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

The complex impedance of a CuO/ZnO oxide semiconductor heterocontact w as studied by applying a dc bias voltage in a mixture of CO or H-2 (40 00 ppm) in air between 250 degrees and 400 degrees C. The frequency de pendence of reactance showed a minimum peak, and the absolute value of the reactance and tbe frequency giving the peak both changed in a dif ferent manner, depending on the presence of a reducing gas of either C O 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 de grees C. Additional resistance and capacitance specific to the ambient gases were introduced to the interface impedance of the semiconductor s, and then were estimated from impedance measurements. Tunable gas se nsing was possible when the product of the resistance and the capacita nce components in the interface had varying values, dependent on gas s pecies and dc bias. This paper proposes this new method of tunable gas sensing that uses the complex impedance characteristics of a heteroco ntact. In addition, the mechanisms of the tuning function are discusse d.