NEW APPROACH TO SELECTIVE SEMICONDUCTOR GAS SENSORS USING A DC-BIASED PN HETEROCONTACT

The complex impedance of a CuO/ZnO oxide semiconductor heterocontact was studied by applying a dc bias voltage in a mixture of CO or H-2 (4000 ppm) in air between 250 degrees and 400 degrees C. The frequency dependence of reactance showed a minimum peak, and the absolute value of the reactance and tbe frequency giving the peak both changed in a different manner, depending on the presence of a reducing gas of either CO and H-2, and also the dc bias. Sensing properties could be tuned to detect both CO and H-2 gases in air or to detect only H-2 gas in air, by selecting the measuring frequency and the applied de bias at 400 degrees C. Additional resistance and capacitance specific to the ambient gases were introduced to the interface impedance of the semiconductors, and then were estimated from impedance measurements. Tunable gas sensing was possible when the product of the resistance and the capacitance components in the interface had varying values, dependent on gas species and dc bias. This paper proposes this new method of tunable gas sensing that uses the complex impedance characteristics of a heterocontact. In addition, the mechanisms of the tuning function are discussed.