs charging/discharging current. Increase of grain boundary charge leads to negative polarization, which gives a serially coupled negative resistor and capacitor. In the samples (ii) is dominating.
[1] Varghese et al., J. App. Phys., vol. 87, pp. 7457– 7465, 2000. |