SENSOR FOR MEASURING A GAS CONCENTRATION

Abstract

The disclosure relates to a sensor for measuring a gas concentration of a target gas in a sample of ambient air. The sensor comprises a first gas sensitive component comprising a first gas sensitive layer being arranged between a first pair of measuring electrodes, a second gas sensitive component comprising a second gas sensitive layer being arranged between a second pair of measuring electrodes and one or more heating elements to heat the first gas sensitive layer and the second gas sensitive layer. The first gas sensitive layer and the second gas sensitive layer comprise a metal oxide semiconductor. The first gas sensitive component is configured to measure the gas concentration of the target gas in a first concentration band and the second gas sensitive component is configured to measure the gas concentration of the target gas in a second concentration band. According to an embodiment, the sensor is configured to operate for both the measuring of the first concentration band and for the measuring of the second concentration band in a respective transition regime. The transition regime is situated between a perturbation regime and a saturation regime. The transition regime is characterized by a higher sensitivity to the target gas than the perturbation regime and the saturation regime.

Further aspects of the disclosure relate to a corresponding method, a computer program product and an electronic device.

Claims

1. A sensor for measuring a gas concentration of a target gas in a sample of ambient air, the sensor comprising a first gas sensitive component comprising a first gas sensitive layer being arranged between a first pair of measuring electrodes, the first gas sensitive layer comprising a metal oxide semiconductor; a second gas sensitive component comprising a second gas sensitive layer being arranged between a second pair of measuring electrodes, the second gas sensitive layer comprising a metal oxide semiconductor; one or more heating elements configured to heat the first gas sensitive layer and the second gas sensitive layer; wherein the first gas sensitive component is configured to measure the gas concentration of the target gas in a first concentration band; and the second gas sensitive component is configured to measure the gas concentration of the target gas in a second concentration band; wherein the sensor is configured to operate for both the measuring of the first concentration band and for the measuring of the second concentration band in a respective transition regime, the transition regime being situated between a perturbation regime and a saturation regime, wherein the transition regime is characterized by a higher sensitivity to the target gas than the perturbation regime and the saturation regime.

2. The sensor of claim 1, wherein one or more layer parameters of the first gas sensitive layer and the second gas sensitive layer are configured to operate the sensor for both the measuring of the first concentration band and for the measuring of the second concentration band in the respective transition regime.

3. The sensor of claim 1, wherein the slopes of a resistance-concentration curve of the first gas sensitive layer and the slopes of a resistance-concentration curve of the second gas sensitive layer are greater in the transition regime than in the saturation regime and the perturbation regime.

4. The sensor of claim 1, wherein the sensor is configured the measure the gas concentration in the first concentration band and in the second concentration band in a continuous manner.

5. The sensor of claim 1, wherein the first gas sensitive component is configured to detect in the first concentration band a first threshold concentration; and the second gas sensitive component is configured to detect in the second concentration band a second threshold concentration.

6. The sensor of claim 1, wherein the first and the second gas sensitive layer comprise an n-doped metal oxide semiconductor material; and the sensor is configured to measure a concentration of an oxidizing target gas.

7. The sensor of claim 6, wherein the oxidizing target gas is ozone or nitrogen dioxide.

8. The sensor of claim 1, wherein the first and the second gas sensitive layer comprise a p-doped metal oxide semiconductor material; and the sensor is configured to measure a concentration of a reducing target gas.

9. The sensor of claim 1, wherein the first gas sensitive component and the second gas sensitive component differ in at least one layer parameter, the layer parameter being selected from the group consisting of: a thickness of the first gas sensitive layer and the second gas sensitive layer; the morphology of the first gas sensitive layer and the second gas sensitive layer, in particular the porosity of the first gas sensitive layer and the second gas sensitive layer; the material of the first gas sensitive layer and the second gas sensitive layer, in particular different additives added to the same base material; and a respective operating temperature of the first gas sensitive layer and the second gas sensitive layer.

10. The sensor of claim 1, wherein the first gas sensitive layer has a first thickness and the second gas sensitive layer has a second thickness, the second thickness being different from the first thickness; the first thickness and the second thickness are chosen such that the sensor operates for both the detection of the first threshold concentration and for the detection of the second threshold concentration in a transition regime, the transition regime being situated between a perturbation regime and a saturation regime, wherein the transition regime is characterized by a higher sensitivity to the target gas than the perturbation regime and the saturation regime.

11. The sensor of claim 2, wherein the one or more layer parameters are selected from the group consisting of: a thickness of the first gas sensitive layer and the second gas sensitive layer; the morphology of the first gas sensitive layer and the second gas sensitive layer, in particular the porosity of the first gas sensitive layer and the second gas sensitive layer; the material of the first gas sensitive layer and the second gas sensitive layer, in particular different additives added to the same base material; and a respective operating temperature of the first gas sensitive layer and the second gas sensitive layer.

12. The sensor of claim 1, wherein the sensor comprises a first operation mode configured to operate the first gas sensitive component and the second gas sensitive component simultaneously.

13. The sensor of claim 1, wherein the sensor comprises a second operation mode configured to operate only the first gas sensitive component or only the second gas sensitive component.

14. The sensor of claim 1, wherein the thickness of the first and the second gas sensitive layer is between 100 nm and 5000 nm.

15. The sensor of claim 1, wherein the sensor is configured to operate the first and the second gas sensitive component at predefined reactivities.

16. The sensor of claim 1, wherein the sensor is configured to provide a fine-tuning of the transition regime via one or more reactivity control parameters, in particular by the operating temperature of the first and the second gas sensitive layer, in particular by selecting a plateau region of a respective resistivity-reactivity curve for the first gas sensitive layer and the second gas sensitive layer.

17. A method for measuring a gas concentration of a target gas in a sample of ambient air, the method comprising: providing a first gas sensitive component comprising a first gas sensitive layer being arranged between a first pair of measuring electrodes, the first gas sensitive layer comprising a metal oxide semiconductor; providing a second gas sensitive component comprising a second gas sensitive layer being arranged between a second pair of measuring electrodes, the second gas sensitive layer comprising a metal oxide semiconductor; measuring, by the first gas sensitive component, the gas concentration of the target gas in a first concentration band; and measuring, by the second gas sensitive component, the gas concentration of the target gas in a second concentration band; operating the sensor for both the measuring of the first concentration band and for the measuring of the second concentration band in a respective transition regime, the transition regime being situated between a perturbation regime and a saturation regime, wherein the transition regime is characterized by a higher sensitivity to the target gas than the perturbation regime and the saturation regime.

18. A computer program product for operating a sensor as claimed in claim 1, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by the sensor to cause the sensor to perform a method comprising: measuring, by the first gas sensitive component, the gas concentration of the target gas in a first concentration band; and measuring, by the second gas sensitive component, the gas concentration of the target gas in a second concentration band.

19. An electronic device comprising a sensor as claimed claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] The invention will be better understood and objects other than those set forth above will become apparent from the following detailed description thereof. Such description refers to the annexed drawings, wherein:

[0078] FIG. 1a shows a 3-dimensional view of a sensor comprising a first gas sensitive component and a second gas sensitive component with different layer thicknesses according to an embodiment of the invention;

[0079] FIG. 1b shows a cross section/side view of the first gas sensitive component of FIG. 1a;

[0080] FIG. 1c shows a cross section/side view of the second gas sensitive component of FIG. 1a;

[0081] FIG. 2a shows a cross sectional view of a gas sensitive component operating in a perturbation regime;

[0082] FIG. 2b shows a cross sectional view of a gas sensitive component operating in a transition regime;

[0083] FIG. 2c shows a cross sectional view of a gas sensitive component operating in a saturation regime;

[0084] FIG. 3 shows a sensor having four gas sensitive components having different layer thicknesses;

[0085] FIG. 4 shows a sensor having four gas sensitive components having the same layer thickness, but a different porosity;

[0086] FIG. 5 shows a sensor having four gas sensitive components having the same layer thickness, but a different reactivity;

[0087] FIG. 6 shows a schematic illustration of the operation of a sensor according to an embodiment of the invention;

[0088] FIG. 7a illustrates a resistance-concentration curve for a first gas sensitive layer;

[0089] FIG. 7b illustrates a resistance-concentration curve for a second gas sensitive layer having a greater thickness than the first gas sensitive layer;

[0090] FIG. 8 shows a diagram of an exemplary simulation illustrating resistivity-reactivity curves for three different layer thicknesses;

[0091] FIG. 9 shows the diagram of FIG. 8 with an additional illustration of a fine-tuning of a transition regime via one or more reactivity control parameters;

[0092] FIG. 10 shows a sensor having four gas sensitive components being operated at different temperatures; and

[0093] FIG. 11 illustrates a basic mode of operation of a sensor configured to measure the gas concentration in a continuous manner.

[0094] The drawings are provided to aid the understanding of embodiments of the invention and are not to scale.

MODES FOR CARRYING OUT THE INVENTION

[0095] FIG. 1a shows a 3-dimensional view of a sensor 100 according to an embodiment of the invention. The sensor 100 is configured to measure a gas concentration of a target gas in a sample of ambient air that surrounds the gas sensor 100.

[0096] The sensor 100 comprises a first gas sensitive component 110 and a second gas sensitive component 120.

[0097] The first gas sensitive component comprises a first gas sensitive layer 111 and the second gas sensitive component 120 comprises a second gas sensitive layer 121. The gas sensitive layers 111, 121 comprise a gas sensitive sensing material. The sensing material is a material that changes at least one electrical property (in particular the real or imaginary part of its electrical impedance) as a function of the composition of the gas that surrounds it. The change of the property can be measured in order to obtain information on said composition. The sensing material of the gas sensitive layers 111, 121 can e.g. contain at least one metal oxide material. Such metal oxide material may include one or more of tin oxide, zinc oxide, titanium oxide, tungsten oxide, indium oxide and gallium oxide. Such metal oxides may be used for the detection of various analytes as target gas such as ozone, VOCs, carbon monoxide, nitrogen dioxide, methane, ammonia, hydrogen, or hydrogen sulphide.

[0098] Metal oxide sensors are based on the concept that gaseous analytes interact with the metal oxide layer at elevated temperatures of the sensitive layer in the range of more than 100 C., and specifically between 250 C. and 450 C. As a result of the catalytic reaction, the conductivity of the gas sensitive layers may change, which change can be measured. Hence, such sensors are also denoted as high temperature chemoresistors for the reason that a chemical property of the analyte is converted into an electrical resistance at high temperatures of the gas sensitive layers.

[0099] FIG. 1b shows a cross section/side view of the first gas sensitive component 110 and FIG. 1c a cross section/side view of the second gas sensitive component 120. The sensor 100 is integrated with CMOS circuitry (not shown) on a single chip.

[0100] The sensor 100, also referred to as chemical sensor or gas sensor, comprises CMOS layers 130 and a handle layer 140. Parts of the CMOS layers 130 and of the handle layer 140 are etched away to form a cavity 150 at the location of the first gas sensitive layer 111 and the second gas sensitive layer 121. The remaining CMOS layers 130 form a thin membrane to support the first gas sensitive component 110 and the second gas sensitive component 120.

[0101] Embedded within the CMOS layers 130 are heating elements 135 forming a heater to provide a local source of heat to heat the first gas sensitive component 110 and the second gas sensitive component 120 during operation of the sensor 100. The temperature can rise rapidly around the first gas sensitive component 110 and the second gas sensitive component 120, while the thicker part of the chip reacts due to its thermal inertia with a slower rise of temperature. By controlling the heating elements 135 accordingly, the material, in particular the metal oxide, of the first gas sensitive layer 111 and the second gas sensitive layer 121 can be activated for a measurement and subsequent regeneration.

[0102] The first gas sensitive layer 111 is contacted by a first pair of measuring electrodes 160 and the second gas sensitive layer 121 is contacted by a second pair of measuring electrodes 161 and hence both act as a resistor. In the presence of a target gas as analyte, the respective resistance of the first gas sensitive layer 111 and the second gas sensitive layer 121 changes and thereby provides a measure of the concentration of the target gas in the sample of ambient air in the immediate vicinity of the first gas sensitive layer 111 and the second gas sensitive layer 121.

[0103] The electrodes 160 form an electrode layer 162 and the electrodes 161 form an electrode layer 163. The electrode layers 162, 163 may be defined as the layers of the first and the second gas sensitive layer respectively that are situated directly between the measuring electrodes and that provide in a perturbation regime a well conducting path between the measuring electrodes that dominates the resistance of the gas sensitive layers during this perturbation regime. The electrode layers 162, 163 are indicated by dotted lines. It should be noted that the indicated dimensions of the electrode layers 162, 163 are just an exemplary embodiment and may vary in dependence of various factors, such as the material composition, geometry etc. In this respect the electrode layers 162, 163 are in particular defined by the functionality that they provide (well conducting path in the perturbation regime) rather than by their dimensions.

[0104] The first gas sensitive component 110 is generally configured to measure the gas concentration of the target gas in a first concentration band and more particularly to detect a first threshold concentration of the target gas. The second gas sensitive component 120 is generally configured to measure the gas concentration of the target gas in a second concentration band and more particularly to detect a second threshold concentration of the target gas. In order to enhance the sensitivity for the detection of the first threshold concentration and the second threshold concentration, the first gas sensitive layer 111 and the second gas sensitive layer 121 have a different thickness.

[0105] More particularly, the first gas sensitive layer 111 has a first thickness th.sub.1 and the second gas sensitive layer has a second thickness th.sub.2. The second thickness th.sub.2 is larger than the first thickness th.sub.1. The different thicknesses th.sub.1, th.sub.2 are optimized for the respective threshold concentration of the respective gas sensitive component.

[0106] As according to this illustrated embodiment the electrodes 160, 161 are placed at the bottom of the gas sensitive layers 111, 121, the thicknesses th.sub.1, th.sub.2 correspond approximately to the diffusion distance between the surface of the gas sensitive layers 111, 121 that is in contact with the sample of ambient air and the electrode layers 162, 163 comprising the measuring electrodes 160, 161. In other words, the thickness of the first and the second gas sensitive layer determines the diffusion distance that the target gas has to pass to reach the electrode layers 162, 163.

[0107] Both the measuring electrodes 160, 161 and the heating elements 135 are connected to a control unit/control circuitry (not explicitly shown), which can be part of the CMOS circuitry arranged on the same handle layer 140. The function of the control unit/control circuitry is to control the amount of electrical power flowing into the heating elements 135 and to perform and register measurements to determine the changes in the resistance of the gas sensitive layers 111, 121.

[0108] When initialising a measurement, the control unit raises the temperature of the first gas sensitive layer 111 and the second gas sensitive layer 121 to an activation temperature for a predetermined period of time, herein referred to as the activation period. After the activation period, the control unit lets the temperature of the first gas sensitive layer 111 and the second gas sensitive layer 121 drop to a measurement or operation temperature and maintains the sensitive layers 120, 121 at the measurement temperature for a duration herein referred to as measurement period. During the measurement period, the resistance of the gas sensitive layers 111, 121 is measured and converted into a value representative of the concentration of the target gas. A typical operating temperature is in a range between 250 to 600 degrees Celsius.

[0109] According to some embodiments the first gas sensitive layer 111 and the second gas sensitive layer 121 may comprise in particular a n-doped metal oxide semiconductor (MOX) material. According to such an embodiment the sensor 100 may be provided to measure a concentration of an oxidizing target gas, in particular ozone. Another oxidizing gas may be NO.sub.2.

[0110] According to other embodiments the first and the second gas sensitive layer 111, 121 may comprise a p-doped metal oxide semiconductor material and the sensor 100 is provided to measure a concentration of a reducing target gas, e.g. carbon monoxide, VOCs, formaldehyde, methane, ammonia, hydrogen or hydrogen sulphide.

[0111] As mentioned above, the different thicknesses th.sub.1, th.sub.2 are optimized to detect the respective threshold concentration with enhanced sensitivity. More particularly, the different thicknesses th.sub.1, th.sub.2 are configured such that the sensor 100 operates for both the detection of the first threshold concentration and for the detection of the second threshold concentration in a transition regime. The transition regime is situated between a perturbation regime and a saturation regime. This is in the following further illustrated in a schematic and simplified way with reference to FIGS. 2a, 2b and 2c.

[0112] FIG. 2a shows a cross sectional view of a gas sensitive component 200 operating in a perturbation regime. The gas sensitive component 200 may represent both the first gas sensitive component 110 of FIG. 1b as well as the gas sensitive component 120 of FIG. 1c.

[0113] The perturbation regime may be defined as or considered as a regime in which the electrical resistance measured between the measuring electrodes 160, 161 is still dominated by the ambient air. More particularly, a target gas 210 to be measured, which is illustrated by a dotted pattern, has not reached yet or does not substantially influence yet the electrode layers 162, 163 between the measuring electrodes 160, 161. Accordingly, the electrode layers 162, 163 are still well conducting and provide a well conducting path between the measuring electrodes 160, 161.

[0114] As the resistance measured between the measuring electrodes 160, 161 may be considered as a parallel connection/parallel circuit of the electrode layers 162, 163 and a top layer 166 situated above the electrode layers 162 and 163, the total resistance of the layers 111, 121 is still mainly dominated by the electrode layers 162, 163.

[0115] FIG. 2b shows a cross sectional view of a gas sensitive component operating in a transition regime. In the transition regime the target gas 210 has reached the electrode layers 162, 163 and does partly penetrate or at least substantially influence the resistance of the electrode layers 162, 163. However, the target gas 210 has not completely penetrated the electrode layers 162, 163.

[0116] In the transition regime the sensor 200 shows an increased sensitivity to the target gas 210.

[0117] FIG. 2c shows a cross sectional view of a gas sensitive component operating in the saturation regime. In the saturation regime, the target gas 210 has already penetrated the whole gas sensitive layer 111, 121 including the electrode layers 162, 163. Hence in the saturation regime the electrical resistance of the gas sensitive layers 111, 121 is dominated by the target gas.

[0118] It should be noted that FIGS. 2a to 2c are provided to illustrate the regimes according to embodiments of the invention in a simplified manner and to ease the understanding of the invention. However, the underlying physical principles of the different regimes may be more complex. Furthermore, the borders between the different regimes may not be exactly defined, but may vary and may be considered to have sliding or floating transitions.

[0119] In order to operate the sensor for the respective threshold concentration in the transition regime, one or more layer parameters of the gas sensitive layers may be used.

[0120] This is illustrated with reference to FIGS. 3 to 5 and 10.

[0121] FIG. 3 shows a sensor 300 having four gas sensitive components 310, 320, 330 and 340. The gas sensitive components 310, 320, 330 and 340 comprise gas sensitive layers 311, 321, 331, 341 having different layer thicknesses th1, th2, th3 and th4 respectively in a y-direction. The gas sensitive layers 311, 321, 331, 341 have according to embodiments the same width in a x-direction. Each of the gas sensitive layers 310, 320, 330 and 340 may be heated by a heating element 335. Each of the gas sensitive layers 310, 320, 330 and 340 is configured to operate in a transition regime to detect a different threshold concentration c1, c2, c3 and c4 respectively, wherein c4>c3>c2>c1.

[0122] FIG. 4 shows a sensor 400 having four gas sensitive components 410, 420, 430 and 440. The gas sensitive components 410, 420, 430 and 440 comprise gas sensitive layers 411, 421, 431, 441 having the same layer thickness th. Each of the gas sensitive layers 410, 420, 430 and 440 may be heated by a heating element 435. Each of the gas sensitive layers 410, 420, 430 and 440 comprise a layer material of a different porosity p1, p2, p3 and p4 respectively. The porosity, which may also be denoted as void fraction, is a measure of the void spaces in the layer material. More particularly, it indicates the fraction of the volume of voids over the total volume. It may be indicated by a number between 0 and 1, or as a percentage between 0 and 100%. In this example it is assumed that p1>p2>p3>p4. Each of the gas sensitive layers 410, 420, 430 and 440 is configured to operate in a transition regime to detect a different threshold concentration c1, c2, c3 and c4 respectively, wherein again c4>c3>c2>c1.

[0123] FIG. 5 shows a sensor 500 having four gas sensitive components 510, 520, 530 and 540. The gas sensitive components 510, 520, 530 and 540 comprise gas sensitive layers 511, 521, 531, 541 having the same layer thickness th. Each of the gas sensitive layers 510, 520, 530 and 540 may be heated by a heating element 535. Each of the gas sensitive layers 410, 420, 430 and 440 comprise a layer material of a different reactivity r1, r2, r3 and r4 respectively with respect to the target gas. The reactivity may be defined as the tendency of the material of the gas sensitive layers 510, 520, 530 and 540 to undergo a chemical reaction with the target gas. In other words, it may be defined as the relative capacity of the material of the gas sensitive layers 510, 520, 530 and 540 to undergo a chemical reaction with the target gas. The reactivity does not only depend on the material and composition of the gas sensitive layers, but also on the operating temperature of the gas sensitive layers. In this example r4>r3>r2>r1 which is also indicated by an arrow with increasing thickness. Each of the gas sensitive layers 510, 520, 530 and 540 is configured to operate in a transition regime to detect a different threshold concentration c1, c2, c3 and c4 respectively, wherein again c4>c3>c2>c1.

[0124] FIG. 10 shows a sensor 1000 having four gas sensitive components 1010, 1020, 1030 and 1040. The gas sensitive components 1010, 1020, 1030 and 1040 comprise gas sensitive layers 1011, 1021, 1031, 1041 having the same layer thickness th. Each of the gas sensitive layers 1011, 1021, 1031 and 1041 can be individually heated by heating elements 1035. The heating elements 1035 are configured to heat the gas sensitive components 1010, 1020, 1030 and 1040 to different heating temperatures T1, T2, T3 and T4 respectively. As an example, T4>T3>T2>T1. Each of the gas sensitive components 1010, 1020, 1030 and 1040 may be then configured to operate in a transition regime to detect a different threshold concentration c1, c2, c3 and c4 respectively, wherein c4>c3>c2>c1.

[0125] It should be noted that according to further embodiments the layer parameters illustrated with reference to FIGS. 3 to 5 and 10 may also be combined individually to operate the sensor in the transition regime for a desired threshold concentration.

[0126] FIG. 6 shows a schematic illustration of the operation of a sensor 600 according to an embodiment of the invention. The sensor 600 comprises three gas sensitive components 610, 620 and 630. The three gas sensitive components 610, 620 and 630 are in the following also denoted as gas sensitive pixels px.sub.1, px.sub.2 and px.sub.3. The gas sensitive pixels px.sub.1, px.sub.2 and px.sub.3 may be generally characterized by layer thicknesses th.sub.1, th.sub.2 and th.sub.3, by operating temperatures T.sub.1, T.sub.2 and T.sub.3 and by layer materials MOX.sub.1, MOX.sub.2 and MOX.sub.3.

[0127] The sensor 600 is configured to measure the concentration of the target gas in a plurality of concentration bands. A first concentration band is configured for a concentration of less than 25 ppb of the target gas, e.g. of ozone. A second concentration band is configured for a concentration of the target gas between 25 and 50 ppb. A third concentration band is configured for a concentration of the target gas between 50 and 75 ppb. A fourth concentration band is configured for a concentration of the target gas above 75 ppb.

[0128] The sensor 600 is configured to extract the concentration of the target gas in a hierarchical way from the gas sensitive components 610, 620 and 630. More particularly, for the first concentration band all pixels px.sub.1, px.sub.2 and px.sub.3 operate below their respective threshold concentrations and hence do not show any sensor signal. For the second concentration band only pixel px.sub.1 operates above its threshold concentrations and hence shows a sensor signal which is indicated by a chess grid. For the third concentration band pixels px.sub.1 and px.sub.2 operate above their threshold concentrations and hence show a sensor signal indicated by corresponding chess grids. And finally for the fourth concentration band all pixels px.sub.1, px.sub.2 and px.sub.3 operate above their threshold concentrations and hence show a sensor signal. The switching between the concentration bands is triggered only by one of the respective pixels, namely by the pixel that is configured for the corresponding threshold concentration. As an example, the switching from the second concentration band to the third concentration band is triggered by the pixel px.sub.2, while the switching to the third concentration band is triggered by the pixel px.sub.3. The other pixels may be used as redundant pixels for the respective concentration band, but they may also be switched off. Hence according to some embodiments the sensor 600 may be configured to operate all the sensor components 610, 620 and 630 simultaneously and according to other embodiments the sensor 600 is configured to operate the sensor components 610, 620 and 630 in an alternating way or in other words only a subset of the sensor components. According to the latter embodiment, only the sensor components that define the threshold for a current concentration band may be operated, while the other sensor components may be switched off. As an example, in the beginning of the sensor operation only the first sensor component 610 may be operated/switched on. Only if the first sensor component 610 has detected that the first threshold concentration of 25 ppb has been reached, the second sensor component 620 is also switched on to monitor whether the second threshold of 50 ppb has been reached. If the second threshold of 50 ppb has been reached, the third gas sensitive component 630 is switched on to monitor the next threshold of 75 ppb. Then the first gas sensitive component 610 may be switched off to save power. This may include to switch off the heater for the first gas sensitive component 610, thereby saving additional power.

[0129] According to embodiments the thicknesses th.sub.i of the gas sensitive layers may be preferably in a range between 100 nm and 5 m.

[0130] More generally, according to embodiments a number n of gas sensitive components may define n different threshold concentrations of the target gas. These n threshold concentrations may split the concentration range [0, infty) into n1 concentration bands that can be identified by the n gas sensitive components. The larger the number n of gas sensitive components, the larger the resolution.

[0131] FIG. 7a and FIG. 7b illustrate in a schematic and simplified way a basic mode of operation of a sensor according to an embodiment of the invention.

[0132] The sensor comprises a first gas sensitive layer 711 and a second gas sensitive layer 721. The second gas sensitive layer 721 is thicker than the first gas sensitive layer 711.

[0133] FIG. 7a illustrates a resistance-concentration curve for the first gas sensitive layer 711 and FIG. 7b a resistance-concentration curve for the second gas sensitive layer 721. The x-axis denotes the concentration of the target gas in a sample of ambient air and the y-axis the resistance of the gas sensitive layers 711, 721 in relation to the resistance with no target gas in the sample of ambient air.

[0134] Referring to FIG. 7a, starting from zero, the resistance is increasing with a rising concentration of the target gas rather slowly during the perturbation regime. Then, approximately at a concentration C1, the gas sensitive layer 711 enters the transition regime having a steeper increase/slope of the resistance. Finally, approximately at a concentration C2, the gas sensitive layer 711 enters the saturation regime having again a lower increase/slope of the resistance.

[0135] Referring to FIG. 7b, starting from zero, the resistance is increasing again with a rising concentration of the target gas rather slowly during the perturbation regime. Then, approximately at a concentration C3, the gas sensitive layer 721 enters the transition regime having a steeper increase/slope of the resistance. The concentration C3 is greater than the concentration C1. In other words, the second gas sensitive layer 721 enters the saturation regime only at a higher concentration than the first gas sensitive layer 711. This is caused by the larger thickness of the second gas sensitive layer 721. Finally, approximately at a concentration C4, the gas sensitive layer 721 enters the saturation regime having again a lower increase/slope of the resistance.

[0136] As the sensitivity of the sensor is higher in the transition regime than in the perturbation regime and the saturation regime, both the first gas sensitive layer 711 and the second gas sensitive layer 721 are configured to operate in the transition regime. Accordingly, the second gas sensitive layer 721 is preferably configured to detect a second threshold concentration C.sub.th2 that is higher than a first threshold concentration C.sub.th1 that the first gas sensitive layer is supposed to detect. Thereby the accuracy of the sensor may be increased and the error rate may be decreased.

[0137] The slope characteristics of the resistance-concentration curve shown in FIG. 7a and the slope characteristics of the resistance-concentration curve shown in FIG. 7b are equal or substantially equal. More particularly, the resistance-concentration-curve of FIG. 7b is shifted in the x-axis to the right with respect to the resistance curve of FIG. 7a. In other words, the curves are parallel or substantially parallel to each other.

[0138] FIG. 8 illustrates some background of principles according to embodiments of the invention with the help of a simulation based on a simplified phenomenological model of the resistance of a gas sensitive MOX layer. The target gas is assumed to be ozone. The simplified phenomenological model of the MOX resistance is based on the following formulas:

[0139] A reactiondiffusion equation with the parameters reactivity r and diffusion D is supposed to govern the entry of ozone into the gas sensitive MOX layer and leads to an exponentially decreasing ozone concentration profile c(x) of penetration depth x:

[00001]$c\left(x\right)=c_0.Math.e^{-.Math..Math.x},.Math..Math.{}^2=\frac{r}{D}.$

The total conductivity of the gas sensitive layer may be derived as a sum of infinitesimal layers:

[00002]$=\underset{0}{\stackrel{l}{}}.Math.\left(x\right).Math.\mathrm{dx}=\underset{0}{\stackrel{l}{}}.Math.\frac{1}{\left(c\left(x\right)\right)}.Math.\mathrm{dx}.$

The resistivity (inverse of conductivity) of a single layer may be modelled as follows (assuming a background of 500 ppb hydrogen plus zero air at 50% relative humidity):

(c)=R.sub.1(500c).sup.n.sup.H2, for the perturbation regime;

(c)=R.sub.2(1+b c).sup.r, for the saturation regime;

wherein R.sub.1 and n are calibration constants (power law fitting constants) of the sensor response to hydrogen. More particularly, R.sub.1 is the resistance at 1 ppm hydrogen. Furthermore, is a fitting parameter quantifying the selectivity between ozone and hydrogen, and R.sub.2, b, and are fitting parameters for the ozone response.

[0140] Based on the above formulas, FIG. 8 shows a simulation illustrating to resistivity-reactivity curves for three different layer thicknesses. The transition regime between the perturbation regime and the saturation regime was derived by an interpolation. More particularly, FIG. 8 shows the resistance R in dependence on the reactivity r for 5 different concentrations of the target gas ozone in relation to a reference sample of ambient air comprising 0.5 ppm H.sub.2 and no ozone. The 5 different concentrations are 25 ppb ozone, 50 ppb ozone, 75 ppb ozone, 100 ppb ozone and 200 ppb ozone. The left diagram was simulated for a nominal layer thickness of the gas sensitive layer of 1, e.g. 1 m, the middle diagram for a nominal layer thickness of 0.5, e.g. 500 nm and the right diagram for a nominal layer thickness of 0.25, e.g. 250 nm.

[0141] As illustrated with the arrow, for a given fixed reactivity of e.g. 2.9, with decreasing layer thickness a characteristic ohmic jump that corresponds to the transition regime having a higher sensor sensitivity occurs at a decreasing ozone concentration. For example, for the layer with the nominal layer thickness 1 the ohmic jump occurs for a concentration between 100 ppb and 200 ppb. Hence this curve may be e.g. used for detecting a threshold of 200 ppb. Furthermore, for the layer with the nominal layer thickness 0.5 the ohmic jump occurs for a concentration between 25 ppb and 50 ppb. Hence this curve may be e.g. used for detecting a threshold of 50 ppb. Finally, for the layer with the nominal layer thickness 0.25 the ohmic jump occurs for a concentration between 0 ppb and 25 ppb. Hence this curve may be e.g. used for detecting a threshold of 25 ppb.

[0142] Hence according to embodiments, for a given reactivity, fixed by MOX material and MOX temperature, MOX layers of different thicknesses are created as different pixels. The layer thickness on each pixel is adapted such that the largest signal amplitude (characteristic ohmic jump) is achieved for the targeted ozone concentration. Each pixel of different thickness may define an ozone threshold concentration. Thus the output of each pixel is binary: above or below the threshold concentration. The combination of pixels (thicknesses) yields concentration bands. The more pixels (thicknesses) are implemented, the narrower the concentration bands.

[0143] FIG. 9 shows the same diagrams of the simulations according to FIG. 8 with some additional information. More particularly, it shows that according to an embodiment the sensor may be configured to provide a fine-tuning of the transition regime via one or more reactivity control parameters. As indicated by the horizontal arrows 910, for a fixed layer thickness (nominal layer thicknesses 1, 0.5 and 0.25), the response of the sensor towards the target gas (in this example ozone) can be fine-tuned via the temperature of the gas sensitive MOX layers and the material composition of the gas sensitive MOX layers, which both influence/enter the MOX reactivity.

[0144] The reactivity tuning can be done such that the dependence of the sensor signal on changes of the reactivity is minimized for the targeted ozone concentration. This may be achieved by selecting a plateau-regions 920 of a respective resistivity-reactivity curve for the respective gas sensitive layer. The plateau regions 920 are indicated by a circle. Since the reactivity of the MOX material can change over time, e.g. through aging, poisoning or other factors, the selection of the plateau regions 920 increases the stability of the sensor signal over time.

[0145] FIG. 11 illustrates in a schematic and simplified way a basic mode of operation of a sensor according to another embodiment of the invention.

[0146] The sensor comprises a first gas sensitive layer 1111 and a second gas sensitive layer 1121. The second gas sensitive layer 1121 is thicker than the first gas sensitive layer 1111.

[0147] FIG. 11 shows both a resistance-concentration curve 1130 for the first gas sensitive layer 1111 and a resistance-concentration curve 1131 for the second gas sensitive layer 1121. The x-axis denotes the concentration of the target gas in a sample of ambient air and the y-axis the resistance of the gas sensitive layers 1111, 1121 in relation to the resistance with no target gas in the sample of ambient air.

[0148] According to embodiments, the first gas sensitive layer 1111 and the second gas sensitive layer 1121 may measure the gas concentration in a concentration band 1140 and in a concentration band 1141 in a continuous manner.

[0149] A threshold concentration C.sub.th1 of the resistance-concentration curve 1130 defines the upper limit of the concentration band 1140. The threshold concentration C.sub.th1 may correspond to the threshold concentration C.sub.th1 of FIG. 7a. In a concentration band 1140a, which may also be denoted as sub-concentration band 1140a, the sensor shows an increased sensitivity to the target gas and can hence measure the target gas in a continuous/analog way with increased sensitivity.

[0150] The threshold concentration C.sub.th1 also defines the lower limit of the concentration band 1141. The concentration band 1141 comprises a sub-concentration band 1141a. A threshold concentration C.sub.th2 of the resistance-concentration curve 1131 defines the upper limit of the sub-concentration band 1141a. The threshold concentration C.sub.th2 may correspond to the threshold concentration C.sub.th2 of FIG. 7b. In the subconcentration band 1141a, the sensor shows an increased sensitivity to the target gas and can hence measure the target gas in a continuous/analog way with increased sensitivity.

[0151] By increasing the number of layers, a corresponding increase in the number of concentration bands may be achieved.

[0152] Accordingly, the concentration bands may be engineered in such a way that adjacent configuration bands are contiguous. This can be used to achieve improved accuracy. In other words, a respective on-off configuration of the gas sensitive components defines the actual concentration band, and the lowest off gas sensitive components can be used to measure with increased sensitivity, in particular in the sub-concentration bands, e.g. in the sub-concentration bands 1140a and 1141a. The improved accuracy reading may thus be provided by the lowest off gas sensitive component, i.e. by the gas sensitive component that is still operated just below its respective threshold concentration, i.e. below C.sub.th1 and C.sub.th2 respectively in the above example.

[0153] In a preferred embodiment of the invention, a sensor in accordance with this invention is integrated as a component within stationary or portable electronic devices having further uses other than chemical sensing. The electronic devices may be e.g. car infotainment devices, household appliance devices, smart phones, handheld computers, laptops, electronic readers, tablet computers etc. Its housing is typically a shell of metal, glass, or plastic material and can be assembled as a unibody or from several parts. Enclosed in the housing are typically processors, drivers for parts such as screens, antennae, cameras, microphones and speakers as well as batteries to provide power to the device and its parts. A screen is typically arranged as a part of the housing or mounted behind a transparent window of the housing.

[0154] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.