Semiconductor gas sensor

 A semiconductor gas sensor for use in equipment for detecting small amounts of H2 S. The method of sensor fabrication comprises spray deposition of a mixture of metal oxides mixed together with various metal and non-metal materials which serve in the finished product as activators, dopants, and/or film binder materials, and including in suspension a molecular sieve material, for enhancing and defining porosity on a scale of molecular dimensions in the finished sensor.

All of the foregoing materials are suspended in a suitable solution and preferably sprayed onto a heated insulating substrate to form the finished product. The example sensor, capable of selective detection of H2 S in air and a sensitivity of less than 1 PPM (part per million), is comprised of a platinum activated alumina, tin oxide, and zeolite molecular sieve material.

FIELD OF THE INVENTION
This invention relates to semiconductor gas sensors and to methods of fabrication thereof, and more particularly to a unique spray deposition method wherein an improved semiconductor sensor is fabricated which comprises preselected gas sensor components in combination with a molecular sieve material to enhance and define porosity in the final semiconducting film. A specific example of a sensor and its method of fabrication is described which is capable of a selective detection of H2 S by changes in the conductivity of the sensor relative to the concentration of H2 S in the gas sample.

BACKGROUND OF THE INVENTION
Various semiconducting metal oxides have been used in conjunction with a variety of metal and non-metal additives in the fabrication of gas sensitive films suitable for use in gas detection apparatus. Exposure of such gas sensitive films to the gas of interest generally is detected as a change in conductivity of the film. In general, these prior devices exhibited inherent deficiencies in sensitivity, selectivity, response and recovery times, and/or calibration stability.

The electrical characteristics and subsequent gas response characteristics of such materials when employed as gas sensors in previous gas sensing equipment have been found to be highly dependent upon film properties such as thickness, uniformity of composition, purity, film porosity, and density. Since it has previously been difficult to adequately control the foregoing factors this art has been seeking a technique of fabrication which would be capable of producing films with the above mentioned and other properties well controlled.

In addition it is of course desireable that any new technique should be reproducible and cost effective. Further, the previous sensors were sometimes of limited utility if they were not capable of low temperature operation. This property is advantageous when sensing flammable gases in that there would be a reduced hazard of flammable gas ignition by the operating sensor, as well as an increased realiability and sensor life, reduced sensor power requirements, and better compatibility with on-chip integrated signal processing circuitry.

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