New Approach to Selective Semiconductor Gas Sensors Using a de‐Biased pn Heterocontact

Abstract

The complex impedance of a CuO/ZnO oxide semiconductor heterocontact was studied by applying a dc bias voltage in a mixture of CO or H2 (4000 ppm) in air between 250° and 400°C. The frequency dependence of reactance showed a minimum peak, and the absolute value of the reactance and the frequency giving the peak both changed in a different manner, depending on the presence of a reducing gas of either CO and H2, and also the dc bias. Sensing properties could be tuned to detect both CO and H2 gases in air or to detect only H2 gas in air, by selecting the measuring frequency and the applied dc bias at 400°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.

Hydrogen sensitivity of doped CuO/ZnO heterocontact sensors

“…The heterocontact sensor is one such system with its intelligent gas and humidity sensing properties [2,3]. The most common heterocontact system, CuO/ZnO, is unique in that it has inherent selectivity characteristics [4]. The sensing properties can be tuned to detect CO and/or H 2 gases in air by selecting the measuring frequency and the applied dc bias using an impedance analyzer.…”

Gas Sensors Based on Ceramic p-n Heterocontacts

“…Furthermore, contrary to Li doping, changing the amount of Na resulted in a slight change in the forward and reverse currents. The ac capacitance increased in the presence of hydrogen, which can be attributed to a modification in the concentration of charge carriers by adsorption and surface reactions of gas molecules [4]. The ratio of the capacitance in hydrogen to the capacitance in air was found to be approximately 10 over the entire range of measurement li-equency 100 Hz to 5…”

“…The heterocontact sensor is one such system with its intelligent gas and humidity sensing properties [2,3]. The most common heterocontact system, CuO/ZnO, is unique in that it has inherent selectivity characteristics [4], The sensing properties can be tuned to detect CO andor Hz gases in air by selecting the measuring frequency and the applied dc bias using an impedance analyzer.…”

“…In 1950's it was discovered that the electrical properties of some metal oxides are changed when they are exposed to reducing gases, especially at elevated temperatures [l, 21. By applying this phenomenon, in 1962 Seiyama et al proposed the idea of gas sensing using ZnO thin films [3]. The other commonly used sensor material, Sn02, was proposed as a gas sensor in the same year by Taguchi [4], and since 1968 these sensors have been commercially available through Figaro Engineering [SI. The Taguchi Gas Sensor is a partially sintered SnO2 bulk device whose resistance in air is very high and drops when exposed to reducing gases such as combustibles (H2, COY CH4, c3wg) or volatile organic vapors and it has enjoyed a substantial popularity because of its ease of fabrication and stability relative to other competitive systems such as polymers and organic films.…”

Electrical Properties of Nanocrystalline n-SnO2 to Single Crystal p-Si Interfaces under Gas Adsorption Conditions

“…Therefore most of the data concerning application of heterostructures as gas sensors were obtained for structures composed of thin metal contacts deposited on the semiconductor substrate, which are transparent to gas molecules. Such sensors, as well as sensors, for which the gas sensitivity originates from a dipole layer formation at the heterojunction, are normally used for the detection of hydrogen [2][3][4][5][6][7].…”