Gas sensor module

Abstract

The present invention improves the accuracy of gas concentration estimation in a gas sensor module.　In a state where drain-source currents of a sensor FET that reacts to gas and a reference FET that does not react to gas are each at a target current value, a gate voltage detector detects gate voltages of the sensor FET and the reference FET. On the basis of the gate voltage of the sensor FET, a gate voltage estimator uses a correlation between the gate voltage of the reference FET and the gate voltage of the sensor FET when the gas concentration is 0% to obtain an estimated gate voltage value of the sensor FET when the gas concentration is 0%. On the basis of a shift amount of the detected gate voltage of the sensor FET with respect to the estimated gate voltage value, a gas concentration estimator uses a correspondence between the shift amount and the gas concentration to estimate the gas concentration.

Description

Gas Sensor Module

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

[0002] In recent years, gas sensors manufactured using semiconductor processes have become known. For example, Patent Document 1 (JP-A-2003-102666) describes a field effect transistor (FET) gas sensor and a method for detecting gas concentrations using the gas sensor. In this method, an FET with a gate electrode exposed to the atmosphere and an FET with a gate electrode not exposed to the atmosphere are integrated on the same substrate. The concentration of gas in the atmosphere is detected based on the difference in gate voltage when the same current is passed between the drain and source of both FETs.

[0003] JP 2016-85124 A

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

[0005] An object of the present invention is to provide a gas sensor module that can detect gas concentrations with higher accuracy.

[0006] a gate voltage detector that controls the gate voltages of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET become target currents, and detects the gate voltages of the sensor FET and the reference FET in a state in which the drain-source currents of the sensor FET and the reference FET are the target currents; a gate voltage estimator that determines an estimated gate voltage of the sensor FET when the concentration of the gas is 0% based on the detected gate voltage of the reference FET, using a 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%; and a gas concentration estimator that estimates the concentration of the gas based on the detected shift amount of the gate voltage of the sensor FET from the determined estimated gate voltage value of the sensor FET, using the correspondence relationship between the amount of shift in the gate voltage of the sensor FET and the concentration of the gas.

[0007] A representative embodiment of the present invention includes a sensor FET disposed on a substrate and responsive to a gas in the atmosphere, a reference FET disposed on the substrate and unresponsive to 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 so that the temperatures of the sensor FET and the reference FET become target temperatures at which the detection sensitivity of the sensor FET to the gas becomes equal to or exceeds a certain level, and a temperature controller for controlling the gate voltages of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET become target currents, and a temperature controller for controlling the gate voltages of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET become the target currents and the temperature of the sensor FET and the reference FET becomes equal to or exceeds the target currents. a gate voltage detector that detects the gate voltages of the sensor FET and the reference FET when the temperature of the sensor FET is the target temperature; a gate voltage estimator that determines an estimated gate voltage of the sensor FET when the concentration of the gas is 0% based on the detected gate voltage of the reference FET and using a 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%; and a gas concentration estimator that estimates the concentration of the gas based on a shift amount that is the difference between the determined estimated gate voltage of the sensor FET and the detected gate voltage of the sensor FET and using the correspondence relationship between the shift amount of the gate voltage of the sensor FET and the concentration of the gas when the temperatures of the sensor FET and the reference FET are the target temperatures.

[0008] A representative embodiment of the present invention includes a sensor FET disposed on a substrate and responsive to a gas in the atmosphere, a reference FET disposed on the substrate and unresponsive to the gas, a controller that selects a gas to be detected from a plurality of types of gas, a temperature sensor that acquires the temperatures of the sensor FET and the reference FET, a heater that heats 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 reach target temperatures at which the detection sensitivity of the sensor FET to the gas to be detected is at or above a certain level, and a temperature controller that controls gate voltages of the sensor FET and the reference FET so that drain-source currents of the sensor FET and the reference FET reach target currents, and controls a temperature controller that controls the temperature of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET reach the target currents and the temperature controller ... temperature controller controls the temperature of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET reach the target currents and the temperature controller controls the temperature controller a gate voltage detector that detects the gate voltages of the sensor FET and the reference FET when the temperature of the sensor FET is at the target temperature; a gate voltage estimator that calculates an estimated gate voltage of the sensor FET when the concentration of the gas to be detected is 0% based on the detected gate voltage of the reference FET and using a 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%; and a gas concentration estimator that estimates the concentration of the gas to be detected based on a shift amount that is the difference between the calculated estimated gate voltage of the sensor FET and the detected gate voltage of the sensor FET and using the correspondence relationship between the shift amount of the gate voltage of the sensor FET and the concentration of the gas to be detected when the temperatures of the sensor FET and the reference FET are at the target temperatures.

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

[0010] 1 is a diagram showing an example of a configuration using functional blocks of a gas sensor module according to a first embodiment. FIG. 2 is a diagram showing an example of a configuration using hardware of a gas sensor module according to the first embodiment. FIG. 3 is a diagram showing an example of a structure of a sensor FET and a reference FET according to the first embodiment. FIG. 4 is a diagram showing an example of changes in current-voltage characteristics of the sensor FET and the reference FET according to the first embodiment depending on the gas concentration. FIG. 5 is a diagram showing an example of changes in gate voltage with temperature at a constant current in a plurality of samples of the sensor FET. FIG. 6 is a diagram showing an example of a correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET. FIG. 7 is a diagram showing a first example of a configuration of a gate voltage detector according to the first embodiment. FIG. 8 is a diagram showing a second example of a configuration of a gate voltage detector according to the first embodiment. FIG. 9 is a diagram showing a gas concentration estimation sequence in the gas sensor module according to the first embodiment. FIG. 10 is a diagram showing a method of making the gate voltage estimator according to the first embodiment learn a gate voltage estimation equation. FIG. 11 is a diagram showing a method of making the gate voltage estimator according to the first embodiment learn a gate voltage estimation equation. FIG. 12 is a diagram showing a method of making the gas concentration estimator according to the first embodiment learn a gas concentration estimation equation. FIG. 13 is a diagram showing a method of making the gas concentration estimator according to the first embodiment learn a gas concentration estimation equation. 1 is a diagram showing a method for making the gate voltage estimator according to embodiment 2 learn a gate voltage estimation equation. FIG. 2 is a diagram showing a method for making the gate voltage estimator according to embodiment 2 learn a gate voltage estimation equation. FIG. 3 is a diagram showing a method for making the gas concentration estimator according to embodiment 2 learn a gas concentration estimation equation. FIG. 4 is a diagram showing a method for making the gas concentration estimator according to embodiment 2 learn a gas concentration estimation equation. FIG. 5 is a diagram showing a configuration example of a gas sensor module according to embodiment 3. FIG. 6 is a diagram showing a method for making the gas concentration estimator according to embodiment 3 learn a gas concentration estimation equation for each temperature. FIG. 7 is a diagram showing a method for making the gas concentration estimator according to embodiment 3 learn a gas concentration estimation equation for each temperature. FIG. 8 is a diagram showing a method for making the gas concentration estimator according to embodiment 3 learn a gas concentration estimation equation for each temperature.

[0011] (Background of the Inventors' Study) The embodiment described below relates to a gas sensor module constituting a MOSFET gas sensor. MOSFET gas sensors are used in gas leak detectors, gas concentration meters, etc. That is, MOSFET gas sensors are used to detect target gases contained in the atmosphere and to measure the concentration of target gases in the atmosphere. Gases to be detected include, for example, hydrogen, hydrogen sulfide, carbon dioxide, and carbon monoxide.

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

[0013] It is known that the current-voltage characteristics of a sensor FET, i.e., the characteristics showing the relationship between the drain-source current and the gate voltage, shift in the gate voltage direction depending on 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 currents of both FETs are the same constant current. The concentration of the gas in the atmosphere is then detected based on the difference in the gate voltages of both FETs after this adjustment, i.e., a value corresponding to the shift in the current-voltage characteristics of the sensor FET in the gate voltage direction.

[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, and fluctuate with the temperature of the FET, i.e., the ambient temperature. If the current-voltage characteristics of the FET vary due to slight differences in the structure or composition of the FET element, or fluctuate due to the ambient temperature, the accuracy of detecting the concentration of the gas to be detected in the atmosphere will decrease.

[0015] Therefore, the present inventors conducted various studies to suppress a decrease in the accuracy of detecting the concentration of a target gas in an atmosphere, which is caused by manufacturing variations or temperature fluctuations in the current-voltage characteristics of an FET. As a result, the present inventors found that when the gas concentration is 0%, 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 is passed between the drain and source of the sensor FET and the reference FET. The present inventors utilized this correlation to invent a technique for substantially correcting manufacturing variations or temperature fluctuations in the current-voltage characteristics of an FET, and a method for obtaining information such as the correlation required for such 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 will be simply referred to as "gas," and the concentration of the gas to be detected in the atmosphere will be simply referred to as "gas concentration."

[0017] (Embodiment 1) The gas sensor module according to embodiment 1 has information that represents a correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET when the gas concentration is 0%, the information being based on the assumption that the drain-source currents of the sensor FET and the reference FET are the same constant value. Furthermore, the gas sensor module according to embodiment 1 estimates a corrected gate voltage of the sensor FET when the gas concentration is 0% using the correlation based on the detected gate voltage of the reference FET. The gas sensor module according to embodiment 1 then calculates the difference between the estimated gate voltage, i.e., the estimated gate voltage of the sensor FET when 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 that represents a correspondence relationship between the shift amount and the gas concentration. The gas sensor module according to embodiment 1 estimates the gas concentration based on the calculated shift amount and the correspondence relationship. The "correlation" and "correspondence relationship" are calculated in advance using a predetermined method.

[0018] <Example of Functional Block Configuration of Gas Module According to Embodiment 1> Fig. 1A is a diagram showing an example of a functional block configuration of a gas sensor module according to Embodiment 1. 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 an FET that reacts to a gas to be detected and changes its current-voltage characteristics according to the concentration of the gas. The reference FET 102 is an FET that does not react to the gas. The reference FET 102 has a structure similar to that of the sensor FET 101, but the gate electrode that reacts to the gas is not exposed to the atmosphere.

[0020] Gate voltage detector 103 controls the gate voltages of sensor FET 101 and reference FET 102 so that the drain-source currents in these FETs become constant values ​​that are set target currents. Gate voltage detector 103 also detects the gate voltages of sensor FET 101 and reference FET 102 when the drain-source currents in these FETs converge to the constant values.

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

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

[0023] The controller 106 appropriately controls the gate voltage detector 103, the gate voltage estimator 104, and the gas concentration estimator 105. The controller 106 performs control necessary to, for example, estimate the gate voltage of the sensor FET 101 when the gas concentration is 0%, obtain correlation information R11 required for the estimation, estimate the gas concentration, and obtain correspondence information R12 required for the estimation.

[0024] The drain, source, and backgate voltages of the sensor FET 101 and the reference FET 102 are fixed to their respective set voltage values, and the gate voltages are controlled so that a target current flows between the drain and source. The sensor FET 101 and the reference FET 102 are arranged close to each other on the same substrate (chip) and formed using the same semiconductor formation process to ensure that the current-voltage characteristics of the sensor FET 101 and the reference FET 102 are substantially equivalent in terms of manufacturing variations and the way they fluctuate in response to temperature changes. However, the difference between the two FETs is whether or not their gas-sensitive gates are exposed to the atmosphere. The gate of the sensor FET 101 is exposed to the atmosphere. On the other hand, the gate of the reference FET 102 is not exposed to the atmosphere.

[0025] The gate voltage detector 103 receives a signal representing the target current J0 and an ON signal to start operation from the controller 106, and supplies a drain voltage, a source voltage, a back gate voltage, and a gate voltage to each of the sensor FET 101 and the reference FET 102. The gate voltage detector 103 also detects the drain-source currents of the sensor FET 101 and the reference FET 102 and controls the gate voltages of the respective FETs so that the currents become a constant value, which is the target current J0. The gate voltage detector 103 then detects the gate voltages of the sensor FET 101 and the reference FET 102 when the drain-source currents of the sensor FET 101 and the reference FET 102 converge to the constant value, which 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 in accordance with the mode signal. The modes of the gate voltage estimator 104 include a normal mode and a learning mode.

[0027] In 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 uses correlation information R11 stored internally to determine an estimated gate voltage value of the sensor FET 101 when the gas concentration is 0%. The gate voltage estimator 104 outputs the determined estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% to the gas concentration estimator 105.

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

[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 obtained under multiple conditions from the gate voltage detector 103, determines the correlation between the gate voltage of the reference FET 102 and the gate voltage of the sensor FET 101, and generates a correlation equation representing this correlation as a gate voltage estimation equation.

[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 in accordance with the mode signal. The gas concentration estimator 105 has two modes: a normal mode and a learning mode.

[0031] In the normal mode, the gas concentration estimator 105 receives from the gate voltage estimator 104 an estimated gate voltage value of the sensor FET 101 when the gas concentration is 0%, and receives from the gate voltage detector 103 the detected gate voltage of the sensor FET 101. The gas concentration estimator 105 calculates the difference between the received estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the received gate voltage of the sensor FET 101 as the amount of shift in the gate voltage of the sensor FET 101. The gas concentration estimator 105 estimates the gas concentration based on the calculated amount of shift in the gate voltage using correspondence information R12 stored inside.

[0032] 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 in which the shift amount of the gate voltage of the sensor FET 101 is used as an input parameter and the gas concentration is used as an output parameter. Here, an example is shown in which the gas concentration estimation formula is used as the correspondence information R12.

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

[0034] <Example of Hardware Configuration of Gas Sensor Module> Fig. 1B is a diagram showing an example of the hardware configuration 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 read-only memory (ROM) 1512, a random access memory (RAM) 1513, an input / output (I / O) interface 1514, an electronic circuit 1515, and the like. The processor 1511 is, for example, a central processing unit (CPU), a microprocessor unit (MPU), or a microcontroller unit (MCU).

[0035] The sensor FET 101, the reference FET 102, and the integrated circuit 151 are formed on the same substrate 150. Note that it is assumed here that the sensor FET 101 and the reference FET 102 are formed on the same chip. A 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 or a glass epoxy substrate. The sensor FET 101, the reference FET 102, and the integrated circuit 151 are formed, for example, by a CMOS formation process, which is one of the well-known semiconductor formation processes. For example, the processor 1511 executes a predetermined program and controls the electronic circuit 1515, causing the integrated circuit 151 to function as the gate voltage detector 103, the gate voltage estimator 104, the gas concentration estimator 105, and the controller 106 described above.

[0036] <Structural Example of Sensor FET and Reference FET> FIG. 2 is a diagram showing structural examples of the sensor FET and reference FET according to the first embodiment. As shown in FIG. 2, the sensor FET 101 and the reference FET 102 both have a typical FET structure. That is, in these FETs, a well 10W is formed on a substrate 10SB, and a backgate 10BG, a drain 10D, and a source 10S are formed thereon. In addition, in these FETs, an insulating layer 10X is formed between the drain and source, and a 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 so that the gate 10G is exposed to the atmosphere. In contrast, the reference FET 102 is formed so that the gate 10G is covered with a protective film 10P so that it is not exposed to the atmosphere. Therefore, the sensor FET 101 reacts to the gas to be detected contained in the atmosphere, while the reference FET 102 does not react to the gas to be detected contained in the atmosphere.

[0037] <Example of Current-Voltage Characteristics of Sensor FET and Reference FET> Figure 3 is a diagram showing an example of changes in the current-voltage characteristics of the sensor FET and the reference FET according to embodiment 1 according to the gas concentration. In Figure 3, graph F31 on the right side represents the current-voltage characteristics of the reference FET 102, and graph F32 on the left side represents the current-voltage characteristics of the sensor FET 101. In the graph showing the current-voltage characteristics shown in Figure 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 versus the gate voltage Vg is drawn.

[0038] As shown in graph F32, the curve representing the current-voltage characteristics of the sensor FET 101 shifts by ΔV in the gate voltage direction from the solid line to the dotted line when the gas concentration changes. This phenomenon occurs when the potential energy of the FET changes as the gas ionizes and adheres to the gate. Therefore, the amount of shift in the current-voltage characteristics of the sensor FET 101 in the gate voltage direction changes depending on the concentration of the gas to be detected in the atmosphere. On the other hand, as shown in 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 determining the voltage change in the gate voltage when the drain-source current in the sensor FET 101 converges to a constant value, it is possible to estimate the gas concentration from the amount of shift.

[0039] <Example of temperature variation in voltage-current characteristics in multiple sensor FET samples> However, in a 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 change in gate voltage with temperature at a constant current for multiple sensor FET samples. The graph in Fig. 4 shows the change in gate voltage for seven sensor FET samples #1 to #7 when the drain-source current is controlled to a constant 10 µA and the temperature of the sensor FET is changed in stages from 25°C to 100°C. In the graph shown in Fig. 4, the horizontal axis represents the temperature of the sensor FET (ambient temperature), and the vertical axis represents the gate voltage when the drain-source current is 10 µA. It can be seen from the graph in Fig. 4 that if the sensor FET elements are different or the temperature of the sensor FET (ambient temperature) changes, the gate voltage can vary significantly even if the drain-source current of the sensor FET is constant.

[0041] Here, we consider the relationship between the gate voltage of the sensor FET 101 and the gate voltage of the reference FET 102. As shown in FIG. 2, the reference FET 102 and the sensor FET 101 have substantially the same structure, but their gate configurations are slightly different. As a result, the gate voltage of the reference FET 102 is not the same as the gate voltage of the sensor FET 101. Therefore, the difference in gate voltage of the sensor FET 101 relative to the reference FET 102 differs for each FET element and also varies depending on the ambient temperature. In other words, the difference in gate voltage between the sensor FET 101 and the reference FET 102 cannot be used directly to compensate for variations due to manufacturing or temperature fluctuations.

[0042] However, as a result of investigations by the present inventors, it was 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 Correlation Between Gate Voltage of Sensor FET and Gate Voltage of 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 gate voltage of the sensor FET on the horizontal axis and the gate voltage of the reference FET on the vertical axis, and plots data for seven samples of combinations of sensor FETs and reference FETs when the environmental temperature is changed at three points.

[0044] 5, there is a strong correlation between the gate voltage of the sensor FET and the gate voltage of the reference FET, regardless of differences in the temperature (environmental temperature) of the FET or differences in the sample. This is because the sensor FET and the reference FET have similar structures, and therefore, although the absolute values ​​of their sensitivities to external influences are different, the ratios are approximately the same.

[0045] Therefore, by understanding 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 determine the gate voltage of the sensor FET 101 when the gas concentration is 0% from the gate voltage of the reference FET 102, regardless of variations in the current-voltage characteristics caused by manufacturing 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 made up 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 from an external device connected to the microcomputer 603, for example, by the external device executing a predetermined program.

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

[0048] The amplifier 612 controls the gate voltage so that the source voltage is constant, and the gate voltage is converted into a digital value by an ADC (Analog to Digital Converter) 613 and transmitted to the microcomputer 603. The amplifier 612 is, for example, an operational amplifier. 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, receiving the value of the target current J0 from the microcomputer 603, stabilizing the drain-source current and the source voltage, and transmitting 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, a microcomputer 703 directly controls the gate voltage and detects the drain-source current on the drain side. This has the advantage of being able to handle cases where the element isolation between the sensor FET 101 and the reference FET 102 is insufficient.

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

[0052] The microcomputer 703 similarly controls the reference FET-side gate voltage detection circuit 702. The microcomputer 703 receives a target current value by, for example, executing a predetermined program, and causes the drain-source currents of the sensor FET 101 and the reference FET 102 to converge to a constant value that is the target current value.

[0053] 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] 8 , first, the controller 106 turns on the gate voltage detector 103, sets a target current J0, and sets the gate voltage estimator 104 and the gas concentration estimator 105 to 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 a value of the target current J0 to the gate voltage detector 103, and sets the target current J0 for the drain-source current of the sensor FET 101 and the reference FET 102. Furthermore, the controller 106 transmits a mode signal for the normal mode to the gate voltage estimator 104 and the gas concentration estimator 105 to operate the gate voltage estimator 104 and the gas concentration estimator 105 in normal mode (S101).

[0055] 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 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 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 (S103). 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 (S103: Yes), the process 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, which is the target current J0 (S103: No), the process returns to step S102, and control of the gate voltage continues.

[0057] After 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, the gate voltage detector 103 detects and acquires the 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 calculates and outputs an estimated value of the gate voltage of the sensor FET 101 when the gas concentration is 0% by using 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% (S105).

[0059] 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 in an environment with a gas concentration of 0% and the gate voltage of the sensor FET when the gas concentration is 0%, as shown in FIG. 5 . This correlation can be expressed by a table or a formula. In this example, this correlation is expressed by a formula, which will be referred to as a gate voltage estimation formula. The gate voltage estimation formula can be expressed, 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 second or higher order may also be assumed. The gate estimation formula is assumed to be derived, for example, by a learning method described later, and saved and stored in the gate voltage estimator 104.

[0062] When mass-producing gas sensor modules 1, it is conceivable to derive a gate voltage estimation formula for each individual gas sensor module 1 using the learning method and store the derived gate voltage estimation formula in the gate voltage estimator 104. In this case, although it requires more man-hours, since the gate voltage estimation formula is derived for each gas sensor module 1, it is possible to derive the gate voltage estimation formula with high accuracy. As another method, it is conceivable to derive a gate voltage estimation formula for one gas sensor module 1 using the learning method and store the derived gate voltage estimation formula uniformly in the gate voltage estimators 104 of multiple gas sensor modules 1. In this case, the accuracy of the gate voltage estimation formula may be slightly reduced, but it is possible to reduce the number of man-hours.

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

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

[0065] B=f(ΔVsg) (2) B: gas concentration ΔVsg: shift amount of gate voltage of sensor FET f(ΔVsg): function with ΔVsg as a parameter

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

[0067] When mass-producing gas sensor modules 1, it is conceivable to derive a gas concentration estimation equation for each individual gas sensor module 1 using the learning method and save and store the derived gas concentration estimation equation in the gas concentration estimator 105. In this case, although it requires more man-hours, since the gas concentration estimation equation is derived for each gas sensor module 1, it is possible to derive the gas concentration estimation equation with high accuracy. As another method, it is conceivable to derive a gas concentration estimation equation for one gas sensor module 1 using the learning method and save the derived gas concentration estimation equation uniformly in the individual gas concentration estimators 105 of multiple gas sensor modules 1. In this case, the accuracy of the gas concentration estimation equation may be slightly reduced, but it is possible to reduce the number of man-hours.

[0068] <Method for Learning Gate Voltage Estimation Formula in Gate Voltage Estimator> Next, a method for 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 to derive the gate voltage estimation formula increases. Here, for simplicity of explanation, it is assumed that the number of samples of the sensor FET used to derive the gate voltage estimation formula is two.

[0069] 9A and 9B are diagrams illustrating a method for 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 and 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 a target current J0, and sets the gate voltage estimator 104 to 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. The controller 106 also transmits 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. Furthermore, the controller 106 transmits a learning mode mode signal to the gate voltage estimator 104 to operate the gate voltage estimator 104 in learning mode (L101).

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

[0072] The gate voltage detector 103 determines whether the drain-source currents of the first sensor FET 101a and the first reference FET 102a have converged to a constant value, which is the target current J0 (L103). If 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, which is the target current J0 (L103: Yes), the process proceeds to the next step, L104. On the other hand, if 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, which is the target current J0 (L103: No), the process returns to step L102, and control of the gate voltage continues.

[0073] After the drain-source currents of the first sensor FET 101a and the first reference FET 102a converge to a constant value, which is the target current J0, the gate voltage detector 103 detects and acquires the 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 (L105). After that, 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, causing the gate voltage detector 103 to fall (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 J. More specifically, the controller 106 transmits the target current J and an ON signal to the gate voltage detector 103 to start up the gate voltage detector 103, sends the target current J to the gate voltage detector 103, and sets the target current J for the drain-source current of the sensor FET 101 and the reference FET 102 (L108).

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

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

[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 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 (L112). After that, the controller 106 turns off the gate voltage detector 103.

[0081] Then, based on a data group including the gate voltages of the first sensor FET 101 a and the first reference FET 102 a and the gate voltages of the second sensor FET 101 b and the second reference FET 102 b, the gate voltage estimator 104 derives a 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%, i.e., a gate voltage estimation formula, and stores the equation in the gate voltage estimator 104 (L113). To derive the gate voltage estimation formula, for example, a linear approximation to the above-mentioned data group of gate voltages is used.

[0082] In this example, the correlation information R11 is a gate voltage estimation formula, and a linear function is assumed as the gate voltage estimation formula. However, a quadratic or higher order function may also be assumed. This example illustrates a case where the number of sample points of the sensor FET 101 is two. However, even if the number of sample points of the sensor FET 101 is three or more, the basic part of the method for learning the gate voltage estimation formula is the same. In this example, one reference FET 102 is combined with one sensor FET 101. However, one same reference FET 102 may be combined with multiple sensor FETs 101. The greater the number of sample points of the sensor FET 101, the higher the accuracy of the gate voltage estimation formula, but the greater the man-hours. Therefore, the number of samples should be determined taking into consideration the balance between accuracy and man-hours.

[0083] <Learning Sequence of Gas Concentration Estimation Formula According to First Embodiment> Next, a method for causing the gas concentration estimator 105 to learn the gas concentration estimation formula will be described.

[0084] 10A, 10B, and 10C are diagrams illustrating a method for causing the gas concentration estimator according to the first embodiment to learn a gas concentration estimation equation.

[0085] 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 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. The controller 106 also 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. Furthermore, the controller 106 sends a mode signal for the learning mode to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to 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, 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 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 (L124). 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 (L124: Yes), the process proceeds to the next step L125. 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 (L124: No), the process returns to step L123, and control of the gate voltage continues.

[0089] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L125).

[0090] 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 based on the detected gate voltage of the reference FET 102 (L126).

[0091] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the first concentration C1 as the shift amount of the gate voltage of the sensor FET 101 at the first 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, which is the value of the target current J0 (L129).

[0094] The gate voltage detector 103 determines whether 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 (L130). 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 (L130: Yes), the process 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, which is the target current J0 (L130: No), the process returns to step L129, and control of the gate voltage continues.

[0095] Incidentally, while the gas concentration is being changed, conditions such as the environmental 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 converge 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 (L131).

[0097] 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 based on the detected gate voltage of the reference FET 102 (L132).

[0098] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the second concentration C2 as the shift amount of the gate voltage of the sensor FET 101 at the second 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, a 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 currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current J0 (L135).

[0101] The gate voltage detector 103 determines whether 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 (L136). 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 (L136: Yes), the process proceeds to the next step L137. 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 (L136: No), the process returns to step L135, and control of the gate voltage continues.

[0102] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L137).

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

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

[0105] Thereafter, the gas concentration estimator 105 performs high-order polynomial approximation to derive a gate voltage estimation equation for the values ​​of the first concentration C1, the second concentration C2, and the third concentration C3 of the gas with respect to the shift amount of the gate voltage of the sensor FET 101 at the first concentration C1, the second concentration C2, and the third concentration C3, respectively.The gas concentration estimator 105 then obtains the gas concentration estimation equation from the result of the polynomial approximation as correspondence relationship information R12 that represents the correspondence relationship between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration (L140).

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

[0107] <Example of Effects of First Embodiment> According to the first embodiment, 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 calculated in advance and stored according to the learning sequence. Then, in the first embodiment, this correlation is used based on the detected gate voltage of the reference FET to calculate an estimated gate voltage of the sensor FET when the gas concentration is 0%. Therefore, in the first embodiment, the gate voltage of the sensor FET when the gas concentration is 0% can be estimated, correcting for variations in the current-voltage characteristics of the sensor FET due to manufacturing and deviations due to temperature fluctuations, and the shift amount of the gate voltage of the sensor FET corresponding to the gas concentration can be calculated more accurately. As a result, according to the first embodiment, the concentration of the gas to be detected can be calculated with higher accuracy.

[0108] Second Embodiment A gas sensor module according to a second embodiment is different from the first embodiment in that it further includes a temperature control mechanism for controlling the temperatures of the sensor FET and the reference FET.

[0109] The inventors have confirmed that the temperature range of the sensor FET at which the gas detection sensitivity of the sensor FET is at or above a certain level differs depending on the type of gas to be detected. Therefore, in the gas sensor module according to the second embodiment, the temperature control mechanism is used to control the temperatures of the sensor FET and the reference FET to be constant at a predetermined target temperature so that the gas detection sensitivity of the sensor FET is at or above a certain level or at its maximum depending on the type of gas to be detected.

[0110] In the second embodiment, the gate voltage estimation formula is obtained by performing a learning sequence that utilizes temperature fluctuations in the gate voltages of the sensor FET and the reference FET. The gas concentration estimation formula is obtained by performing the learning sequence while controlling the temperature of the sensor FET to be constant at the target temperature.

[0111] <Configuration Example of Gas Sensor Module According to Embodiment 2> Fig. 11 is a diagram showing a configuration example of a 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 in addition to the components of 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 the first embodiment, but 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 a physical property, such as a voltage or a resistance value. The temperature sensor 1102 is, for example, a thermocouple, a platinum resistance thermometer, or a thermistor thermometer.

[0114] The amount of heat generated by the heater 1103 changes depending on whether the heater 1103 is turned on or off by a temperature controller 1104 or on the power supplied thereto. The heater 1103 is, for example, a resistor or a heating wire.

[0115] Temperature controller 1104 controls heater 1103 so that the temperature detected by temperature sensor 1102 approaches a set target temperature T0. Target temperature T0 is a temperature determined so that the detection sensitivity of sensor FET 101 to the detection target gas is at or above a certain level or is maximized. Temperature sensor 1102 and heater 1103 are disposed near sensor FET 101 and reference FET 102. Temperature controller 1104 controls heater 1103 to heat sensor FET 101 and reference FET 102, and maintains the temperatures of these FETs constant at target temperature T0.

[0116] As mentioned above, the inventors have confirmed that the temperature range of the sensor FET 101 at which the detection sensitivity reaches a certain level or higher differs depending on the type of gas to be detected. The gas sensor module 2 also includes a temperature control mechanism 1100 for controlling the temperatures of the sensor FET 101 and the reference FET 102. Therefore, the gas sensor module 2 can detect a desired type of gas from among multiple types of gases with high sensitivity by changing the temperature of the sensor FET 101 in accordance with the gas to be detected.

[0117] Furthermore, in the gas sensor module 2, the gate voltage estimator 104 is made to learn the temperature dependency of the current-voltage characteristics of the sensor FET 101, which is expected to have an effect of improving the estimation accuracy of the gate voltage of the sensor FET when the gas concentration is 0%. Similarly, the gas concentration estimator 105 is made to learn the temperature dependency of the fluctuation of the current-voltage characteristics of the sensor FET 101 due to the gas concentration, which is expected to have an effect of improving the estimation accuracy of the gas concentration.

[0118] Furthermore, the inventors have confirmed that the performance of the sensor FET in a gas sensor module changes or deteriorates with use or over time. As described above, the gas sensor module 2 includes the temperature control mechanism 1100. Therefore, by temporarily heating the sensor FET 101 to a high temperature using the temperature control mechanism 1100, foreign matter adhering to the gate of the sensor FET 101 can be removed, and the changed or deteriorated performance of the sensor FET 101 can be expected to be restored to a state closer to its initial state than its current state. 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, solid, or liquid.

[0119] 11 , a controller 1101 sends an ON signal and a value of a target temperature T0 to a temperature controller 1104. The temperature controller 1104 compares the target temperature T0 with the temperature indicated by information from a temperature sensor 1102, and controls a 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] 12A and 12B are diagrams showing a method for causing a gate voltage estimator according to embodiment 2 to learn a gate voltage estimation formula. 12A and 12B show a learning sequence for obtaining the gate voltage estimation formula as correlation information R21 based on a data group of the obtained gate voltages by changing the temperature of the FET for a pair of a sensor FET and a reference FET to a plurality of temperatures.

[0121] 12A and 12B, the controller 1101 turns on the gate voltage detector 103, sets a target current J0, and sets the gate voltage estimator 104 to 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. The controller 1101 also sends a 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. Furthermore, the controller 1101 sends a learning mode mode signal to the gate voltage estimator 104 to operate the gate voltage estimator 104 in 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 a first temperature T1. More specifically, the controller 1101 transmits an ON signal to the temperature controller 1104 to start up the temperature controller 1104. The controller 1101 also sends the value of the first temperature T1 to the temperature controller 1104 as the target current value, and sets 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 the temperatures of the sensor FET 101 and the reference FET 102 have converged to the first temperature T1, which is the target temperature (L204). 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 process 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 process returns to step L203, and control of the heater 1103 continues.

[0125] 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 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 have converged to a constant value, which is the target current J0 (L206). 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 (L206: Yes), the process proceeds to the next step, L207. 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 (L206: No), the process returns to step L205, and 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 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 (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 as the target temperature to the temperature controller 1104, 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 the second temperature T2, which is the target temperature (L211). 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 process 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 process returns to step L210, and 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 currents of the sensor FET 101 and the reference FET 102 become the set target current J0 (L212).

[0133] The gate voltage detector 103 determines whether 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 (L213). 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 (L213: Yes), the process proceeds to the next step, L214. 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 (L213: No), the process returns to step L212, and control of the gate voltage continues.

[0134] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (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, the gate voltage estimator 104 determines the correlation between the gate voltage of the sensor FET 101 and the gate voltage of the reference FET 102 based on the data sets of the gate voltages of the sensor FET 101 and the reference FET 102 at the first temperature T1 and the second temperature T2, and stores correlation information R21 representing the correlation internally (L216).

[0137] 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. In this example, the correlation is calculated by plotting two points of the sensor FET gate voltage at a constant current versus temperature, but it may also be calculated by plotting three or more points. In this example, only one sample is used as the sensor FET, but multiple samples may also be used. When multiple samples are used, all but the final processing of the sequence in FIG. 12B is performed for each of the multiple samples, and the final processing of the sequence is performed only for the last sample. The more samples of the sensor FET 101 or the number of temperatures set, the higher the accuracy of the gate voltage estimation formula, but the greater the amount of work required. Therefore, the number of samples of the sensor FET 101 or the number of temperatures set should be determined by considering the balance between accuracy and work required.

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

[0139] 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 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. The controller 1101 also 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. Furthermore, the controller 1101 sends a mode signal for the learning mode to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to 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 at which the sensitivity of the sensor FET 101 to the gas to be detected is at or above a certain level or at its maximum.

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

[0142] The temperature controller 1104 determines whether the temperatures of the sensor FET 101 and the reference FET 102 have converged to a constant value, which is the target temperature T0 (L224). If it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have converged to a constant value, which is the target temperature T0 (L224: Yes), the process proceeds to the next step, L225. 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 constant value, which is the target temperature T0 (L224: No), the process returns to step L223, and 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 that is the target current J0 (L226).

[0145] The gate voltage detector 103 determines whether 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). 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 (L227: Yes), the process proceeds to the next step L228. 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 (L227: No), the process returns to step L226, and control of the gate voltage continues.

[0146] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L228).

[0147] 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 based on the detected gate voltage of the reference FET 102 (L229).

[0148] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the first concentration C1 as the shift amount of the gate voltage of the sensor FET 101 at the first 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, a 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 currents of the sensor FET 101 and the reference FET 102 converge to a constant value that is the target current J0 (L232).

[0151] The gate voltage detector 103 determines whether 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 (L233). 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 (L233: Yes), the process proceeds to the next step L234. 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 (L233: No), the process returns to step L232, and control of the gate voltage continues.

[0152] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L234).

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

[0154] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the second concentration C2 as the shift amount of the gate voltage of the sensor FET 101 at the second 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, a 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 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 (L239). 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 (L239: Yes), the process proceeds to the next step L240. 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 (L239: No), the process returns to step L238, and control of the gate voltage continues.

[0158] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L240).

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

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

[0161] Thereafter, the gas concentration estimator 105 performs high-order polynomial approximation to derive a gate voltage estimation equation for the values ​​of the first concentration C1, the second concentration C2, and the third concentration C3 of the gas with respect to the shift amount of the gate voltage of the sensor FET 101 at the first concentration C1, the second concentration C2, and the third concentration C3, respectively.The gas concentration estimator 105 then obtains the gas concentration estimation equation from the result of the polynomial approximation as correspondence relationship information R22 that represents the correspondence relationship between the shift amount of the gate voltage of the sensor FET 101 and the gas concentration (L243).

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

[0163] Note that this example shows a case where the number of FET samples is one. If the number of FET samples is multiple, the processes other than the final process (L244) of the sequence in Fig. 13C are performed for each of the multiple samples, and the final process (L244) of the sequence is performed only when processing the last sample.

[0164] <Example of Effect of Second Embodiment> According to the second embodiment, the temperature of the sensor FET is controlled using a temperature control mechanism so that it reaches a target temperature at which the detection sensitivity for the gas to be detected is at or above a certain level or at its maximum. Also, according to the second embodiment, a learning sequence is performed in a state in which 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 the second embodiment, the concentration of the gas to be detected can be estimated with higher accuracy.

[0165] Furthermore, according to the second embodiment, a learning sequence is executed in which the temperatures of the sensor FET and the reference FET are changed to acquire the gate voltages of the respective FETs, and the gate voltage estimation formula is obtained based on the acquired gate voltages. Therefore, according to the second embodiment, the gate voltage estimation formula can be obtained as correlation information without replacing the FET elements, and the correlation information can be obtained without requiring much time or effort.

[0166] (Embodiment 3) Compared to Embodiment 2, the gas sensor module according to Embodiment 3 stores a plurality of gas concentration estimation equations corresponding to a plurality of target temperatures of the sensor FET. When there are a plurality of gases that can be detected as detection candidates, the plurality of target temperatures are configured as temperatures of the sensor FET at which the detection sensitivity for each of the plurality of types of gas is at or above a certain level.

[0167] As described above, the inventors have confirmed that the temperature range of the sensor FET at which the gas detection sensitivity of the sensor FET is at or above a certain level varies depending on the type of gas to be detected. Therefore, in the gas sensor module according to the third embodiment, multiple types of gases are selected as detection candidate gases, and a gas to be detected is selected from among the multiple types of gases. The temperature of the sensor FET is then controlled using a temperature control mechanism so that it reaches a 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 to the selected gas to be detected is at or above a certain level or at its maximum. A gas concentration estimation equation at the target temperature corresponding to the selected gas type is used to estimate the gas concentration.

[0168] In the third embodiment, the gate voltage estimation formula is obtained by implementing a learning sequence similar to that in the second embodiment. A gas concentration estimation formula is prepared for each target temperature corresponding to a plurality of types of gases that are detection candidates, and the learning sequence is implemented in each temperature state in which the temperature of the sensor FET is controlled to each of a plurality of target temperatures corresponding to the plurality of types of gases.

[0169] Fig. 14 is a diagram showing an example of the configuration of a gas sensor module according to embodiment 3. As shown in Fig. 14, in the gas sensor module 3 according to embodiment 3, compared to the gas sensor module 2 according to embodiment 2, the correspondence relationship information R32 held by the gas concentration estimator 105 includes a gas concentration estimation formula for each target temperature corresponding to the type of gas. The gas concentration estimation formula for each target temperature is a formula that expresses the correspondence relationship between the gate voltage of the sensor FET and the gas concentration estimation value for each target temperature.

[0170] 14, gas concentration estimator 105 receives a target temperature in addition to a mode signal from controller 1101. Then, gas concentration estimator 105 estimates the gas concentration by switching the gas concentration estimation formula to be used depending on the target temperature using the sensor FET gate voltage estimate value for a gas concentration of 0% output from gate voltage estimator 104, the sensor FET gate voltage output from gate voltage detector 103, and the target temperature. The operation of each other component is the same as in the second embodiment. The learning method of gate voltage estimator 104 is also the same as in the second embodiment (FIG. 13).

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

[0172] As shown in Figures 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 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. The controller 1101 also sends a 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. The controller 1101 also sends a learning mode signal to the gas concentration estimator 105 to set the mode of the gas concentration estimator 105 to learning mode. The controller 1101 also 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 the Nth temperature TT(N) suitable for detecting the Nth gas as the target temperature. More specifically, the controller 1101 transmits an ON signal and the Nth temperature TT(N) as the target temperature to the temperature controller 1104 (L302). The Nth temperature TT(N) as the target temperature is, for example, the temperature of the sensor FET 101 at which the sensitivity of the sensor FET 101 to the Nth gas to be detected is at or above a certain level or at its maximum. Therefore, when the processing of step L302 is performed for the first time, N=1, and so 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 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 constant value, which is the Nth temperature TT(N) (L303).

[0175] The temperature controller 1104 determines whether the temperatures of the sensor FET 101 and the reference FET 102 have converged to a constant value, which is the Nth temperature TT(N) (L304). If it is determined that the temperatures of the sensor FET 101 and the reference FET 102 have converged to a constant value, which is the Nth temperature TT(N) (L304: Yes), the process 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 constant value, which is the Nth temperature TT(N) (L304: No), the process returns to step L303, and control of the heater 1103 continues.

[0176] After the temperatures of the sensor FET 101 and the reference FET 102 converge to a constant 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, 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 that is the target current J0 (L306).

[0178] The gate voltage detector 103 determines whether 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). 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 process 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 process returns to step L306, and control of the gate voltage continues.

[0179] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L308).

[0180] 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 based on the detected gate voltage of the reference FET 102 (L309).

[0181] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the first concentration C1 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, a 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 that is the target current J0 (L312).

[0184] The gate voltage detector 103 determines whether 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). 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 (L313: Yes), the process proceeds to the next step, L314. 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 (L313: No), the process returns to step L312, and control of the gate voltage continues.

[0185] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L314).

[0186] 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 based on the detected gate voltage of the reference FET 102 (L315).

[0187] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the second concentration C2 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 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 (L319). 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 (L319: Yes), the process 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, which is the target current J0 (L319: No), the process returns to step L318, and control of the gate voltage continues.

[0191] After 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, the gate voltage detector 103 detects and acquires the gate voltages of the sensor FET 101 and the reference FET 102 (L320).

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

[0193] The gas concentration estimator 105 calculates and stores the difference between the estimated gate voltage value of the sensor FET 101 when the gas concentration is 0% and the gate voltage of the sensor FET 101 detected at the third concentration C3 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 high-order polynomial approximation to derive a gate voltage estimation equation for the first concentration C1, the second concentration C2, and the third concentration C3 of the gas, corresponding to the shift amounts of the gate voltage of the sensor FET 101 at the first concentration C1, the second concentration C2, and the third concentration C3. Then, from the results of the polynomial approximation, the gas concentration estimator 105 obtains the gas concentration estimation equation as correspondence relationship information R32 that represents the correspondence relationship between the shift amounts of the gate voltage of the sensor FET 101 and the gas concentrations (L323).

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

[0196] The controller 1101 determines whether there is another gas concentration estimation equation to be learned, i.e., whether to learn a gas concentration estimation equation corresponding to a next gas other than the gases learned so far (L325). If it is determined that a gas concentration estimation equation corresponding to the next gas is to be learned (L325: Yes), the controller 1101 increments the parameter N by 1 and executes the process of N←N+1 (L326), and the process returns to step L302. On the other hand, if it is determined that a gas concentration estimation equation corresponding to the next gas is not to be learned (L325: No), the controller 1101 ends learning.

[0197] In this way, by performing a similar learning sequence for each of multiple target temperatures TT(1), TT(2), ... corresponding to multiple types of gas, it is possible to obtain a gas concentration estimation formula at a temperature suitable for each of multiple types of gas.

[0198] <Example of Effect of Embodiment 3> According to the third embodiment, the temperature of the sensor FET is controlled and maintained in accordance with the type of target gas so that the detection sensitivity of the sensor FET for that target gas is at or above a certain level or at its maximum. Also, according to the third embodiment, gas concentration estimation equations are determined and stored for each of a plurality of target temperatures corresponding to a plurality of target gases. Therefore, according to the third embodiment, the gas concentrations of a plurality of types of gas can be estimated with high accuracy.

[0199] Although the embodiments have been described above, the present invention is not limited to these embodiments, and various modifications are possible within the scope of the invention. Furthermore, the numbers and names used in the embodiments are merely examples, and other numbers and names may be used.

[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 gas sensor module comprising: a sensor FET disposed on a substrate and responsive to a gas in an atmosphere; a reference FET disposed on the substrate and unresponsive to the gas; a gate voltage detector that controls gate voltages of the sensor FET and the reference FET so that drain-source currents of the sensor FET and the reference FET become target currents, and detects the gate voltages of the sensor FET and the reference FET in a state in which the drain-source currents of the sensor FET and the reference FET are the target currents; a gate voltage estimator that determines an estimated gate voltage of the sensor FET when the concentration of the gas is 0% based on the detected gate voltage of the reference FET, using a 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%; and a gas concentration estimator that estimates the concentration of the gas based on an amount of shift in the detected gate voltage of the sensor FET relative to the determined estimated gate voltage value of the sensor FET, using the correspondence between an amount of shift in the gate voltage of the sensor FET and the concentration of the gas.

2. A gas sensor module as described in claim 1, wherein the correlation is determined by detecting gate voltages of the sensor FET and the reference FET in an environment in which the gas concentration is 0% for a plurality of samples of the sensor FET and one or a plurality of samples of the reference FET, and based on a group of data on the detected gate voltages.

3. A gas sensor module as described in claim 1, wherein the correspondence relationship is determined based on a data group of the shift amounts obtained by obtaining a difference between an estimated gate voltage value of the sensor FET when the gas concentration is 0% in each environment adjusted to each of a plurality of known concentrations and the detected gate voltage of the sensor FET.

4. A sensor FET disposed on a substrate and responsive to a gas in the atmosphere; a reference FET disposed on the substrate and unresponsive to the gas; a temperature sensor for acquiring 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 becomes equal to or higher than a certain level; and a gate voltage detector for controlling the gate voltages of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET become target currents, and for detecting the gate voltages of the sensor FET and the reference FET when the drain-source currents of the sensor FET and the reference FET are the target currents and the temperatures of the sensor FET and the reference FET are the target temperatures. a gate voltage estimator that calculates an estimated gate voltage value of the sensor FET when the concentration of the gas is 0% by using a 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% based on the detected gate voltage of the reference FET; and a gas concentration estimator that estimates the concentration of the gas based on an amount of shift that is a difference between the calculated estimated gate voltage value of the sensor FET and the detected gate voltage of the sensor FET by using a correspondence relationship between an amount of shift in the gate voltage of the sensor FET and the concentration of the gas when the temperatures of the sensor FET and the reference FET are at the target temperature.

5. A gas sensor module as described in claim 4, wherein the correlation is determined by detecting gate voltages of the sensor FET and the reference FET in an environment in which the gas concentration is 0% in each temperature state in which the temperatures of the sensor FET and the reference FET are adjusted to each of a plurality of temperatures, and based on a data group of the detected gate voltages.

6. A gas sensor module as described in claim 4, wherein the correspondence is determined based on a group of data on the shift amounts obtained by obtaining the shift amounts of the gate voltage of the sensor FET in each environment adjusted to each of a plurality of known concentrations of the gas when the temperatures of the sensor FET and the reference FET are at the target temperature.

7. A sensor FET disposed on a substrate and responsive to a gas in the atmosphere; a reference FET disposed on the substrate and unresponsive to the gas; a controller for selecting a gas to be detected from a plurality of types of gas; a temperature sensor for acquiring 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 becomes equal to or higher than a certain level; a gate voltage detector for controlling the gate voltages of the sensor FET and the reference FET so that the drain-source currents of the sensor FET and the reference FET become target currents, and for detecting the gate voltages of the sensor FET and the reference FET when the drain-source currents of the sensor FET and the reference FET are the target currents and the temperatures of the sensor FET and the reference FET are the target temperatures; a gate voltage estimator that calculates an estimated gate voltage value of the sensor FET when the concentration of the gas to be detected is 0% by using a 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 that estimates the concentration of the gas to be detected based on an amount of shift that is a difference between the calculated estimated gate voltage value of the sensor FET and the detected gate voltage of the sensor FET by using a correspondence relationship between an amount of shift in the gate voltage of the sensor FET and the concentration of the gas to be detected when the temperatures of the sensor FET and the reference FET are at the target temperature.

8. A gas sensor module as described in claim 7, wherein the gas concentration estimator has information indicating the correspondence relationship corresponding to each detection candidate gas contained in the plurality of types of gas, and the correspondence relationship corresponding to the detection candidate gas is determined based on a group of data on the obtained shift amounts in the gate voltage of the sensor FET in each environment adjusted to each of a plurality of known concentrations of the detection candidate gas when the temperatures of the sensor FET and the reference FET are at target temperatures at which the detection sensitivity of the sensor FET to the detection candidate gas is at or above a certain level.

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

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

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

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