Gas sensor with protective gate, method of forming the sensor, and method of sensing

#The invention is related to gas sensors. In particular, the invention is related to a gas sensor with a protective gate.

#Gas and vapor sensors have many industrial applications. These applications include the detection of hazardous gas in the workplace for worker safety, enhanced control of the air-fuel mixture and feed in combustion of fuel, increased product yield and reduced waste in a product stream. These applications generally rely on individual sensors, which are installed to detect a single gas or vapor whose concentration may approach a harmful level. The examples of these gases include, but are not limited to, hydrogen sulfide, carbon monoxide, chlorine, ammonia, hydrogen, and methane.

#Methods for industrial gas and vapor sensing have evolved basically along two technology paths. The first path involves use of complex analytical instruments, for example infrared spectroscopy, gas chromatography, and mass spectrometry. The development of microcontrollers and microcomputers has led to smaller versions of these analytical instruments. These instruments are very powerful, but they have significant disadvantages, such as being very maintenance intensive. Further, these instruments are sensitive to adverse, corrosive environments and must be located far from the gas or vapor source in a climate controlled enclosure. Thus, a gas (vapor) sample must be transported to the analyzer, as a result real time information is not available. Also, these instruments are expensive, and are thus not affordable candidates for real-time, in-situ applications.

#FIG. 1 is a cross-sectional view of a sensor with a protective gate; and

#FIG. 2 is a cross-sectional view of a sensor schematically illustrating one example of the operation of the sensor.

#Gas sensors, such as, but not limited to, a metal-insulator-semiconductor (MIS) diode gas sensor and a field effect transducer (FET) gas sensor, are known in the art. The sensors comprise a semiconductor substrate; a thin insulator layer mounted on the semiconductor substrate; and a catalytic metallic gate (gate) mounted on the thin insulator layer. These known sensors have been generally used for monitoring gas in clean, dry, noncorrosive environments.

#A generalized operation of an MIS gas sensor will now be provided, using hydrogen gas as the detected gas. The sensor, its operation and the hydrogen gas are merely exemplary, and are not meant to limit the invention. Initially, hydrogen gas molecules (H2) are adsorbed onto the metallic gate from the surrounding ambient environment. The adsorbed molecules are altered, such as by being catalytically dissociated from each other on one of a molecular or atomic level. For hydrogen gas (H2), the molecules (H2) are dissociated into individual hydrogen atoms (H−). Next, the atomic hydrogen (H−)diffuses through the metallic gate to a metal-oxide interface at the semiconductor substrate. This diffusion forms a dipole layer that modulates electrical conductance of the sensor and changes electrical conductivity of the substrate. Many individual gases containing hydrogen, such as, but not limited to, amines, mercaptans, hydrocarbons, and alcohols, can be detected in this manner by a sensor.

#The level of sensitivity for each hydrogen-containing gas is different for each particular gate material. A sensor is tuned to a particular gas by virtue of the particular gate material. Known sensors provide adequate operation for a single hydrogen-containing gas in a clean, dry, noncorrosive environment. These known sensors, however, do not operate effectively if the environment is not clean and contains foreign matter (matter not intended to be associated with the sensor), such as particulates (minute separate particles that often result when oil degrades), or contains water vapor. Also, known sensors do not operate effectively if the sensor is placed in a corrosive gaseous (chlorine and hydrogen sulfide) environment, or if the sensor does not have a satisfactory tuned (sensitivity) level for the designated gas.