Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
  • G01N27/4141—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS specially adapted for gases
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
  • B82B1/00—Nanostructures formed by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects

Definitions

• the present invention relates in general to gas sensors. More particularly the present invention relates to gas sensors that operate by applying a gate voltage so as to tune detection of a current through a layer of a compound that is capable of chemical interaction with an analyte gas. Additionally, other methods could be used to tune the detection of an analyte, including, optical excitation, chemical dopants, surface chemical layers and combinations thereof. Additionally, the application of an external force, such as a gate bias, helps to eliminate or greatly reduce the requirement for a heated substrate surface. Still more particularly, the present invention relates to thin film gated metal oxide detectors adapted for detection of analyte gases, such a carbon monoxide.
• Metal oxides typically have an amorphous crystal structure. This means there will be individual crystalline grains but that there is no long-range order to the surface. The interface between each crystallite creates a grain boundary. Conduction through a metal oxide is limited by the energy barrier created at each grain boundary, hi addition to the change in surface energy heat changes the energy level of barriers created at these individual grain boundaries.
• CO carbon monoxide
• Carbon monoxide has about 210 times the affinity to bind to hemoglobin compared to oxygen.
• CO is an odorless, tasteless, colorless gas that causes hypaemic hypoxia wherein there is a reduced oxygen carrying capacity of the blood.
• Carbon monoxide in the blood creates carboxyhaemoglobin (COHb) which prevents oxygen uptake.
• COHb carboxyhaemoglobin
• increased levels of COHb cause various symptoms ranging from headache to unconsciousness.
• CO at sea level causes a headache (equivalent to 15-20% COHb content in the body).
• the effects of CO poisoning and altitude hypoxia are cumulative, driving a need for a continuous low-level monitoring of sub-200 ppm levels of CO in aircraft cabins.
• Heated molybdenum oxide (MoO 3 ) CO sensor Heated molybdenum oxide (MoO 3 ) CO sensor.
• molybdenum oxide (MoO 3 ) thin films prepared by sol-gel and RF magnetron sputtering processes were previously employed in the development of CO sensors' as described in "Carbon Monoxide response of molybdenum oxide thin films deposited by different techniques," by E. Comini, G. Faglia, G. Sberveglieri, C. Cantalini, M. Passacantando, S.
• the RF deposited films had a needle-like structure with longitudinal dimension ranging from
• Reference 3 demonstrated that monodisperse nanoparticles of molybdenum dioxide can be grown on a conductive surface using a pulsed voltammetric technique.
• Figure 5 of Reference 3 shows the scanning electron micrograph of molybdnum dioxide metal nanoparticles on graphite basal plane surfaces. As shown in that Figure, the nanoparticles have with an apparent size, as indicated by a 1 micrometer scale line, of 100-200 nm. It is also possible to oxidize an existing metal film.
• any known metal oxide sensor that operates due to thermal activation can be preferably operated without a thermal excitation by application of alternative energy activation methods.
• Application of alternative energy methods provides similar changes in surface energy states and conductivity mechanisms of metal oxides.
• application gate bias in a thin-film transistor (TFT) architecture eliminates the thermal requirements of described sensors.
• FIG. 1 shows how the gate bias can manipulate the metal oxide surface energy and change the energy barriers through a grain boundary.
• the excitation methods could be optical, magnetic or combinations thereof.
• the sensing mechanism of the above-mentioned metal oxide sensor is independent of operating temperature.
• the semiconducting channel is electronically manipulated.
• the sensor operates at - 6OF and 140F under the same gate bias.
• the TFT architecture may include a semiconducting thin film that includes a compound capable of chemical interaction with an analyte gas.
• the compound is preferably a metal oxide.
• the chemical interaction is preferably electron transfer.
• a gas sensor includes a metal oxide.
• the metal is preferably a transition metal, more preferably a Group 6B element, still more preferably molybdenum.
• TFT thin film transistor
• a gas sensor operates with an electron transfer from an analyte gas.
• This electron transfer could be the result of a catalytic reaction on the surface of the sensor.
• the electron transfer may be electron donating.
• the electron transfer may be electron withdrawing.
• the analyte gas is preferably carbon monoxide.
• a gas sensor includes a sensor architecture using a semiconductor metal oxide.
• the metal oxide may be in the form of a thin film.
• the metal oxide may be in the form of a nanoparticle or a network of nanoparticles.
• the sensor architecture preferably includes a thin film transistor architecture.
• a gas sensor includes at least one metal oxide nanoparticle made by a growth method.
• the thin film may grown in situ by first depositing a metal followed by its oxidation.
• the metal layer growth method may include electrochemical growth, sputtering, metal evaporation, solution processing, including sol gel and nanoparticle solutions.
• the metal layer deposition method could be combinations of the above mentioned techniques. After depositions of the metal layer, a post process oxidation is required to convert the metal to its corresponding metal oxide. This oxidation can be performed thermally, chemically, electrochemically or combinations thereof. Electrical conductivity through this metal oxide film may be dominated by grain boundary interfaces.
• a gas sensor includes a microprocessor.
• the present invention relates to gas sensors that operate by applying an external force to tune the sensor by changing the surface energy of the semiconducting layer and change the electrical transport through the semiconducting layer. Tuning the surface energy could be accomplished by applying a gate. Additionally, other methods could be used to tune the detection of an analyte, including, optical excitation, chemical dopants, surface chemical layers, magnetic fields and combinations thereof. Additionally, the application of an external force, such as a gate bias, helps to eliminate or greatly reduce the requirement for a heated substrate surface.
• a gas sensor includes RF integration for remote sensing.
• the gas sensor may act as a dosimeter.
• a gas sensor for detecting an analyte gas may include a first contact, a second contact, a semiconducting layer, an insulating layer, a substrate, and a third contact.
• the third contact preferably serves as a gate contact.
• the gate contact may be the substrate.
• the semiconducting layer preferably includes a compound capable of chemical interaction with the analyte gas. The chemical interaction is preferably electron transfer.
• the insulating layer, the semiconducting layer, first contact, second contact, and third contact are preferably arranged in an architecture predetermined such that the sensor detects a variation in the level of the analyte gas (labeled e.g.
• CO as a variation in a current between the first and second contacts occurring when an electron transfer event takes place between the compound and the analyte.
• CO is oxidized to carbon dioxide (CO 2 ).
• CO 2 carbon dioxide
• This reaction takes place an electron is transferred from CO into the metal oxide surface.
• This electron transfer can be facilitated when a gate voltage is applied across the first contact and the third contact .
• the transport of the electron (or hole) created through this electron transfer reaction between the analyte and compound can be manipulated when a gate voltage is applied across the first contact and the third contact. Manipulation of the electron transport may be due to changing energy barrier heights at the grain boundary layers between nanoparticles.
• Application of a gate bias may reduce thermal requirements for the sensor and allow operation at room temperature and below without an external or internal heated substrate.
• a gas sensor may include a signal amplifier that includes a thin film transistor that includes a semiconducting thin film that includes an oxide of molybdenum.
• the signal amplifier may be external.
• the signal amplifier may be the thin-film transistor sensor.
• FIG. 1 illustrates tuning the surface energy for reaction and the grain boundary dependent electron transport by application of a gate bias
• FIG. 2 illustrates an embodiment of the present invention
• FIG. 3 is a plot illustrating the sensitivity based on internal amplification of a gas sensor according to an embodiment of the present invention.
• FIG. 4 illustrates the gated metal oxide sensor architecture;
• FIG. 5 is a schematic diagram illustrating additional processing steps for making a gas sensor using the substrate as a third gate contact according to an embodiment of the present invention
• FIG. 6 is a schematic diagram illustrating a gas sensor for remote applications according to an embodiment of the present invention.
• FIG. 7 is a plot of the response of an exemplary gas sensor according to an embodiment of the present invention.
• FIG. 8 is another plot of the response of an exemplary gas sensor according to an embodiment of the present invention.
• FIG. 1 denotes the tunability of the sensor.
• FIG. IA shows an exaggerated surface topography of a metal oxide between two contacts. The metal oxide and its contacts are present on top of an insulating layer that separates a third contact. The surface topography creates a grain boundary where conduction of an electron or hole is dominated by the energy barrier to get from one grain to an adjacent grain.
• FIG. IB we can see that there is an energy barrier associated with the grain boundaries depicted in FIG. IA.
• the Inventors reduce the barrier height by application of an electrical bias to contact 3, thereby eliminating the requirement of heat. Referring to FIG. 1C, the Inventors show how the surface reaction energy can be tuned.
• the metal oxide is semiconducting and has a band gap.
• This band gap is the energy it takes to promote an electron from the valence band up into the conduction band.
• high heat is required to create a favorable distribution of electrons within the valence and conduction band for a surface reaction to take place, such as the oxidation of CO to CO 2 .
• the distribution of electrons witibdn the valence and conduction bands is tuned by application of a gate bias.
• the gas sensor shown in FIG. 2 exemplifies a gas sensor for detecting an analyte gas that includes a first contact (labeled e.g. metal contact pad), a second contact, a semiconducting layer (labeled e.g. metal oxide nanoparticle layer), an insulating layer (labeled e.g. dielectric insulator), a substrate (labeled e.g. Si gate), and a third contact (not shown) contacting the substrate.
• the third contact preferably serves as a gate contact.
• the semiconducting layer preferably includes a compound capable of chemical interaction with the analyte gas. Still referring to FIG.
• the insulating layer, the semiconducting layer, first contact, second contact, and third contact are preferably arranged in an architecture predetermined such that the sensor detects a variation in the level of the analyte gas (labeled e.g. CO) as a variation in conductivity between the first and second contacts occurring when a gate voltage is applied across the first contact and the third contact.
• the analyte gas labeled e.g. CO
• a miniaturized, low power, rapid responsive CO sensor based on metal oxide nanoparticle networks applied to thin-film transistor ("TFT") architecture.
• Certain metal oxides are n-type semiconductors that show an increase in conductance due to the transfer of electrons resulting from oxidation or reduction of an analyte gas. The change in conductance is proportional to the concentration of the analyte gas.
• the nanoparticle network approach provides considerable improvement in sensitivity and selectivity over the most successful commercial technology of CO detectors based on metal oxide films due to the following reasons.
• the nanostructured interaction of a metal oxide nanoparticle network provides higher sensitivity compared to commercial detectors.
• the metal oxide nanoparticle network can be refreshed by ambient oxygen.
• the TFT design enables increased sensitivity due to the built-in gain. The gain comes from a non-linear current vs. voltage curve, characteristic of semiconductors.
• salient characteristics of the proposed device that present an improvement over existing technologies include the following advantages: built-in gain through TFT architecture; on-chip design and integration; fast response and continuous monitoring; built in refresh through chemistry and gate voltage; quantitative response; and time integrated response for cumulative exposure.
• the TFT architecture can be made using CMOS processing for highly parallel and low-cost manufacturing.
• This change in carrier concentration is the same thing that happens when a gate bias is applied to any semiconductor and is by definition a change in current by field effect.
• This previous devices operates as a ChemFET. Their device will not operate in ambient environmental conditions due to surface saturations at normal oxygen concentrations. It is not a sensor, but rather a physical change in transistor response due to a change in environment.
• the present invention operates via chemical reactions at a metal oxide surface. These chemical reactions can add (through oxidation) or remove (through reduction) electrons from the semiconducting layer of the sensor. The change in number of electrons will change the current through the device. The removal of electrons will always cause a change in current even if the material is a poor semiconductor whereas oxygen adsorption changing electron density or distribution will not.
• Dalin J. "Fabrication and characterization of a novel MOSFET gas sensor" Final Thesis at Linkopings Institute of Technology, Fraunhofer Institute for Physical Measurement Techniques, Frieburg, Germany, 6-5-2002, LiTH-ISY-EX-3184, herein denoted Ref.
• the gated metal oxide sensor of the present invention does not require heat for operation. This gated metal oxide sensor operates from -6O 0 C to greater than 100 0 C. It demonstrates an increased response at lower temperatures.
• Technical approach - chemistry :
• Metal oxide chemistry is a driving force for this invention to sense an electron transfer from a surface reaction of an analyte gas.
• the analyte gases include, but are not limited to, carbon monoxide (CO) and other electron donating and or electron accepting species.
• CO carbon monoxide
• Any metal oxide thin film/nanoparticle system is deemed suitable with the present invention, but a more specifically transitional metal oxide such as molybdenum oxide (MoO 3 ) is used due to its unique properties towards CO.
• MoO 3 molybdenum oxide
• Metal oxides exist in several forms. For example, molybdenum oxide could be MoO, MoO 2 , MoO 3 depending on the oxidation state of the metal.
• MoO 3 is an n-type semiconductor that will oxidize CO through electron transfer, which causes a measurable change in resistance.
• Molybdenum trioxide contains molybdenum in its hexavalent state. Hexavalent molybdenum has no electrons in its 4d orbitals.
• oxidation of carbon monoxide involves an electron transfer of an electron from CO to Mo +6 . Following this initial electron transfer step, several reaction pathways are possible for the subsequent oxidation of carbon monoxide to carbon dioxide.
• the invention includes, but is not limited to, molybdenum oxide nanoparticles for CO detection.
• Molybdenum oxide presents certain unique properties suitable for the present invention.
• Other metal oxides may be used with solid state sensor design, but none have equivalent properties to molybdenum oxide.
• the two other metals in Group 6B are Chromium (Cr) and Tungsten (W). They have a similar chemistry to molybdenum oxide.
• the top of the period (CrO 3 ) will be more reactive. This reactivity comes at a cost. The more reactive species will create a more stable product increasing the difficulty of reversing the reaction, i.e., refreshing of the sensor.
• Spherical nanoparticle films have several advantages compared with nanoparticles of other morphologies.
• Spherical nanoparticles have an increased percentage of active surface atoms (diameters ranging from 5-300 nm).
• the atoms in the middle of the particle, called the "bulk,” do not contribute electronically to any reactions or binding events.
• the invention employs a thin-film transistor (TFT) architecture to maximize the signal output of the CO sensor.
• Molybdenum Oxide is an n-type semiconducting material. This means that conduction through the material can be manipulated by a third terminal contact commonly called a gate. Because it is an n-type semiconductor, the resistance will decrease as we move to a positive gate voltage. An electron transfer from the oxidation of CO to CO 2 will increase the number of electrons in the MoO 3 film, therefore increasing the number of carriers and also the current through the device. This is effectively the same as applying a positive gate voltage.
• FIG. 2 illustrates a metal oxide semiconductor thin film transistor for CO detection. An electron transfer takes place when CO is oxidized on the surface of the metal oxide. This surface reaction changes the carrier concentration of the n-type semiconductor and is measured as a change in current.
• the semiconducting nature of the MoO 3 contributes to an increased sensitivity in the sensor.
• the TFT architecture has inherent gain, which is to say that a very small change in gate voltage (for example an electron transfer caused by an oxidation of CO) creates change in current.
• An electron transfer event from an oxidation of a gas like CO into the metal oxide nanoparticle framework is depicted in FIG. 1. As seen in FIG. 1, this electron transfer event causes a change in current that can be up to several orders of magnitude depending on the slope of the current vs. gate voltage curve. This curve is based on an n-type semiconducting material.
• the invention can be manipulated for a reducible gas by making the semiconducting channel p-type.
• the surface would provide an electron to allow an oxidized chemical species to be reduced.
• Changing from n-type to p-type would require a change in the chemistry of the metal oxide by changing the base metal, alloys of other metals or doping with small amounts of additional materials.
• FIG. 2 illustrates Current Vs Gate Voltage for an n-type semi-conductor showing the high inherent change in current ( ⁇ I) by changing a small gate voltage ( ⁇ V) due to the electron transfer from CO.
• the TFT architecture may be processed by a method that includes electrochemically depositing MoO 3 nanoparticles on a conductive substrate as described below. Further, the metal oxide may be deposited by a solution method. Still further, the metal oxide may be grown by oxidizing a thin metal film or metal nanoparticle film.
• the present inventors contemplate growing nanoparticles with two parallel approaches using, for example, a Gamry Potentiostat with PC interface.
• the process includes direct deposition of the nanoparticles on a conductive substrate, followed by transforming a surface portion of the conductive substrate into an insulating layer.
• the two step process includes indirect deposition of the nanoparticles on a conducting substrate and removal of the nanoparticles from the conducting substrate followed by deposition of the nanoparticles on an insulating substrate. While the indirect and direct deposition are described by way of example as electrochemical growth it will be understood that alternative deposition methods known in the art are contemplated, for example sputtering, thermal evaporation, electron beam evaporation, and the like. Further, in accordance with the deposition method, initial deposition may occur on any suitable surface, selecting from among conductors, insulators, and semiconductors. Processing Steps:
• FIG. 3 illustrates electrochemical processing steps to create a MoO 3 TFT-based CO sensor.
• Step A will electrochemically deposit the nanoparticle film on a Si substrate.
• Step B will thermally grow a Gate oxide.
• the present inventors will start with a conductive silicon substrate and directly grow the nanoparticles until a conductive film is achieved arriving at step B.
• the silicon substrate will eventually serve as a global back gate.
• the electrochemical fabrication of the metal oxide nanostructure will be carried out by a two step procedure involving a nucleation step (applying a high negative voltage for a short period of time ⁇ 10sec) and a prolonged growth step (up to 10 minutes) at lower negative voltages in an aqueous/organic metal oxide solution.
• the electrochemical fabrication may be carried out in a constant voltage mode (chronoamperometry) or in a constant current mode (chronopotentiometry).
• step B The electrochemical growth will be followed by a thermal oxidation to grow silicon oxide under the molybdenum oxide layer (step B).
• This 1000° C thermal step will serve two functions. First, it will produce a high quality, thermal silicon oxide gate insulator, and avoid the alignment issues of photolithography. Second, the thermal layer will anneal the semi conducting molybdenum oxide film to increase conductivity. Higher conductivity will lessen the need for complex electronics to eliminate noise when measuring low current signals. As an alternative to direct electroplating the metal oxide, a second parallel approach is also feasible.
• Approach II Indirect electrochemical growth of metal oxide nanostructures:
• the present invention will involve a procedure, wherein the molybdenum oxide will be nucleated and grown onto a conductive substrate (e.g., freshly cleaved graphite surface) followed by removal of the nanoparticles from the conductive substrate and collection in a solution phase. This will allow us to solution deposit the nanoparticles onto any insulating substrate.
• a conductive substrate e.g., freshly cleaved graphite surface
• FIG. 4 illustrates indirect electrochemical processing steps to create a MoO 3 TFT-based CO sensor.
• Step A will electrochemically deposit the nanoparticle film on a conductive (e.g., graphite) substrate followed by a harvest step wherein the nanoparticles are dispersed in a liquid suspension.
• the nanoparticles could then be deposited by as a dropcast or a Langmuir Blodgett film on an insulating substrate.
• the present invention can involve a procedure, wherein the molybdenum is deposited onto the substrate.
• This metal layer could be deposited as a liquid suspension of metal nanoparticles, a thermal evaporation, sputtering, electron beam evaporation or other technique known in the art.
• This metal layer will be deposited onto an insulating substrate followed by oxidation to the metal oxide.
• the metal layer could be oxidized by heating in an oxygen containing environment. Controlling the temperature and time of oxidation will control the conductivity of the metal layer.
• Metals can oxidize at a temperature between 100 0 C and 1400 0 C. Metals can also be oxidized chemically or electrochemically. Once this layer of metal oxide is formed we can move on to final processing.
• FIG. 5 illustrates photolithography processing steps for the MoO 3 CO sensor.
• first metal contact electrodes creating the source and drain will be evaporated onto the MoO 3 film using standard photolithography and lift-off (step C). These contacts will be used to measure current or resistance through the active area of the device.
• the active area of the device will be patterned using photolithography as shown in Step D.
• the photoresist will create a protective film for the subsequent reactive ion etching. This etch step will remove the molybdenum oxide from unwanted areas on the chip, prevent cross-talk between adjacent sensors, and define the dimension of the actual end device.
• the final step will be to open a hole in the insulating silicon oxide followed by metallization to allow a gate contact with the underlying substrate
• Step E Following completion of processing a wafer it will be cleaved into individual sensor elements. The sensors will then be mounted to a multi-pin header using a suitable epoxy. The three contacts (source, drain and gate) will then be wire bonded to a header pin using packaging methods known to the one skilled in art. Microprocessor Development and device integration:
• the microprocessor developed as a part of this invention will have the ability to manipulate the gate voltage, measure current through our CO sensor, compute a CO concentration, and drive a digital display and output to an alarm.
• the device box will likely contain both a piezo-based audible alarm and an LED based visual alarm.
• the microprocessor will be required to run more than one input channel.
• An inactive reference sensor will be incorporated in the device to cancel aging and temperature drift.
• the reference channel will be measured along with the active sensor during each sampling cycle. Data samples are averaged to filter noise and converted to CO concentration levels.
• the on-board LCD display will be updated every 20 seconds or less.
• FIG. 6 illustrates radiofrequency (RF) integration for remote applications.
• FIG. 6 shows a possible arrangement.
• the software capability would handle normal "I'm alive” and battery status reporting, as well as change-of- measurement readout.
• An approximate duty cycle of 0.1% activity is assumed with reporting of CO changes occurring at approximately 10 second intervals. This holds total power consumption to approximately 2 mW, occurring at the 0.1% interval. This low average power consumption enables long battery life.
• the present invention will be more easily and fully understood by the following example.
• the example is representative of a gas sensor in accordance with one embodiment of the present invention.
• a sensor was prepared by growing molybdenum trioxide, as an exemplary sensing compound, in a thin film arranged as part of a thin film transistor architecture. The growth was via electron beam evaporation of molybdenum, followed by thermal oxidation of molybdenum. The structure of the film was characterized using a scanning electron microscope (SEM). The film had a nanoparticle structure. The deposited metal film had a thickness of less than 20 nm.
• FIG. 7 shows the response of the sensor to a continuous flow of carbon monoxide at 50 ppm inside a sealed chamber.
• the sensor was operated at room temperature.
• the response was measured as the normalized ratio of R, the resistance in the presence of carbon monoxide, to R 0 , to the resistance in the absence of carbon monoxide, as a function of time.
• the response was determined for two different values of gate voltage, +5 V, and -5 V.
• the lines marked ON and OFF indicate the dose of CO being turned on and off.
• the upper curve (-5Vg) shows a response to CO.
• the lower curve (+5Vg) shows no response to CO.
• the results demonstrate that the response of the sensor may be tuned by varying the gate voltage.
• FIG. 8 shows the response of the same sensor to a continuous stream of carbon monoxide at increasing levels of concentration of 2ppm, 5ppm, 10 ppm, and 20 ppm.
• the sensor was operated at room temperature.
• the response was measured as the normalized ratio of R, the resistance in the presence of carbon monoxide, to R 0 , to the resistance in the absence of carbon monoxide, as a function of time.
• the results demonstrate the sensitive response of the sensor to low levels of gas.
• the response to CO is linear.
• the present inventors have discovered that operation of at temperatures lower than room temperature is also possible, for example -60 degrees F.
• the sensor may operate at atmospheric temperatures encountered from ground level to up to 40,000 feet, and thus is adapted for use in an air plane or other high altitude application.
• a method of operating the sensor may include adjusting the gate voltage according to the temperature.
• the gate bias can be tuned to different analyte gases at a wide range of concentrations.
• a method of operating a sensor according to an embodiment of the present invention may include tuning any one or combination of the gate bias and the sensing compound so as to select the analyte.

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