Gas sensor module

Abstract

To increase estimation precision of gas concentration in a gas sensor module.SOLUTION: A gate voltage detector detects gate voltages of a sensor FET and a reference FET in a state in which when between the drain and source of a sensor FET reacting with gas and a reference FET not reacting with gas is a target current value. A gate voltage estimator obtains a gate voltage estimation value of the sensor FET when gas concentration is 0% by using a correlation relationship between the gate voltage of the reference FET and the gate voltage of the sensor FET when gas concentration is 0% on the basis of the gate voltage of the sensor FET. A gas concentration estimator estimates gas concentration by using the correspondence between the amount of shift and gas concentration on the basis of the amount of shift of the detected gate voltage of the sensor FET relative to the gate voltage estimation value.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to a gas sensor module.

Background Art

[0002] In recent years, gas sensors manufactured by semiconductor processes have been known. For example, Patent Document 1 describes a FET (Field Effect Transistor) type gas sensor and a method for detecting the concentration of a gas using the gas sensor. In this method, an FET whose gate electrode is exposed to the atmosphere and an FET whose gate electrode is not exposed to the atmosphere are integrated on the same substrate. Then, based on the difference in gate voltage when the same value of current flows between the drain and source of both FETs, the concentration of the gas in the atmosphere is detected.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a FET type gas sensor, due to variations in manufacturing characteristics, fluctuations in environmental temperature, etc., the relationship between the current flowing between the drain and source and the gate voltage, that is, the current-voltage characteristics, changes. Furthermore, an FET whose gate electrode is exposed to a gas and an FET whose gate electrode is not exposed to the gas are different devices even though their structures are similar to each other. Therefore, even when the concentration of the gas to be detected is 0%, the gate voltages when the same value of current is flowing do not match. Therefore, when detecting the gas concentration based on the difference in gate voltage between the above two types of FETs, the detection accuracy of the gas concentration becomes low.

[0005] An object of the present invention is to provide a gas sensor module capable of detecting a gas concentration with higher accuracy.

Means for Solving the Problem

[0006] A representative embodiment of the present invention includes a sensor FET disposed on a substrate and reacting with a gas in the atmosphere, a reference FET disposed on the substrate and not reacting with the gas, a gate voltage detector that controls the gate voltages of the sensor FET and the reference FET so that the drain-source current between the sensor FET and the reference FET becomes a target current, and detects the gate voltages of the sensor FET and the reference FET when the drain-source current between the sensor FET and the reference FET is the target current, a gate voltage estimator that obtains an estimated value of the gate voltage of the sensor FET when the gas concentration is 0% by using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the gas concentration is 0% based on the detected gate voltage of the reference FET, and a gas concentration estimator that estimates the gas concentration by using the correspondence between the shift amount of the gate voltage of the sensor FET and the gas concentration based on the shift amount of the detected gate voltage of the sensor FET with respect to the estimated value of the gate voltage of the sensor FET. It is a gas sensor module.

[0007] A representative embodiment of the present invention includes a sensor FET disposed on a substrate and reacting with a gas in the atmosphere, a reference FET disposed on the substrate and not reacting with the gas, a temperature sensor for acquiring the temperatures of the sensor FET and the reference FET, a heater for heating the sensor FET and the reference FET, a temperature controller for controlling the heater based on information from the temperature sensor such that the temperatures of the sensor FET and the reference FET reach a target temperature at which the detection sensitivity of the sensor FET to the gas is equal to or higher than a certain level, a gate voltage controller for controlling the gate voltages of the sensor FET and the reference FET such that the drain-source currents of the sensor FET and the reference FET become a target current, a gate voltage detector for detecting the respective gate voltages of the sensor FET and the reference FET in a state where the drain-source currents of the sensor FET and the reference FET are the target current and the temperatures of the sensor FET and the reference FET are the target temperature, a gate voltage estimator for obtaining an estimated value of the gate voltage of the sensor FET when the gas concentration is 0% by using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the gas concentration is 0% based on the detected gate voltage of the reference FET, and a gas concentration estimator for estimating the gas concentration by using the correspondence between the shift amount of the gate voltage of the sensor FET and the gas concentration in a state where the temperatures of the sensor FET and the reference FET are the target temperature based on the shift amount which is the difference between the obtained estimated value of the gate voltage of the sensor FET and the detected gate voltage of the sensor FET.

[0008] One typical embodiment of the present invention includes a sensor FET disposed on a substrate and reacting with a gas in the atmosphere, a reference FET disposed on the substrate and not reacting with the gas, a controller for selecting a gas to be detected from among multiple types of gases, a temperature sensor for acquiring the temperatures of the sensor FET and the reference FET, a heater for heating the sensor FET and the reference FET, a temperature controller for controlling the heater based on information from the temperature sensor such that the temperatures of the sensor FET and the reference FET reach a target temperature at which the detection sensitivity of the sensor FET to the gas to be detected is at a certain level or higher, a gate voltage controller for controlling the gate voltages of the sensor FET and the reference FET such that the drain-source currents of the sensor FET and the reference FET become a target current, a gate voltage detector for detecting the respective gate voltages of the sensor FET and the reference FET in a state where the drain-source currents of the sensor FET and the reference FET are the target current and the temperatures of the sensor FET and the reference FET are the target temperature, a gate voltage estimator for obtaining an estimated value of the gate voltage of the sensor FET when the concentration of the gas to be detected is 0% using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the concentration of the gas to be detected is 0% based on the detected gate voltage of the reference FET, and a gas concentration estimator for estimating the concentration of the gas to be detected using the correspondence between the shift amount of the gate voltage of the sensor FET and the concentration of the gas to be detected in a state where the temperatures of the sensor FET and the reference FET are the target temperature based on the shift amount, which is the difference between the obtained estimated value of the gate voltage of the sensor FET and the detected gate voltage of the sensor FET.

Effect of the Invention

[0009] A gas sensor module capable of detecting a gas concentration with higher accuracy can be provided.

Brief Description of the Drawings

[0010]

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Embodiments for Carrying Out the Invention

[0011] (Background of the Study by the Present Inventors) The embodiments described below relate to a gas sensor module that constitutes a MOSFET type gas sensor. The MOSFET type gas sensor is used in gas leak detectors, gas concentration meters, etc. That is, the MOSFET type gas sensor is used to detect a gas to be detected contained in the atmosphere or to measure the concentration of the gas to be detected in the atmosphere. The gas to be detected is, for example, hydrogen, hydrogen sulfide, carbon dioxide, carbon monoxide, etc.

[0012] A MOSFET type gas sensor has a sensor FET and a reference FET. The sensor FET and the reference FET are formed, for example, through substantially the same MOSFET formation process and have similar structures, physical properties, etc. The sensor FET is a FET in which the gate electrode serving as a gas sensor is exposed to the atmosphere and reacts to the gas. On the other hand, the reference FET is a FET in which the gate electrode is not exposed to the atmosphere and does not react to the gas.

[0013] It is known that the current-voltage characteristics of the sensor FET, that is, the characteristics showing the relationship between the drain-source current and the gate voltage, shift in the direction of the gate voltage according to the concentration of the gas to be detected. Therefore, in a MOSFET type gas sensor, the gate voltages of both the sensor FET and the reference FET are adjusted so that the drain-source current becomes the same constant current. Then, based on the difference in the gate voltages of both FETs after the adjustment, that is, the value corresponding to the shift amount in the direction of the gate voltage of the current-voltage characteristics in the sensor FET, the concentration of the gas in the atmosphere is detected.

[0014] However, the current-voltage characteristics of the sensor FET and the reference FET vary due to slight differences in structure or composition caused by the manufacturing process, or fluctuate depending on the temperature of the FET, that is, the ambient temperature. When the current-voltage characteristics of the FET vary due to slight differences in the structure or composition of the FET element or due to the ambient temperature, the accuracy of detecting the concentration of the gas to be detected contained in the atmosphere decreases.

[0015] Therefore, the present inventors conducted various studies in order to suppress a decrease in the accuracy of detecting the concentration of a gas to be detected in the atmosphere due to manufacturing variations or temperature fluctuations in the current-voltage characteristics of the FET. As a result, the present inventors found that there is a correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the same constant current flows between the drain and source of the sensor FET and the reference FET in the case where the gas concentration is 0%. The present inventors invented a method for substantially correcting manufacturing variations or temperature fluctuations in the current-voltage characteristics of the FET by utilizing the correlation, and a method for obtaining information such as the above-mentioned correlation necessary for the correction.

[0016] Hereinafter, each embodiment of the present invention will be described with reference to the drawings. In the description of each embodiment, the gas to be detected contained in the atmosphere is also simply referred to as "gas", and the concentration of the gas to be detected in the atmosphere is also simply referred to as "gas concentration".

[0017] (Embodiment 1) The gas sensor module according to Embodiment 1 has information based on the premise that the drain-source current of the sensor FET and the reference FET is the same constant value, and represents the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET in the case where the gas concentration is 0%. Further, the gas sensor module according to Embodiment 1 estimates the corrected gate voltage of the sensor FET in the case where the gas concentration is 0% by using the correlation based on the detected gate voltage of the reference FET. Then, the gas sensor module according to Embodiment 1 obtains the difference between the estimated gate voltage, that is, the gate voltage estimated value of the sensor FET in the case where the gas concentration is 0%, and the actually detected gate voltage of the sensor FET as a shift amount. The gas sensor module according to Embodiment 1 has information representing the correspondence relationship between this shift amount and the gas concentration. The gas sensor module according to Embodiment 1 estimates the gas concentration by using the correspondence relationship based on the obtained shift amount. The above-mentioned "correlation" and "correspondence relationship" are obtained in advance by a predetermined method.

[0018] <Functional block configuration example of the gas module according to Embodiment 1> FIG. 1A is a diagram showing a configuration example of a gas sensor module according to Embodiment 1 by functional blocks. As shown in FIG. 1A, the gas sensor module 1 according to Embodiment 1 includes a sensor FET 101, a reference FET 102, a gate voltage detector 103, a gate voltage estimator 104, a gas concentration estimator 105, and a controller 106.

[0019] The sensor FET 101 is a FET that reacts to the gas to be detected and changes its current-voltage characteristics according to the concentration of the gas. The reference FET 102 is a FET that does not react to the gas. The reference FET 102 has substantially the same structure as the sensor FET 101, but has a structure in which the gate electrode that reacts to the gas is not exposed to the atmosphere.

[0020] The gate voltages of these FETs are controlled so that the drain-source currents in the gate voltage detector 103, the sensor FET 101, and the reference FET 102 become a constant value that is the set target current. Further, the gate voltage detector 103 detects the respective gate voltages when the drain-source currents in the sensor FET 101 and the reference FET 102 converge to the constant value.

[0021] The gate voltage estimator 104 stores and memorizes correlation relationship information R11 representing the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101 when the gas concentration is 0% when the drain-source current converges to a constant value that is the target current. The gate voltage estimator 104 estimates the gate voltage of the sensor FET when the gas concentration is 0% using the correlation relationship information R11 based on the detected gate voltage of the reference FET 102. Details of the method for estimating the gate voltage of the sensor FET when the gas concentration is 0% will be described later.

[0022] The gas concentration estimator 105 stores and remembers the correspondence information R12 representing the correspondence between the gate voltage of the sensor FET 101 when the drain-source current of the sensor FET 101 converges to a constant value which is the target current, and the gas concentration around the sensor FET 101. The gas concentration estimator 105 estimates the gas concentration around the sensor FET 101 by using the correspondence information R12 based on the estimated value of the gate voltage of the sensor FET 101 in the case of 0% gas concentration obtained from the gate voltage estimator 104 and the detected gate voltage of the sensor FET 101 obtained from the gate voltage detector 103. Details of the method for estimating the gas concentration will be described later.

[0023] The controller 106 appropriately controls the gate voltage detector 103, the gate voltage estimator 104, and the gas concentration estimator 105. For example, the controller 106 performs control necessary for estimating the gate voltage of the sensor FET 101 in the case of 0% gas concentration, obtaining the correlation information R11 necessary for the estimation, estimating the gas concentration, and obtaining the correspondence information R12 necessary for the estimation.

[0024] The gate voltages of the sensor FET 101 and the reference FET 102 are controlled so that the voltages of the drain, source, and back gate are fixed to their respective set voltage values and a target current flows between the drain and the source. The sensor FET 101 and the reference FET 102 are arranged close to each other on the same substrate (chip) and formed by the same semiconductor formation process so that their current-voltage characteristics have substantially the same variation manner according to manufacturing variations and temperature changes. However, between these two, they differ in whether the gate that reacts to the gas is formed so as to be exposed to the atmosphere, that is, so as to be exposed to the atmosphere. The gate of the sensor FET 101 is formed so as to be exposed to the atmosphere, that is, so as to be exposed to the atmosphere. On the other hand, the gate of the reference FET 102 is formed so as not to be exposed to the atmosphere, that is, so as not to be exposed to the atmosphere.

[0025] The gate voltage detector 103 receives a signal representing the target current J0 and an ON signal for starting the operation from the controller 106, and supplies the drain voltage, source voltage, back gate voltage, and gate voltage to each of the sensor FET 101 and the reference FET 102. Further, the gate voltage detector 103 detects the drain-source current of each of the sensor FET 101 and the reference FET 102, and controls the gate voltage of each so that the current becomes a constant value that is the target current J0. Then, the gate voltage detector 103 detects the gate voltage of each when the drain-source current of the sensor FET 101 and the reference FET 102 converges to a constant value that is the target current J0. The gate voltage detector 103 outputs the detected gate voltage of the reference FET 102 to the gate voltage estimator 104, and outputs the detected gate voltage of the sensor FET 101 to the gas concentration estimator 105.

[0026] The gate voltage estimator 104 receives a mode signal from the controller 106, and sets the mode of the gate voltage estimator 104 itself according to the mode signal. The mode of the gate voltage detector 104 includes a normal mode and a learning mode.

[0027] In the normal mode, the gate voltage estimator 104 receives the gate voltage of the reference FET 102 from the gate voltage detector 103. Based on the received gate voltage of the reference FET 102, the gate voltage estimator 104 obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% using the correlation information R11 stored inside. The gate voltage estimator 104 outputs the obtained estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% to the gas concentration estimator 105.

[0028] Note that the correlation information R11 may be, for example, a table in which the gate voltage of the reference FET 102 and the estimated gate voltage of the sensor FET 101 in the case of a gas concentration of 0% are associated with each other for each gate voltage of the reference FET 102. The correlation information R11 may also be, for example, a gate voltage estimation formula that uses the gate voltage of the reference FET 102 as an input parameter and the estimated gate voltage of the sensor FET 101 in the case of a gas concentration of 0% as an output parameter. Here, an example using the gate voltage estimation formula as the correlation information R11 is shown.

[0029] In the learning mode, the gate voltage estimator 104 receives the gate voltage of the sensor FET 101 and the gate voltage of the reference FET 102 from the gate voltage detector 103 obtained under a plurality of conditions, obtains the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101, and generates a correlation formula representing this correlation as a gate voltage estimation formula.

[0030] The gas concentration estimator 105 receives a mode signal from the controller 106 and sets the mode of the gas concentration estimator 105 itself according to the mode signal. The modes of the gas concentration estimator 105 are a normal mode and a learning mode.

[0031] In the normal mode, the gas concentration estimator 105 receives the estimated gate voltage of the sensor FET 101 in the case of a gas concentration of 0% from the gate voltage estimator 104 and receives the detected gate voltage of the sensor FET 101 from the gate voltage detector 103. The gas concentration estimator 105 obtains the difference between the received estimated gate voltage of the sensor FET 101 in the case of a gas concentration of 0% and the received gate voltage of the sensor FET 101 as the shift amount of the gate voltage of the sensor FET 101. The gas concentration estimator 105 estimates the gas concentration using the correspondence information R12 stored therein based on the obtained shift amount of the gate voltage.

[0032] Note that the correspondence information R12 may be, for example, a table in which the shift amount of the gate voltage of the sensor FET 101 is associated with the gas concentration. The correspondence information R12 may also be, for example, a gas concentration estimation formula that uses the shift amount of the gate voltage of the sensor FET 101 as an input parameter and the gas concentration as an output parameter. Here, an example using the gas concentration estimation formula as the correspondence information R12 is shown.

[0033] Also, here, the controller 106 may be incorporated as a sequencer or may be manually controlled.

[0034] <Hardware Configuration Example of Gas Sensor Module> FIG. 1B is a diagram showing a hardware configuration example of the gas sensor module according to Embodiment 1. As shown in FIG. 1B, the gas sensor module 1 according to Embodiment 1 includes a substrate 150, a sensor FET 101, a reference FET 102, and an integrated circuit 151. The integrated circuit 151 includes a processor 1511, a ROM (Read Only Memory) 1512, a RAM (Random Access Memory) 1513, an I / O (Input / Output) interface 1514, an electronic circuit 1515, and the like. The processor 1511 is, for example, a CPU (Central Processing Unit), an MPU (Micro Processor Unit), an MCU (Micro Controller Unit), or the like.

[0035] The sensor FET 101, the reference FET 102, and the integrated circuit 151 are formed on the same substrate 150. Here, it is assumed that the sensor FET 101 and the reference FET 102 are formed on the same chip. Part or all of the integrated circuit 151 including the processor 1511 may be formed on the same chip as the chip on which the sensor FET 101 and the reference FET 102 are formed, or may be formed on a different chip. The substrate 150 is, for example, a silicon substrate, a glass epoxy substrate, or the like. The sensor FET 101, the reference FET 102, and the integrated circuit 151 are formed by, for example, a CMOS formation process which is one of known semiconductor formation processes. The integrated circuit 151 functions as the gate voltage detector 103, the gate voltage estimator 104, the gas concentration estimator 105, and the controller 106 described above, for example, by the processor 1511 executing a predetermined program or controlling the electronic circuit 1515.

[0036] <Structural Example of Sensor FET and Reference FET> FIG. 2 is a diagram showing a structural example of the sensor FET and the reference FET according to Embodiment 1. As shown in FIG. 2, both the sensor FET 101 and the reference FET 102 have a general FET structure. That is, in these FETs, a well 10W is formed in a substrate (Substrate) 10SB, and a backgate (Backgate) 10BG, a drain (Drain) 10D, and a source (Source) 10S are formed. Further, an insulating layer 10X is formed over the drain-source, and a gate (Gate) 10G is formed thereon. The difference between the sensor FET 101 and the reference FET 102 lies in the configuration of the gate electrode portion. The sensor FET 101 is formed such that the gate 10G is exposed to the atmosphere. In contrast, the reference FET 102 is formed covered with a protective film 10P so that the gate 10G is not exposed to the atmosphere. Therefore, the sensor FET 101 reacts to the gas which is the detection target contained in the atmosphere, while the reference FET 102 does not react to the gas which is the detection target contained in the atmosphere.

[0037] <Example of Current-Voltage Characteristics of Sensor FET and Reference FET> FIG. 3 is a diagram showing an example of changes in current-voltage characteristics according to the gas concentration of the sensor FET and the reference FET according to Embodiment 1. In FIG. 3, the graph F31 on the right represents the current-voltage characteristics of the reference FET 102, and the graph F32 on the left represents the current-voltage characteristics of the sensor FET 101. In the graph representing the current-voltage characteristics shown in FIG. 3, the horizontal axis represents the gate voltage Vg, and the vertical axis represents the drain-source current Id, and a curve representing the drain-source current Id with respect to the gate voltage Vg is drawn.

[0038] As shown in the graph F32, the curve representing the current-voltage characteristics of the sensor FET 101 shifts by ΔV in the gate voltage direction from the position of the solid line to the position of the dotted line when the gas concentration changes. This phenomenon occurs because the potential energy of the FET changes when gas ionizes and adheres to the gate. Therefore, the shift amount in the gate voltage direction of the current-voltage characteristics of the sensor FET 101 changes according to the concentration of the gas to be detected in the atmosphere. On the other hand, as shown in the graph F31, the current-voltage characteristics of the reference FET 102 do not shift and remain the same even when the gas concentration changes. From the above, by obtaining the voltage change of the gate voltage when the drain-source current in the sensor FET 101 converges to a certain value, it becomes possible to estimate the gas concentration from the shift amount.

[0039] <Example of Temperature Variation of Voltage-Current Characteristics in Multiple Samples of Sensor FET> However, in the sensor FET, it is known that the gate voltage when the drain-source current is a predetermined constant value varies from FET element to FET element or changes depending on the ambient temperature.

[0040] FIG. 4 is a diagram showing an example of the temperature change of the gate voltage at a constant current in a plurality of samples of the sensor FET. The graph in FIG. 4 shows the change in the gate voltage when the drain-source current is controlled to be constant at 10 μA and the temperature of the sensor FET is gradually changed from 25° C. to 100° C. for seven samples #1 to #7 of the sensor FET. In the graph shown in FIG. 4, the horizontal axis is the temperature (ambient temperature) of the sensor FET, and the vertical axis is the gate voltage when the drain-source current is 10 μA. From the graph of FIG. 4, it is understood that even if the drain-source current of the sensor FET is constant, the gate voltage varies greatly or changes when the elements of the sensor FET are different or the temperature (ambient temperature) of the sensor FET changes.

[0041] Here, the relationship between the gate voltage of the sensor FET 101 and the gate voltage of the reference FET 102 will be examined. As shown in FIG. 2, although the structures of the reference FET 102 and the sensor FET 101 are substantially the same, the gate configurations are slightly different. Therefore, the gate voltage of the reference FET 102 does not become the same as the gate voltage of the sensor FET 101. Accordingly, the difference in the gate voltage of the sensor FET 101 with respect to the reference FET 102 varies for each FET element and also varies depending on the ambient temperature. That is, the difference in the gate voltage between the sensor FET 101 and the reference FET 102 cannot be directly used for compensating for variations due to manufacturing and temperature fluctuations.

[0042] However, as a result of studies by the present inventors, it has been found that there is a strong correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the drain-source current is controlled to a constant value, for example, 10 μA.

[0043] <Example of the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET> FIG. 5 is a diagram showing an example of the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET. The graph shown in FIG. 5 has the date voltage of the sensor FET on the horizontal axis and the gate voltage of the reference FET on the vertical axis, and plots the data when the environmental temperature is changed at three points for seven sets of samples as a combination of the sensor FET and the reference FET.

[0044] As can be understood from FIG. 5, it can be seen that there is a strong correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET, regardless of the difference in the temperature (environmental temperature) of the FET and the difference in the samples. This is because the structures of the sensor FET and the reference FET are similar, so although the absolute values are different in terms of sensitivity to external influences, the ratio is almost the same.

[0045] Therefore, by grasping the correlation between the gate voltage of the sensor FET 101 and the gate voltage of the reference FET 102 (the ratio of the gate voltage of the sensor FET to the gate voltage of the reference FET), it is possible to obtain the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% from the gate voltage of the reference FET 102, regardless of variations due to manufacturing of the current-voltage characteristics or temperature fluctuations.

[0046] <First Configuration Example of Gate Voltage Detector> FIG. 6 is a diagram showing a first configuration example of the gate voltage detector according to Embodiment 1. As shown in FIG. 6, in the first configuration example, the gate voltage detector 103 is composed of a sensor FET side gate voltage detection circuit 601, a reference FET side gate voltage detection circuit 602, and a microcomputer 603. The microcomputer 603 is controlled by an external device connected to the microcomputer 603 when the external device executes a predetermined program, for example.

[0047] The sensor FET side gate voltage detection circuit 601 and the reference FET side gate voltage detection circuit 602 have the same circuit configuration. The sensor FET side gate voltage detection circuit 601 supplies a constant voltage to the drain-back gate, converts the value of the target current J0 set by the microcomputer 603 into a voltage by the DAC (Digital to Analog Converter) 611, and the constant current circuit 610 draws a constant current from the source.

[0048] The amplifier 612 controls the gate voltage so that the source voltage becomes constant, and the gate voltage is converted into a digital value by the ADC (Analog to Digital Converter) 613 and transmitted to the microcomputer 603. Note that the amplifier 612 is, for example, an operational amplifier. Also, the ADC 613 and the DAC 611 may be included in the microcomputer 603.

[0049] The reference FET side gate voltage detection circuit 602 operates in the same manner as the sensor FET side gate voltage detection circuit 601, receives the value of the target current J0 from the microcomputer 603, makes the source voltage constant while making the drain-source current constant, and transmits the gate voltage at that time to the microcomputer 603.

[0050] <Second Configuration Example of Gate Voltage Detector> FIG. 7 is a diagram showing a second configuration example of the gate voltage detector according to Embodiment 1. As shown in FIG. 7, in the second configuration example, the microcomputer 703 directly controls the gate voltage and detects the drain-source current on the drain side. In this case, there is an advantage that it can cope even when the element isolation between the sensor FET 101 and the reference FET 102 is insufficient.

[0051] The sensor FET side gate voltage detection circuit 701 and the reference FET side gate voltage detection circuit 702 have the same circuit configuration. The sensor FET side gate voltage detection circuit 701 receives a gate voltage set value from the microcomputer 703, and supplies the gate voltage to the sensor FET 101 by the DAC 712. At that time, the gate voltage is digitized by the ADC 713, the drain-source current of the sensor FET 101 is obtained by the current detection circuit 710, and digitized by the ADC 711. The digitized gate voltage and the drain-source current are transmitted to the microcomputer 703, and the microcomputer 703 filters and averages them, and changes the gate voltage set value so as to converge the drain-source current to a constant value which is the target current J0.

[0052] The microcomputer 703 performs the same control on the reference FET side gate voltage detection circuit 702. The microcomputer 703 receives a target current value, for example, by executing a predetermined program, and converges the drain-source currents of the sensor FET 101 and the reference FET 102 to a constant value which is the target current value.

[0053] <Gas Concentration Estimation Sequence> FIG. 8 is a diagram showing a gas concentration estimation sequence in the gas sensor module according to Embodiment 1. In this sequence, the controller 106 controls the operations of the gate voltage detector 103, the gate voltage estimator 104, and the gas concentration estimator 105.

[0054] As shown in FIG. 8, first, the controller 106 turns on the gate voltage detector 103, sets the target current J0, and sets the gate voltage estimator 104 and the gas concentration estimator 105 to the normal mode. More specifically, the controller 106 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103, sends the value of the target current setting J0 to the gate voltage detector 103, and sets the target current J0 of the drain-source currents of the sensor FET 101 and the reference FET 102. Further, the controller 106 transmits a mode signal in the normal mode to the gate voltage estimator 104 and the gas concentration estimator 105, and operates the gate voltage estimator 104 and the gas concentration estimator 105 in the normal mode (S101).

[0055] Next, the gate voltage detector 103 controls the respective gate voltages so that the drain-source currents of the sensor FET 101 and the reference FET 102 become the set target current J0 (S102).

[0056] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (S103). Here, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (S103: Yes), the processing step proceeds to the next step S104. If it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value that is the target current J0 (S103: No), the processing step returns to step S102 and the control of the gate voltage continues.

[0057] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the respective gate voltages of the sensor FET 101 and the reference FET 102 (S104).

[0058] Next, based on the acquired gate voltage of the reference FET 102, the gate voltage estimator 104 uses the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% to calculate and output an estimated value of the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% (S105).

[0059] Note that the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101 when the gas concentration is 0% is, for example, the correlation between the gate voltage of the reference FET and the gate voltage of the sensor FET when the gas concentration is 0% in an environment with a gas concentration of 0% as shown in FIG. 5. This correlation can be represented by a table or a mathematical formula. In this example, this correlation will be represented by a mathematical formula, and the mathematical formula will be called the gate voltage estimation formula. The gate voltage estimation formula can be represented, for example, by the following formula (1).

[0060] Vgs = a·Vgr + b ···(1) Vgs: Gate voltage of the sensor FET when the gas concentration is 0% Vgr: Gate voltage of the reference FET a: Coefficient b: Constant

[0061] Here, a linear function is assumed as the gate voltage estimation formula, but a higher-order function of the second order or higher may also be assumed. The gate estimation formula is assumed to be derived, for example, by the learning method described later and stored in the gate voltage estimator 104 for storage and memory.

[0062] Note that when mass-producing the gas sensor module 1, for example, for each individual gas sensor module 1, it is conceivable to derive the gate voltage estimation formula by the learning method and store and memorize the derived gate voltage estimation formula in the gate voltage estimator 104. In this case, although it takes man-hours, since the gate voltage estimation formula is derived for each gas sensor module 1, the gate voltage estimation formula can be derived with high accuracy. As another method, for one gas sensor module 1, the gate voltage estimation formula is derived by the learning method, and the derived gate voltage estimation formula is uniformly stored in the individual gate voltage estimators 104 of a plurality of gas sensor modules 1. In this case, the accuracy of the gate voltage estimation formula may slightly decrease, but man-hours can be reduced.

[0063] Return to the description of the gas concentration estimation sequence. Next to step S105, the gas concentration estimator 105 calculates the difference between the estimated gate voltage of the sensor FET 101 when the gas concentration is 0% and the detected gate voltage of the sensor FET 101, and obtains this difference as the shift amount of the gate voltage from the case where the gas concentration is 0% (S106).

[0064] Then, based on the obtained shift amount of the gate voltage, the gas concentration estimator 105 estimates and outputs the gas concentration using the correspondence relationship between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration (S107). Note that the correspondence relationship between the shift amount of the gate voltage and the gas concentration can be represented by a table or a mathematical formula. In this example, this correspondence relationship will be represented by a mathematical formula, and this mathematical formula will be called the gas concentration estimation formula. The gas concentration estimation formula can be represented by, for example, the following formula (2).

[0065] B = f(ΔVsg) ···(2) B: Gas concentration ΔVsg: Shift amount of the gate voltage of the sensor FET f(ΔVsg): Function with ΔVsg as a parameter

[0066] Here, a higher-order function of the second order or higher is assumed as the gas concentration estimation formula, but a first-order function may also be assumed. The gas concentration estimation formula is assumed to be derived, for example, by the learning method described later and stored in the gas concentration estimator 105 for storage.

[0067] When mass-producing the gas sensor module 1, for example, for each individual gas sensor module 1, it is conceivable to derive a gas concentration estimation formula by the learning method and store and memorize the derived gas concentration estimation formula in the gas concentration estimator 105. In this case, although it takes man-hours, since the gas concentration estimation formula is derived for each gas sensor module 1, the gas concentration estimation formula can be derived with high accuracy. As another method, for one gas sensor module 1, a gas concentration estimation formula is derived by the learning method, and the derived gas concentration estimation formula is uniformly stored in the individual gas concentration estimators 105 of a plurality of gas sensor modules 1. In this case, although the accuracy of the gas concentration estimation formula may slightly decrease, man-hours can be reduced.

[0068] <Learning Method of Gate Voltage Estimation Formula in Gate Voltage Estimator> Next, a method of causing the gate voltage estimator 104 to learn the gate voltage estimation formula will be described. Note that the accuracy of the gate voltage estimation formula improves as the number of samples of the sensor FET used for deriving the gate voltage estimation formula increases. Here, for the sake of simplicity, it is assumed that the number of samples of the sensor FET used for deriving the gate voltage estimation formula is two.

[0069] <Learning Sequence of Gate Voltage Estimation Formula According to Embodiment 1> FIGS. 9A and 9B are diagrams showing a method of causing the gate voltage estimator according to Embodiment 1 to learn the gate voltage estimation formula. As described above, the gate voltage estimation formula is an example of the correlation information R11.

[0070] As shown in FIGS. 9A to 9B, first, the first sensor FET 101a and the first reference FET 102a are connected to the gas sensor module 1. Then, the controller 106 turns on the gate voltage detector 103, sets the target current J0, and sets the gate voltage estimator 104 to the learning mode. More specifically, the controller 106 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103. Also, the controller 106 sends the value of the target current J0 to the gate voltage detector 103 to set the target current J0 for the drain-source current of the first sensor FET 101a and the first reference FET 102a. Further, the controller 106 sends a mode signal in the learning mode to the gate voltage estimator 104 to operate the gate voltage estimator 104 in the learning mode (L101).

[0071] Next, the gate voltage detector 103 controls the respective gate voltages so that the drain-source currents of the first sensor FET 101a and the first reference FET 102a become the set target current J0 (L102).

[0072] The gate voltage detector 103 determines whether or not the drain-source currents of the first sensor FET 101a and the first reference FET 102a have converged to a constant value that is the target current J0 (L103). Here, when it is determined that the drain-source currents of the first sensor FET 101a and the first reference FET 102a have converged to a constant value that is the target current J0 (L103: Yes), the processing step proceeds to the next step L104. On the other hand, when it is determined that the drain-source currents of the first sensor FET 101a and the first reference FET 102a have not converged to a constant value that is the target current J0 (L103: No), the processing step returns to step L102 and the control of the gate voltage is continued.

[0073] After the drain-source currents of the first sensor FET 101a and the first reference FET 102a have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the respective gate voltages of the first sensor FET 101a and the first reference FET 102a (L104).

[0074] Next, the gate voltage estimator 104 stores the detected gate voltages of the first sensor FET 101a and the first reference FET 102a, respectively (L105). Then, the controller 106 turns off the gate voltage detector 103. More specifically, the controller 106 transmits an OFF signal, which is an operation end signal, to the gate voltage detector 103 to shut down the gate voltage detector 103 (L106).

[0075] Thereafter, the first sensor FET 101a and the first reference FET 102a are removed from the gas sensor module 1, and the second sensor FET 101b and the second reference FET 102b are connected to the gas sensor module 1 (L107).

[0076] Thereafter, the controller 106 turns on the gate voltage detector 103 and sets the target current J0. More specifically, the controller 106 transmits the target current J0 and an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103, and sends the target current setting J0 to the gate voltage detector 103 to set the target current J0 of the drain-source current of the sensor FET 101 and the reference FET 102 (L108).

[0077] Next, the gate voltage detector 103 controls the respective gate voltages so that the drain-source currents of the second sensor FET 101b and the second reference FET 102b become the set target current J0 (L109).

[0078] The gate voltage detector 103 determines whether or not the drain-source currents of the second sensor FET 101b and the second reference FET 102b converge to a constant value which is the target current J0 (L110). Here, when it is determined that the drain-source currents of the second sensor FET 101b and the second reference FET 102b converge to a constant value which is the target current J0 (L110: Yes), the processing step proceeds to the next step L111. On the other hand, when it is determined that the drain-source currents of the second sensor FET 101b and the second reference FET 102b do not converge to a constant value which is the target current J0 (L110: No), the processing step returns to step L109 and the control of the gate voltage is continued.

[0079] After the drain-source currents of the second sensor FET 101b and the second reference FET 102b converge to a constant value which is the target current J0, the gate voltage detector 103 detects and acquires the respective gate voltages of the second sensor FET 101b and the second reference FET 102b (L111).

[0080] Next, the gate voltage estimator 104 stores the detected gate voltages of the second sensor FET 101b and the second reference FET 102b respectively (L112). Thereafter, the controller 106 turns off the gate voltage detector 103.

[0081] Then, based on the data group including the respective gate voltages of the first sensor FET 101a and the first reference FET 102a and the respective gate voltages of the second sensor FET 101b and the second reference FET 102b, the gate voltage estimator 104 derives the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101 when the gas concentration is 0%, that is, the gate voltage estimation formula, and stores it in the gate voltage estimator 104 (L113). For the derivation of the gate voltage estimation formula, for example, a linear approximation to the above data group of gate voltages is used.

[0082] Note that, in this example, the correlation information R11 is a gate voltage estimation formula. As the gate voltage estimation formula, a linear function is assumed, but a higher-order function of the second order or higher may be assumed. Further, this example is an example in the case where the number of samples of the sensor FET 101 is two, but even when the number of samples of the sensor FET 101 is three or more, the basic part in the learning method of the gate voltage estimation formula is the same. Further, in this example, one reference FET 102 is combined for each sensor FET 101, but one same reference FET 102 may be combined for a plurality of sensor FETs 101. Note that, the larger the number of samples of the sensor FET 101, the higher the accuracy of the gate voltage estimation formula, but the man-hour increases. Therefore, the number of samples may be determined in consideration of the balance between the accuracy and the man-hour.

[0083] <Learning sequence of gas concentration estimation formula according to Embodiment 1> Next, a method for causing the gas concentration estimator 105 to learn the gas concentration estimation formula will be described.

[0084] FIGS. 10A, 10B, and 10C are diagrams showing a method for causing the gas concentration estimator according to Embodiment 1 to learn the gas concentration estimation formula.

[0085] As shown in FIGS. 10A to 10C, first, the controller 106 turns on the gate voltage detector 103, sets the target current J0, and sets the gas concentration estimator 105 to the learning mode. More specifically, the controller 106 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103. Further, the controller 106 sends the value of the target current J0 to the gate voltage detector 103 to set the target current J0 of the drain-source current of the sensor FET 101 and the reference FET 102. Further, the controller 106 sends a mode signal in the learning mode to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to the learning mode (L121).

[0086] The gas concentration around the sensor FET 101 is adjusted to a known first concentration C1 (L122). The first concentration C1 is, for example, a 1% concentration.

[0087] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current J0 (L123).

[0088] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L124). Here, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L124: Yes), the processing step proceeds to the next step L125. On the other hand, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value that is the target current J0 (L124: No), the processing step returns to step L123 and the control of the gate voltage is continued.

[0089] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L125).

[0090] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 in the case where the gas concentration is 0% using a gate voltage estimation formula (L126).

[0091] The gas concentration estimator 105 obtains and stores, as a shift amount of the gate voltage of the sensor FET 101 at the first concentration C1, the difference between the estimated value of the gate voltage of the sensor FET 101 in the case where the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the first gas concentration C1 (L127).

[0092] Next, the gas concentration around the sensor FET 101 is adjusted to a known second concentration C2 (L128). The second concentration C2 is, for example, a 10% concentration.

[0093] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current value J0 (L129).

[0094] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L130). Here, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L130: Yes), the processing step proceeds to the next step L131. On the other hand, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value that is the target current J0 (L130: No), the processing step returns to step L129 and the control of the gate voltage continues.

[0095] Incidentally, while the gas concentration is being changed, conditions such as the ambient temperature may change. That is, when the gas concentration is changed, it is better to detect the gate voltage of the reference FET 102 again and obtain a new estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% based on the gate voltage of the reference FET 102. Therefore, in this example, the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% is recalculated from the gate voltage of the reference FET 102 detected after the gas concentration is changed.

[0096] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L131).

[0097] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% by using a gate voltage estimation formula (L132).

[0098] The gas concentration estimator 105 obtains and stores, as the shift amount of the gate voltage of the sensor FET 101 at the second concentration C2, the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the second gas concentration C2 (L133).

[0099] Next, the gas concentration around the sensor FET 101 is adjusted to a known third concentration C3 (L134). The third concentration C3 is, for example, 90% concentration.

[0100] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source current between the sensor FET 101 and the reference FET 102 converges to a constant value which is the target current J0 (L135).

[0101] The gate voltage detector 103 determines whether or not the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value which is the target current J0 (L136). Here, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value which is the target current J0 (L136: Yes), the processing step proceeds to the next step L137. On the other hand, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has not converged to a constant value which is the target current J0 (L136: No), the processing step returns to step L135 and the control of the gate voltage is continued.

[0102] After the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value which is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L137).

[0103] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% using a gate voltage estimation formula (L138).

[0104] The gas concentration estimator 105 obtains and stores, as a shift amount of the gate voltage of the sensor FET 101 at the third concentration C3, the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the third gas concentration C3 (L139).

[0105] Thereafter, the gas concentration estimator 105 performs a higher-order polynomial approximation for the values of the first gas concentration C1, the second gas concentration C2, and the third gas concentration C3 of the gas with respect to the shift amounts of the gate voltages of the sensor FET 101 at the first concentration C1, the second concentration C2, and the third concentration C3, respectively, in order to derive a gate voltage estimation formula. Then, the gas concentration estimator 105 obtains a gas concentration estimation formula as correspondence information R12 representing the correspondence between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration from the result of the polynomial approximation (L140).

[0106] The gas concentration estimator 105 stores the obtained gas concentration estimation formula inside the gas concentration estimator 105 (L141).

[0107] <Examples of Effects According to Embodiment 1> According to such Embodiment 1, according to the learning sequence, the correlation between the gate voltage of the reference FET and the gate voltage of the sensor FET when the gas concentration is 0% is obtained and stored in advance. And in Embodiment 1, based on the detected gate voltage of the reference FET, using this correlation, the estimated value of the gate voltage of the sensor FET when the gas concentration is 0% is obtained. Therefore, in Embodiment 1, it is possible to estimate the gate voltage of the sensor FET when the gas concentration is 0%, in which the variations caused by manufacturing in the current-voltage characteristics of the sensor FET and the shifts due to temperature fluctuations are corrected, and it is possible to more accurately obtain the shift amount of the gate voltage of the sensor FET according to the gas concentration. As a result, according to Embodiment 1, the concentration of the gas to be detected can be obtained with higher accuracy.

[0108] (Embodiment 2) The gas sensor module according to Embodiment 2 further includes a temperature control mechanism for controlling the temperatures of the sensor FET and the reference FET as compared with Embodiment 1.

[0109] The inventors have confirmed that depending on the type of gas to be detected, the temperature range of the sensor FET in which the detection sensitivity of the gas in the sensor FET becomes a certain level or higher is different. Therefore, in the gas sensor module according to Embodiment 2, according to the type of gas to be detected, the temperatures of the sensor FET and the reference FET are controlled to be constant at a predetermined target temperature using the temperature control mechanism so that the detection sensitivity of the gas in the sensor FET becomes a certain level or higher, or maximum.

[0110] Also, in Embodiment 2, the gate voltage estimation formula is obtained by implementing a learning sequence using the temperature variations of the gate voltages in the sensor FET and the reference FET. The gas concentration estimation formula is obtained by implementing the learning sequence in a state where the temperature of the sensor FET is controlled to be constant at the above target temperature.

[0111] <Configuration example of the gas sensor module according to Embodiment 2> FIG. 11 is a diagram showing a configuration example of the gas sensor module according to Embodiment 2. As shown in FIG. 11, the gas sensor module 2 according to Embodiment 2 further includes a temperature control mechanism 1100 with respect to the gas sensor module 1 according to Embodiment 1. The temperature control mechanism 1100 includes a controller 1101, a temperature sensor 1102, a heater 1103, and a temperature controller 1104.

[0112] The controller 1101 has substantially the same functions as the controller 106 in Embodiment 1, and further has a function of controlling the temperature controller 1104.

[0113] The temperature sensor 1102 converts the ambient temperature of the temperature sensor 1102 into a physical quantity related to physical properties, such as voltage or resistance value. The temperature sensor 1102 is, for example, a thermocouple, a platinum resistance thermometer, a thermistor thermometer, or the like.

[0114] The heater 1103 changes the calorific value by on / off control by the temperature controller 1104 or control of the supplied power. The heater 1103 is, for example, a resistor, a heating wire, or the like.

[0115] The temperature controller 1104 controls the heater 1103 so that the temperature detected based on the temperature sensor 1102 approaches the set target temperature T0. The target temperature T0 is a temperature determined so that the detection sensitivity for the detection target gas of the sensor FET 101 is at a certain level or higher or maximized. The temperature sensor 1102 and the heater 1103 are arranged in the vicinity of the sensor FET 101 and the reference FET 102. The temperature controller 1104 controls the heater 1103 to heat the sensor FET 101 and the reference FET 102, and keeps the temperatures of these FETs constant at the target temperature T0.

[0116] Incidentally, as described above, the inventors have confirmed that the temperature range of the sensor FET 101 in which the detection sensitivity becomes a certain level or higher varies depending on the type of gas to be detected. Further, the gas sensor module 2 includes a temperature control mechanism 1100 for controlling the temperatures of the sensor FET 101 and the reference FET 102. Therefore, in the gas sensor module 2, by changing the temperature of the sensor FET 101 according to the gas to be detected, it is possible to detect a desired type of gas among a plurality of types of gases with high sensitivity.

[0117] Further, in the gas sensor module 2, by causing the gate voltage estimator 104 to learn the temperature dependence of the current-voltage characteristics of the sensor FET 101, an effect of improving the estimation accuracy of the sensor FET gate voltage in the case where the gas concentration is 0% is also expected. Similarly, by causing the gas concentration estimator 105 to learn the temperature dependence of the variation of the current-voltage characteristics of the sensor FET 101 according to the gas concentration, an effect of improving the estimation accuracy of the gas concentration is also expected.

[0118] Furthermore, the inventors have confirmed that the performance of the sensor FET changes or deteriorates with the use or over time of the gas sensor module. As described above, the gas sensor module 2 includes the temperature control mechanism 1100. Therefore, by heating the sensor FET 101 temporarily to a high temperature by the temperature control mechanism 1100, foreign matter attached to the gate of the sensor FET 101 is removed, and an effect of restoring the changed or deteriorated performance of the sensor FET 101 to a state closer to the initial state than the current state is also expected. Note that the foreign matter is a substance other than the gas to be detected, and may be in any form such as a gas, a solid, or a liquid.

[0119] In FIG. 11, the controller 1101 sends an ON signal and the value of the target temperature T0 to the temperature controller 1104. The temperature controller 1104 compares the target temperature T0 with the temperature represented by the information from the temperature sensor 1102, and controls the heater 1103 so that the temperatures of the sensor FET 101 and the reference FET 102 become the target temperature T0. The operations of the gate voltage detector 103, the gate voltage estimator 104, and the gas concentration estimator 105 are the same as those in the first embodiment.

[0120] <Learning Sequence of Gate Voltage Estimation Formula According to Embodiment 2> FIGS. 12A and 12B are diagrams showing a method of learning a gate voltage estimation formula in the gate voltage estimator according to Embodiment 2. In FIGS. 12A to 12B, for a set of sensor FET and reference FET, the temperature of the FET is changed to a plurality of temperatures to obtain the gate voltage at each temperature, and based on the obtained data group of the gate voltages, a learning sequence for obtaining a gate voltage estimation formula as the correlation information R21 is shown.

[0121] As shown in FIGS. 12A to 12B, the controller 1101 turns on the gate voltage detector 103, sets the target current J0, and sets the gate voltage estimator 104 to the learning mode. More specifically, the controller 1101 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103. Further, the controller 1101 sends the value of the target current J0 to the gate voltage detector 103 to set the target current J0 of the drain-source current of the sensor FET 101 and the reference FET 102. Furthermore, the controller 1101 sends a mode signal in the learning mode to the gate voltage estimator 104 to operate the gate voltage estimator 104 in the learning mode (L201).

[0122] Next, the controller 1101 turns on the temperature controller 1104 and sets the target temperatures of the sensor FET 101 and the reference FET 102 to the first temperature T1. More specifically, the controller 1101 transmits an ON signal to the temperature controller 1104 to start up the temperature controller 1104. Also, the controller 1101 sends the value of the first temperature T1 to the temperature controller 1104 as a target current value to set the target temperatures of the sensor FET 101 and the reference FET 102 to the first temperature T1 (L202). The first temperature T1 is, for example, 40 degrees Celsius (°C).

[0123] Next, the temperature controller 1104 controls the heater 1103 based on the information from the temperature sensor 1102 so that the temperatures of the sensor FET 101 and the reference FET 102 become the first temperature T1 (L203).

[0124] The temperature controller 1104 determines whether or not the temperatures of the sensor FET 101 and the reference FET have converged to the first temperature T1, which is the target temperature (L204). Here, if it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have converged to the first temperature T1 (L204: Yes), the processing step proceeds to the next step L205. On the other hand, if it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have not converged to the first temperature T1 (L204: No), the processing step returns to step L203 and the control of the heater 1103 continues.

[0125] Next, the gate voltage detector 103 controls the respective gate voltages so that the drain-source currents of the sensor FET 101 and the reference FET 102 become the set target current J0 (L205).

[0126] The gate voltage detector 103 determines whether the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value, which is the target current J0 (L206). Here, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value, which is the target current J0 (L206: Yes), the processing step proceeds to the next step L207. On the other hand, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 do not converge to a constant value, which is the target current J0 (L206: No), the processing step returns to step L205, and the control of the gate voltage continues.

[0127] After the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value, which is the target current J0, the gate voltage detector 103 detects and acquires the respective gate voltages of the sensor FET 101 and the reference FET 102 (L207).

[0128] The gate voltage detector 103 stores the gate voltage acquired in step L207 as data on the gate voltages of the sensor FET 101 and the reference FET 102 at the first temperature T1 (L208).

[0129] Next, the controller 1101 sends the value of the second temperature T2 to the temperature controller 1104 as the target temperature, and sets the target temperatures of the sensor FET 101 and the reference FET 102 to the second temperature T2 (L209). The second temperature T2 is, for example, 80 degrees Celsius (°C).

[0130] Next, the temperature controller 1104 controls the heater 1103 based on the information from the temperature sensor 1102 so that the temperatures of the sensor FET 101 and the reference FET 102 become the second temperature T2 (L210).

[0131] The temperature controller 1104 determines whether the temperatures of the sensor FET 101 and the reference FET have converged to a second temperature T2, which is the target temperature (L211). Here, if it is determined that the temperatures of the sensor FET 101 and the reference FET have converged to the second temperature T2 (L211: Yes), the processing step proceeds to the next step L212. On the other hand, if it is determined that the temperatures of the sensor FET 101 and the reference FET have not converged to the second temperature T2 (L211: No), the processing step returns to step L210, and the control of the heater 1103 continues.

[0132] Next, the gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source current between them becomes a set target current J0 (L212).

[0133] The gate voltage detector 103 determines whether the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value, which is the target current J0 (L213). Here, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value, which is the target current J0 (L213: Yes), the processing step proceeds to the next step L214. On the other hand, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has not converged to a constant value, which is the target current J0 (L213: No), the processing step returns to step L212, and the control of the gate voltage continues.

[0134] After the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value, which is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L214).

[0135] The gate voltage detector 103 stores the gate voltage acquired in step L214 as data on the gate voltages of the sensor FET 101 and the reference FET 102 at the second temperature T2 (L215).

[0136] Finally, based on the data groups of the gate voltages of the sensor FET 101 and the reference FET 102 at the first temperature T1 and the second temperature T2, the gate voltage estimator 104 obtains the correlation between the gate voltage of the sensor FET 101 with respect to the gate voltage of the reference FET 102, and internally stores the correlation information R21 representing the correlation (L216).

[0137] Note that in this example, the correlation information R21 is a gate voltage estimation formula, and a linear function is assumed as the gate voltage estimation formula. Also, in this example, the correlation is obtained by plotting two points of the sensor FET gate voltage at a constant current with respect to temperature, but it may also be obtained by plotting three or more points. Further, in this example, only one sample is used as the sensor FET, but a plurality of samples may be used. When using a plurality of samples, the processes other than the last process of the sequence in FIG. 12B are performed for each of the plurality of samples, and the last process of the sequence is performed only for the last sample. Note that the higher the number of samples of the sensor FET 101 or the number of set temperatures, the higher the accuracy of the gate voltage estimation formula, but the man-hours increase. Therefore, the number of samples of the sensor FET 101 or the number of set temperatures may be determined in consideration of the balance between these accuracy and man-hours.

[0138] <Gas Concentration Estimation Formula Learning Sequence According to Embodiment 2> FIGS. 13A, 13B, and 13C are diagrams showing a method of learning a gas concentration estimation formula in the gas concentration estimator according to Embodiment 2.

[0139] As shown in FIGS. 13A to 13C, first, the controller 1101 turns on the gate voltage detector 103, sets the target current J0, and sets the gas concentration estimator 105 to the learning mode. More specifically, the controller 1101 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103. Also, the controller 1101 sends the value of the target current J0 to the gate voltage detector 103 to set the target current J0 for the drain-source current of the sensor FET 101 and the reference FET 102. Further, the controller 1101 sends a mode signal in the learning mode to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to the learning mode (L221).

[0140] The controller 1101 turns on the temperature controller 1104 and sets the target temperature T0. More specifically, the controller 1101 transmits an ON signal and the target temperature T0 to the temperature controller 1104 (L222). The target temperature T0 is, for example, the temperature of the sensor FET 101 such that the sensitivity to the gas to be detected in the sensor FET 101 is at a certain level or maximum.

[0141] The temperature controller 1104 controls the heater 1103 based on the information from the temperature sensor 1102 and the set value of the target temperature T0 so that the temperatures of the sensor FET 101 and the reference FET 102 converge to a certain value that is the target temperature T0 (L223).

[0142] The temperature controller 1104 determines whether or not the temperatures of the sensor FET 101 and the reference FET 102 have converged to a certain value that is the target temperature T0 (L224). Here, when it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have converged to a certain value that is the target temperature T0 (L224: Yes), the processing step proceeds to the next step L225. On the other hand, when it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have not converged to a certain value that is the target temperature T0 (L224: No), the processing step returns to step L223 and the control of the heater 1103 continues.

[0143] After the temperatures of the sensor FET 101 and the reference FET 102 converge to a constant value which is the target temperature T0, the gas concentration around the sensor FET 101 is adjusted to a known first concentration C1 (L225). The first concentration C1 is, for example, 1% concentration.

[0144] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value which is the target current J0 (L226).

[0145] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L227). Here, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L227: Yes), the processing step proceeds to the next step L228. On the other hand, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value which is the target current J0 (L227: No), the processing step returns to step L226 and the control of the gate voltage is continued.

[0146] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L228).

[0147] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% by using a gate voltage estimation formula (L229).

[0148] The gas concentration estimator 105 obtains and stores, as the shift amount of the gate voltage of the sensor FET 101 at the first concentration C1, the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the first gas concentration C1 (L230).

[0149] Next, the gas concentration around the sensor FET 101 is adjusted to a known second concentration C2 (L231). The second concentration C2 is, for example, 10% concentration.

[0150] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source current between the sensor FET 101 and the reference FET 102 converges to a constant value that is the target current J0 (L232).

[0151] The gate voltage detector 103 determines whether or not the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value that is the target current J0 (L233). Here, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value that is the target current J0 (L233: Yes), the processing step proceeds to the next step L234. On the other hand, if it is determined that the drain-source current between the sensor FET 101 and the reference FET 102 has not converged to a constant value that is the target current J0 (L233: No), the processing step returns to step L232 and the control of the gate voltage is continued.

[0152] After the drain-source current between the sensor FET 101 and the reference FET 102 has converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L234).

[0153] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% using the gate voltage estimation formula (L235).

[0154] The gas concentration estimator 105 obtains and stores, as the shift amount of the gate voltage of the sensor FET 101 at the second concentration C2, the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the second gas concentration C2 (L236).

[0155] Next, the gas concentration around the sensor FET 101 is adjusted to a known third concentration C3 (L237). The third concentration C3 is, for example, 90% concentration.

[0156] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current J0 (L238).

[0157] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L239). Here, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L239: Yes), the processing step proceeds to the next step L240. On the other hand, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value that is the target current J0 (L239: No), the processing step returns to step L238, and the control of the gate voltage is continued.

[0158] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L240).

[0159] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% using a gate voltage estimation formula (L241).

[0160] The gas concentration estimator 105 obtains and stores, as the shift amount of the gate voltage of the sensor FET 101 at the third concentration C3, the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the detected gate voltage of the sensor FET 101 at the third gas concentration C3 (L242).

[0161] Thereafter, the gas concentration estimator 105 performs a higher-order polynomial approximation for the values of the first gas concentration C1, the second gas concentration C2, and the third gas concentration C3 of the gas with respect to the shift amounts of the gate voltages of the respective sensor FETs 101 at the first concentration C1, the second concentration C2, and the third concentration C3, in order to derive a gate voltage estimation formula. Then, the gas concentration estimator 105 obtains a gas concentration estimation formula as correspondence information R22 representing the correspondence relationship between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration from the result of the polynomial approximation (L243).

[0162] The gas concentration estimator 105 stores the obtained gas concentration estimation formula inside the gas concentration estimator 105 (L244).

[0163] Here, an example in the case where the number of FET samples is one is shown. When the number of FET samples is plural, the processes other than the last process (L244) of the sequence in FIG. 13C are performed for each of the plural samples, and the last process (L244) of the sequence is performed only when performing the process for the last sample.

[0164] <Example of effects according to Embodiment 2> According to such Embodiment 2, the temperature of the sensor FET is controlled using a temperature control mechanism so that the detection sensitivity for the gas to be detected is at a certain level or higher or becomes the maximum target temperature. Further, according to Embodiment 2, a learning sequence is performed in a state where the temperatures of the sensor FET and the reference FET are controlled to be constant at the target temperature, and a gas concentration estimation formula at the target temperature is obtained. Then, the gas concentration is estimated using the gas concentration estimation formula at the target temperature. Therefore, according to Embodiment 2, the concentration of the gas to be detected can be estimated with higher accuracy.

[0165] Further, according to Embodiment 2, a learning sequence is performed in which the temperatures of the sensor FET and the reference FET are changed to obtain the gate voltages of the respective FETs, and a gate voltage estimation formula is obtained based on the obtained gate voltages. Therefore, according to Embodiment 2, without replacing the FET element, a gate voltage estimation formula as correlation relationship information can be obtained, and correlation relationship information can be obtained without taking time or effort.

[0166] (Embodiment 3) The gas sensor module according to Embodiment 3 stores a plurality of gas concentration estimation formulas corresponding to a plurality of target temperatures of the sensor FET as compared with Embodiment 2. The plurality of target temperatures are composed of the temperatures of the sensor FET at which the detection sensitivity is at a certain level or higher for each of the plurality of types of gases when there are a plurality of candidate gases that can be detected.

[0167] As described above, the inventors have confirmed that depending on the type of gas to be detected, the temperature range of the sensor FET in which the gas detection sensitivity in the sensor FET is equal to or higher than a certain level is different for each. Therefore, in the gas sensor module according to Embodiment 3, a plurality of types of gases to be detection candidates are provided, and the gas to be detected is selected from among the plurality of types of gases. Then, the temperature of the sensor FET is controlled using a temperature control mechanism so as to reach the target temperature corresponding to the selected gas to be detected. This target temperature is the temperature of the sensor FET at which the detection sensitivity of the sensor FET for the selected gas to be detected is equal to or higher than a certain level or is maximum. In the estimation of the gas concentration, a gas concentration estimation formula at the target temperature corresponding to the selected gas type is used.

[0168] Also, in Embodiment 3, the gate voltage estimation formula is obtained by implementing the same learning sequence as in Embodiment 2. The gas concentration estimation formula is obtained by implementing the learning sequence in each of the temperature states in which the temperature of the sensor FET is controlled to each of the plurality of target temperatures corresponding to the plurality of types of gases so as to be prepared for each target temperature corresponding to the plurality of types of gases that are detection candidates.

[0169] FIG. 14 is a diagram showing a configuration example of the gas sensor module according to Embodiment 3. As shown in FIG. 14, in the gas sensor module 3 according to Embodiment 3, compared with the gas sensor module 2 according to Embodiment 2, the correspondence relation information R32 included in the gas concentration estimator 105 includes a gas concentration estimation formula for each target temperature corresponding to the gas type. The gas concentration estimation formula for each target temperature is a formula representing the correspondence relation between the gate voltage of the sensor FET and the estimated gas concentration at each target temperature.

[0170] In FIG. 14, the gas concentration estimator 105 receives a target temperature from the controller 1101 in addition to the mode signal. Then, the gas concentration estimator 105 uses the estimated value of the sensor FET gate voltage when the gas concentration is 0% output from the gate voltage estimator 104, the gate voltage of the sensor FET output from the gate voltage detector 103, and the target temperature to switch the gas concentration estimation formula to be used according to the target temperature, and estimates the gas concentration. The operations of the other components are the same as those in the second embodiment. Also, the learning method of the gate voltage estimator 104 is the same as that in the second embodiment (FIG. 13).

[0171] <Learning Sequence of Gas Concentration Estimation Formula According to Embodiment 3> FIGS. 15A, 15B, and 15C are diagrams showing a method of causing the gas concentration estimator according to Embodiment 3 to learn a gas concentration estimation formula for each temperature.

[0172] As shown in FIGS. 15A to 15C, the controller 1101 turns on the gate voltage detector 103, sets the target current J0, and sets the gas concentration estimator 105 to the learning mode. More specifically, the controller 1101 transmits an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103. Also, the controller 1101 sends the value of the target current J0 to the gate voltage detector 103 to set the target current J0 of the drain-source current of the sensor FET 101 and the reference FET 102. Further, the controller 1101 sends a mode signal in the learning mode to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to the learning mode. Further, the controller 1101 sets the parameter N corresponding to the type of gas to N = 1. (L301).

[0173] The controller 1101 turns on the temperature controller 1104 and sets, as the target temperature, the Nth temperature TT(N) suitable for the detection of the Nth gas. More specifically, the controller 1101 transmits an ON signal and the Nth temperature TT(N) which is the target temperature to the temperature controller 1104 (L302). The Nth temperature TT(N) which is the target temperature is, for example, the temperature of the sensor FET 101 such that the sensitivity to the Nth gas to be detected in the sensor FET 101 is at a certain level or maximum. Therefore, when the process of step L302 is performed for the first time, since N = 1, the first temperature TT(1) suitable for the first gas is set as the target temperature.

[0174] The temperature controller 1104 controls the heater 1103 based on the information from the temperature sensor 1102 and the value of the Nth temperature TT(N) which is the set target temperature, so that the temperatures of the sensor FET 101 and the reference FET 102 converge to a certain value which is the Nth temperature TT(N) (L303).

[0175] The temperature controller 1104 determines whether or not the temperatures of the sensor FET 101 and the reference FET 102 have converged to a certain value which is the Nth temperature TT(N) (L304). Here, if it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have converged to a certain value which is the Nth temperature TT(N) (L304: Yes), the process step proceeds to the next step L305. On the other hand, if it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have not converged to a certain value which is the Nth temperature TT(N) (L304: No), the process step returns to step L303 and the control of the heater 1103 continues.

[0176] After the temperatures of the sensor FET 101 and the reference FET 102 have converged to a certain value which is the Nth temperature TT(N), the gas concentration around the sensor FET 101 is adjusted to a known first concentration C1 (L305). The first concentration C1 is, for example, a 1% concentration.

[0177] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value which is the target current J0 (L306).

[0178] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L307). Here, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L307: Yes), the processing step proceeds to the next step L308. On the other hand, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value which is the target current J0 (L307: No), the processing step returns to step L306 and the control of the gate voltage is continued.

[0179] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L308).

[0180] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% using a gate voltage estimation formula (L309).

[0181] The gas concentration estimator 105 obtains the difference between the estimated value of the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% and the gate voltage of the sensor FET 101 detected at the first gas concentration C1, and saves it as the shift amount of the gate voltage of the sensor FET 101 at the first concentration C1 (L310).

[0182] Next, the gas concentration around the sensor FET 101 is adjusted to a known second concentration C2 (L311). The second concentration C2 is, for example, 10% concentration.

[0183] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value which is the target current J0 (L312).

[0184] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L313). Here, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0 (L313: Yes), the processing step proceeds to the next step L314. On the other hand, when it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value which is the target current J0 (L313: No), the processing step returns to step L312 and the control of the gate voltage is continued.

[0185] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value which is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L314).

[0186] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% using a gate voltage estimation formula (L315).

[0187] The gas concentration estimator 105 obtains the difference between the estimated value of the gate voltage of the sensor FET 101 in the case of a gas concentration of 0% and the gate voltage of the sensor FET 101 detected at the second gas concentration C2, and saves it as the shift amount of the gate voltage of the sensor FET 101 at the second concentration C2 (L316).

[0188] Next, the gas concentration around the sensor FET 101 is adjusted to a known third concentration C3 (L317). The third concentration C3 is, for example, a 90% concentration.

[0189] The gate voltage detector 103 controls the gate voltages of the sensor FET 101 and the reference FET 102 so that the drain-source currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current J0 (L318).

[0190] The gate voltage detector 103 determines whether or not the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L319). Here, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0 (L319: Yes), the processing step proceeds to the next step L320. On the other hand, if it is determined that the drain-source currents of the sensor FET 101 and the reference FET 102 have not converged to a constant value that is the target current J0 (L319: No), the processing step returns to step L318 and the control of the gate voltage is continued.

[0191] After the drain-source currents of the sensor FET 101 and the reference FET 102 have converged to a constant value that is the target current J0, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 respectively (L320).

[0192] Based on the detected gate voltage of the reference FET 102, the gate voltage estimator 104 calculates and obtains an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% using the gate voltage estimation formula (L321).

[0193] The gas concentration estimator 105 obtains the difference between the estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the third gas concentration C3, and saves it as the shift amount of the gate voltage of the sensor FET 101 at the third concentration C3 (L322).

[0194] Thereafter, the gas concentration estimator 105 performs a higher-order polynomial approximation to derive a gate voltage estimation formula for the first gas concentration value C1, the second gas concentration value C2, and the third gas concentration value C3 with respect to the shift amount of the gate voltage of each sensor FET 101 at the first concentration C1, the second concentration C2, and the third concentration C3. Then, the gas concentration estimator 105 obtains a gas concentration estimation formula as correspondence information R32 representing the correspondence between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration from the result of the polynomial approximation (L323).

[0195] The gas concentration estimator 105 stores the obtained gas concentration estimation formula inside the gas concentration estimator 105 as a gas concentration estimation formula corresponding to the Nth temperature TT(N) (L324).

[0196] The controller 1101 determines whether there is a gas concentration estimation formula to be learned, that is, whether to learn a gas concentration estimation formula corresponding to the next gas different from the gases learned so far (L325). Here, when it is determined to learn a gas concentration estimation formula corresponding to the next gas (L325: Yes), the controller 1101 increments the parameter N by 1 and executes the process of N←N + 1 (L326), and the process step returns to step L302. On the other hand, when it is determined not to learn a gas concentration estimation formula corresponding to the next gas (L325: No), the controller 1101 terminates the learning.

[0197] In this way, for each of a plurality of target temperatures TT(1), TT(2), ··· corresponding to a plurality of types of gases, by performing a similar learning sequence, a gas concentration estimation formula at a temperature suitable for each of the plurality of types of gases can be obtained.

[0198] <Example of effects according to Embodiment 3> According to such Embodiment 3, the temperature of the sensor FET is controlled and held so that the detection sensitivity of the sensor FET with respect to the gas to be detected is at a certain level or higher or maximum according to the type of the gas to be detected. Further, according to Embodiment 3, gas concentration estimation formulas at respective target temperatures corresponding to a plurality of gases to be detected are obtained and stored. Therefore, according to Embodiment 3, the gas concentration can be estimated with high accuracy for a plurality of types of gases.

[0199] As described above, each embodiment has been explained. However, the present invention is not limited to these embodiments, and various modifications are possible without departing from the spirit of the invention. Also, the numbers or names etc. used in each embodiment are merely examples, and other numbers or names etc. may be used.

Explanation of Reference Signs

[0200] 1, 2, 3 Gas sensor module 101, 101a, 101b Sensor FET 102, 102a, 102b Reference FET 103 Gate voltage detector 104 Gate voltage estimator 105 Gas concentration estimator 106 Controller 150 Substrate 151 Integrated circuit 601 Sensor FET side gate voltage detection circuit 602 Reference FET side gate voltage detection circuit 603 MCU (Micro Controller Unit) 610 Constant current circuit 611 DAC (Digital to Analog Converter) 612 Amplifier 613 ADC (Analog to Digital Converter) 701 Sensor FET side gate voltage detection circuit 702 Reference FET side gate voltage detection circuit 703 MCU 710 Current Detection Circuit 711 ADC 712 DAC 713 ADC 1100 Temperature Control Mechanism 1101 Controller 1102 Temperature Sensor 1103 Heater 1104 Temperature Controller 1511 Processor 1512 ROM 1513 RAM 1514 I / O Interface 1515 Electronic Circuit

Claims

1. A sensor FET disposed on a substrate and reacting with a gas in the atmosphere, A reference FET disposed on the substrate and not reacting with the gas, A gate voltage detector that controls the gate voltages of the sensor FET and the reference FET so that the drain-source current between the sensor FET and the reference FET becomes a target current, and detects the respective gate voltages of the sensor FET and the reference FET in a state where the drain-source current between the sensor FET and the reference FET is the target current; A gate voltage estimator that obtains an estimated value of the gate voltage of the sensor FET when the gas concentration is 0% by using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the gas concentration is 0% based on the detected gate voltage of the reference FET; A gas concentration estimator that estimates the gas concentration by using the correspondence between the shift amount of the gate voltage of the sensor FET and the gas concentration based on the shift amount of the detected gate voltage of the sensor FET with respect to the estimated value of the gate voltage of the sensor FET obtained; comprising A gas sensor module.

2. In the gas sensor module according to Claim 1, The correlation is For a plurality of samples of the sensor FET and one or more samples of the reference FET, the gate voltages of the sensor FET and the reference FET in an environment where the gas concentration is 0% are detected, and it is obtained based on the detected gate voltage data group. A gas sensor module.

3. In the gas sensor module according to Claim 1, The correspondence is In each of a plurality of environments adjusted to respective known concentrations of the gas, the difference between the estimated value of the gate voltage of the sensor FET when the concentration of the gas is 0% and the detected gate voltage of the sensor FET is obtained as a shift amount, and is obtained based on the data group of the obtained shift amounts. Gas sensor module.

4. A sensor FET disposed on a substrate and reacting to a gas in the atmosphere, A reference FET disposed on the substrate and not reacting to the gas, A temperature sensor for obtaining the temperatures of the sensor FET and the reference FET, A heater for heating the sensor FET and the reference FET, A temperature controller that controls the heater based on information from the temperature sensor so that the temperatures of the sensor FET and the reference FET become a target temperature at which the detection sensitivity of the sensor FET to the gas is equal to or higher than a certain level, A gate voltage detector that controls the gate voltages of the sensor FET and the reference FET so that the drain-source current between the sensor FET and the reference FET becomes a target current, and in a state where the drain-source current between the sensor FET and the reference FET is the target current and the temperatures of the sensor FET and the reference FET are the target temperature, detects the respective gate voltages of the sensor FET and the reference FET, Based on the detected gate voltage of the reference FET, using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the concentration of the gas is 0%, a gate voltage estimator that obtains an estimated value of the gate voltage of the sensor FET when the concentration of the gas is 0%, Based on the shift amount, which is the difference between the obtained estimated value of the gate voltage of the sensor FET and the detected gate voltage of the sensor FET, using the correspondence between the shift amount of the gate voltage of the sensor FET and the concentration of the gas in a state where the temperatures of the sensor FET and the reference FET are the target temperature, a gas concentration estimator that estimates the concentration of the gas. provided with gas sensor module.

5. In the gas sensor module according to claim 4, the correlation is in each temperature state where the temperatures of the sensor FET and the reference FET are adjusted to each of a plurality of temperatures, the gate voltages of the sensor FET and the reference FET in an environment where the concentration of the gas is 0% are detected, and it is obtained based on the detected data group of the gate voltages. gas sensor module.

6. In the gas sensor module according to claim 4, the correspondence is in each environment where the temperature of the sensor FET and the reference FET is the target temperature and the concentration of the gas is adjusted to each of a plurality of known concentrations, the shift amount of the gate voltage of the sensor FET is obtained, and it is obtained based on the obtained data group of the shift amounts. gas sensor module.

7. a sensor FET disposed on a substrate and reacting to a gas in the atmosphere, a reference FET disposed on the substrate and not reacting to the gas, a controller for selecting a gas to be detected from among a plurality of types of gases, a temperature sensor for acquiring the temperatures of the sensor FET and the reference FET, a heater for heating the sensor FET and the reference FET, a temperature controller for controlling the heater based on information from the temperature sensor so that the temperatures of the sensor FET and the reference FET become a target temperature at which the detection sensitivity of the sensor FET to the gas to be detected is equal to or higher than a certain level. Control the gate voltages of the sensor FET and the reference FET so that the drain-source current of the sensor FET and the reference FET becomes the target current. In a state where the drain-source current of the sensor FET and the reference FET is the target current and the temperatures of the sensor FET and the reference FET are the target temperatures, a gate voltage detector that detects the respective gate voltages of the sensor FET and the reference FET; Based on the detected gate voltage of the reference FET, using the correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the concentration of the gas to be detected is 0%, a gate voltage estimator that obtains an estimated value of the gate voltage of the sensor FET when the concentration of the gas to be detected is 0%; Based on the shift amount, which is the difference between the obtained estimated value of the gate voltage of the sensor FET and the detected gate voltage of the sensor FET, using the correspondence between the shift amount of the gate voltage of the sensor FET and the concentration of the gas to be detected in a state where the temperatures of the sensor FET and the reference FET are the target temperatures, a gas concentration estimator that estimates the concentration of the gas to be detected; Comprising; A gas sensor module.

8. In the gas sensor module according to claim 7, The gas concentration estimator has information representing the correspondence corresponding to each gas to be detected included in the plurality of types of gases, The correspondence corresponding to the gas to be detected is obtained by adjusting the shift amount of the gate voltage of the sensor FET in each environment where the temperature of the sensor FET and the reference FET is the target temperature at which the detection sensitivity of the sensor FET to the gas to be detected is equal to or higher than a certain level and the concentration of the gas to be detected is adjusted to each of a plurality of known concentrations, and is obtained based on the obtained data group of the shift amounts. A gas sensor module.

9. In the gas sensor module according to claim 1, The gas is hydrogen, hydrogen sulfide, carbon dioxide, or carbon monoxide. Gas sensor module.

10. In the gas sensor module according to claim 1, The sensor FET and the reference FET are formed by a CMOS formation process. Gas sensor module.

11. In the gas sensor module according to claim 3, The correspondence relationship is obtained by polynomial approximation based on a data group of shift amounts for each concentration of the obtained gas. Gas sensor module.

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