JP7754501B2 - Semiconductor gas sensor - Google Patents

Description

The present invention relates to a semiconductor gas sensor.

Conventionally, semiconductor gas sensors equipped with a gas detection layer made of an oxide semiconductor have been known (see, for example, Patent Document 1).

This type of semiconductor gas sensor is used by heating the gas sensing layer to a temperature of several hundred degrees, causing the oxygen adsorbed on the surface of the gas sensing layer to steal electrons from the gas sensing layer, making it difficult for electricity to flow through the gas sensing layer. When the gas sensing layer is exposed to a reducing gas such as acetone or carbon monoxide in this state, the reducing gas reacts with the oxygen on the surface of the gas sensing layer, returning the electrons captured by the adsorbed oxygen to the gas sensing layer, making it easier for electricity to flow through the gas sensing layer. Semiconductor gas sensors can detect gas by measuring changes in the electrical resistance of the gas sensing layer.

Conventional gas sensors increase the surface area of the gas sensing layer by forming it from an aggregate of numerous columnar crystals, thereby improving sensor sensitivity and gas selectivity. However, in order to increase the surface area of the gas sensing layer, conventional gas sensors require the oxide semiconductor that makes up the gas sensing layer to be formed into columnar crystals, which places limitations on the types of oxide semiconductor that can be formed.

Patent No. 5240767

The objective of the present invention is to provide a semiconductor gas sensor that can increase the surface area of the gas sensing layer, regardless of the type of oxide semiconductor used in the gas sensing layer, thereby improving sensor sensitivity and gas selectivity.

The semiconductor gas sensor of the present invention comprises an electrode layer formed on an insulating substrate, a concave-convex structure layer formed across the substrate and the electrode layer, and a gas detection layer made of an oxide semiconductor formed on the concave-convex structure layer, wherein the concave-convex structure layer has a porous structure, and the gas detection layer has a porous structure resulting from the porous structure of the concave-convex structure layer.

In the semiconductor gas sensor of the present invention, the uneven structure layer may be formed, for example, from an insulating material.

The uneven structure layer may also be formed, for example, from an insulating metal oxide.

The uneven structure layer may also be formed, for example, from an insulating aluminum compound layer obtained by making an alumina layer porous through hot water treatment.

The gas sensing layer may be formed of, for example, tungsten trioxide doped with impurities to give it a biased negative charge. More specifically, the gas sensing layer may be formed of tungsten trioxide doped with nickel. The impurity doped into tungsten trioxide (hereinafter also referred to simply as tungsten oxide) may be any element with a valence of five or less, and may be, in addition to nickel (Ni), for example, silicon (Si), indium (In), aluminum (Al), or two or more elements. The oxide semiconductor forming the gas sensing layer may be a metal oxide other than tungsten trioxide, such as tin dioxide or titanium dioxide, or may be an oxide semiconductor doped with impurities.

The semiconductor gas sensor of the present invention can increase the surface area of the gas sensing layer regardless of the type or crystalline structure of the oxide semiconductor in the gas sensing layer.

1 is a schematic plan view showing an embodiment of a semiconductor gas sensor; FIG. 2 is a schematic exploded perspective view showing the embodiment. FIG. 2 is a schematic cross-sectional view corresponding to the position AA in FIG. 1. 5A to 5C are schematic cross-sectional views for explaining an example of a manufacturing process of the embodiment. 1 is a scanning electron microscope image showing a fracture surface of a concave-convex structure layer. 1 is a scanning electron microscope image of the surface of a concave-convex structure layer. 1 is a scanning electron microscope image of the surface of the gas sensing layer of the same embodiment. 10 is a graph showing the results of using the gas sensor of the example and a conventional gas sensor to detect acetone. 10 is a graph showing the response characteristics of the gas sensor of the example to acetone. 10 is a graph showing the response characteristics of the gas sensor of the example to ethanol. 10 is a graph showing the response characteristics of a gas sensor of a comparative example to acetone. 10 is a graph showing the response characteristics of a gas sensor of a comparative example to ethanol. 1 is a graph showing the response characteristics of the gas sensor of Reference Example 1 to acetone. 1 is a graph showing the response characteristics of the gas sensor of Reference Example 1 to ethanol. 10 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to acetone. 10 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to ethanol. 10 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to acetone. 10 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to ethanol.

An embodiment of a semiconductor gas sensor of the present invention will be described with reference to the drawings. Figure 1 is a schematic plan view showing one embodiment. Figure 2 is a schematic exploded perspective view showing the same embodiment. Figure 3 is a schematic cross-sectional view corresponding to the position A-A in Figure 1.

As shown in Figures 1 and 2, the semiconductor gas sensor 1 of this embodiment includes an insulating substrate 2, an electrode layer 3 formed on the substrate 2, a concave-convex structure layer 4 formed across the substrate 2 and the electrode layer 3, and a gas detection layer 5 made of an oxide semiconductor formed on the concave-convex structure layer 4. The gas sensor 1 is a so-called substrate-type gas sensor.

The insulating substrate 2 is formed from an insulating material such as glass, quartz, or ceramics. There are no particular limitations on the size or shape of the substrate 2.

The electrode layer 3 detects changes in the electrical resistance of the gas detection layer 5 and includes a pair of interdigital electrode layers 3a and 3b. The interdigital electrode layers 3a and 3b can be formed, for example, from platinum (Pt) layers deposited on the substrate 2 by sputtering using a metal mask with openings corresponding to the electrode layer formation areas. However, the shape of the electrode layer 3 is not limited to this and can be any shape, such as a parallel plate or spiral. The material of the electrode layer 3 is also not particularly limited. Examples include platinum, gold, silver, titanium, nickel, aluminum, ruthenium, tantalum, copper, platinum, niobium, zirconium, alloys of these elements, alloys of these elements with carbon, and elemental carbon. The electrode layer 3 can be a single-layer film or a multilayer film consisting of multiple layers stacked together.

As shown in FIG. 3, the concave-convex structure layer 4 is formed across the substrate 2 and the electrode layer 3. The concave-convex structure layer 4 is made of an insulating metal oxide having a porous structure with numerous base pores 4a. In this embodiment, the concave-convex structure layer 4 is made of an aluminum compound layer in which an alumina layer has been made porous by hot water treatment. In this embodiment, the concave-convex structure layer 4 is formed on the upper surface of the substrate 2, covering the areas where the comb-tooth portions of the comb-shaped electrode layers 3a and 3b are formed.

Such a concave-convex structure layer 4 can be formed, for example, by depositing a film of amorphous alumina approximately 30 nm thick on the substrate 2 after the electrode layer 3 has been formed using atomic layer deposition (ALD) or physical vapor deposition (PVD), and then immersing the substrate 2 in hot water at approximately 75-85°C for several minutes to several tens of minutes. The porous aluminum compound layer formed by this hot water treatment has a thickness (height) of approximately 140-150 nm, for example. In this way, a concave-convex structure layer 4 with a porous structure can be formed simply by performing the extremely simple process of hot water treatment, thereby reducing manufacturing costs.

Some of the base pores 4a of the concave-convex structure layer 4 open to the surface of the concave-convex structure layer 4 and reach the electrode layer 3. The gas detection layer 5, described below, is electrically connected to the electrode layer 3 via the base pores 4a.

The porous uneven structure layer 4 is not limited to an aluminum compound layer made porous by hot water treatment, but may be formed from other insulating metal oxides. Furthermore, the uneven structure layer 4 may be formed from an insulating material other than a metal oxide, such as mesoporous silica.

The gas detection layer 5 is formed on the concave-convex structure layer 4 and has a porous structure (numerous detection layer pores 5a) resulting from the porous structure of the concave-convex structure layer 4 (numerous base pores 4a). A gas detection layer 5 with such a porous structure can be formed by stacking an oxide semiconductor on the concave-convex structure layer 4. In this embodiment, tungsten trioxide (WO3, hereinafter simply referred to as tungsten oxide) is used as the material for the gas detection layer 5.

The material of the gas detection layer 5 is not limited to tungsten oxide, but may be other oxide semiconductors such as tin dioxide, titanium dioxide, zinc oxide, indium oxide, or ITO (indium tin oxide), or may be doped (added) with impurity elements.

The porous structure of the gas sensing layer 5 depends on the porous structure of the concave-convex structure layer 4, but for example, the thickness (film thickness) is approximately 20 to 1000 nm. Furthermore, by varying the film thickness of the gas sensing layer 5, it is possible to control the porous structure of the gas sensing layer 5 (such as the opening shape, opening width, and pore depth of the sensing layer pores 5a). This makes it possible to form a gas sensor 1 equipped with a gas sensing layer 5 having a porous structure tailored to the application.

The gas sensing layer 5 is preferably manufactured by vapor deposition, as this allows for highly accurate control of the formation area and film thickness. In this embodiment, the formation area of the gas sensing layer 5 is the same as the formation area of the concave-convex structure layer 4. Vapor deposition methods that can be used here include so-called physical vapor deposition (PVD) methods such as sputtering, vacuum deposition, reactive plasma deposition, and ion plating, as well as so-called chemical vapor deposition (CVD) methods. However, the manufacturing method for each layer is not limited to vapor deposition, and printing methods such as screen printing and inkjet printing may also be used.

An adhesive layer may be formed on the surface of the substrate 2 to prevent peeling between the substrate 2 and the electrode layer 3. Materials for such adhesive layers may be any material that provides good adhesion between the substrate 2 and the electrode layer 3, and examples of such materials include chromium, titanium, and tungsten. An adhesive layer may also be formed on the surface of the substrate 2 to prevent peeling between the substrate 2 and the concave-convex structure layer 4. Such an adhesive layer may be a surface treatment layer formed by subjecting the surface of the substrate 2 to a surface treatment that can improve adhesion to the concave-convex structure layer 4. Examples of such surface treatments include plasma treatment, corona treatment, flame treatment, etching treatment, steam treatment, and ion beam treatment.

The gas sensor 1 has a gas sensing layer 5 with a porous structure, which allows the surface area of the gas sensing layer 5 to be significantly increased compared to a flat surface, improving sensor sensitivity (detection sensitivity). Furthermore, the gas sensing layer 5 has sensing layer pores 5a with a porous structure, which allows gas components to come into contact with the gas sensing layer 5 within the sensing layer pores 5a, while preventing impurities larger than the opening width of the sensing layer pores 5a on the surface of the gas sensing layer 5 from entering the sensing layer pores 5a. This reduces the area over which impurities come into contact with the gas sensing layer 5, improving selective sensitivity to gas components.

Furthermore, because the porous structure of the gas detection layer 5 (detection layer pores 5a) is due to the porous structure (base pores 4a) of the underlying uneven structure layer 4, a gas detection layer 5 having a porous structure can be easily and reliably formed by stacking an oxide semiconductor material on the uneven structure layer 4. In other words, the gas sensor 1 forms a gas detection layer 5 having a porous structure even if the oxide semiconductor layer that forms the gas detection layer 5 does not have a specific crystal structure that increases the surface area, such as columnar crystals. That is, the gas sensor 1 can increase the surface area of the gas detection layer 5 regardless of the type or crystal structure of the oxide semiconductor of the gas detection layer 5, thereby improving sensor sensitivity and gas selectivity.

In addition, by varying the thickness of the gas sensing layer 5, it is possible to control the porous structure of the gas sensing layer 5 (such as the opening shape, opening width, and pore depth of the sensing layer pores 5a). This makes it possible to form a gas sensor 1 equipped with a gas sensing layer 5 having a porous structure tailored to the application.

Next, with reference to Figure 4, an example of the fabrication of gas sensor 1 will be described. An electrode layer 3 having a pair of comb-shaped electrode layers 3a, 3b was formed by sputtering on an insulating substrate 2 made of glass with a thickness of approximately 10 mm (see Figure 4(1)). For example, electrode layer 3 is formed of a platinum layer with a thickness of approximately 150 nm. Furthermore, the line width of the comb-shaped electrode layers 3a, 3b where the comb-shaped portions are alternately arranged is 150 μm, and the comb-shaped electrode layers 3a, 3b are arranged parallel and alternately at a pitch of 300 μm (150 μm intervals).

Next, using a metal mask having an opening pattern corresponding to the formation areas of the comb-teeth electrode portions of the comb-shaped electrode layers 3a and 3b, an alumina layer 6 having a thickness of approximately 30 nm was formed as a metal oxide film for forming the concave-convex structure layer 4 (see Figure 4 (2)). Note that the thickness of the alumina layer 6 may be greater or less than 30 nm.

The substrate 2 on which the electrode layer 3 and alumina layer 6 were formed was immersed in warm water at approximately 75-85°C for approximately 7 minutes to make the alumina layer 6 porous, forming a concave-convex structure layer 4 with numerous underlying pores 4a (porous structure). The substrate 2 was then dried. The porous concave-convex structure layer 4 had a thickness of approximately 200-300 nm (see Figure 4 (3)).

Figure 5 is a scanning electron microscope image showing the fracture surface of the concave-convex structure layer 4. Figure 6 is a scanning electron microscope image of the surface of the concave-convex structure layer 4. As can be seen from Figures 5 and 6, the concave-convex structure layer 4, which is made of an insulating aluminum compound layer in which the alumina layer 6 has been made porous by hot water treatment, has numerous base pores 4a formed therein. The surface of the concave-convex structure layer 4 has an uneven shape. Some of the base pores 4a in the concave-convex structure layer 4 open to the surface of the concave-convex structure layer 4 and reach the substrate 2. Although not shown in Figures 5 and 6, some of the base pores 4a on the electrode layer 3 reach the electrode layer 3.

Next, an oxide semiconductor material was deposited on the concave-convex structure layer 4 by reactive plasma deposition using a metal mask with an opening pattern corresponding to the formation area of the concave-convex structure layer 4 (the formation area of the alumina layer 6). This formed the gas detection layer 5 with numerous detection layer pores 5a (porous structure). The gas detection layer 5 is electrically connected to the electrode layer 3 via the base pores 4a. Here, a 100 nm thick film of tungsten oxide (WO3) doped with 0.5 at% (atomic percent) indium (In) was used as the oxide semiconductor material (see Figure 4 (4)). The gas detection layer 5 was then sintered.

In this way, by forming the metal oxide film for forming the concave-convex structure layer 4 and the gas sensing layer 5 by vapor deposition (here, reactive plasma vapor deposition) using a metal mask with an opening pattern, patterning by etching or lift-off after deposition of each layer is not required, thereby reducing manufacturing costs.

Figure 7 is a scanning electron microscope image of the surface of the porous gas sensing layer 5 in one embodiment of the gas sensor 1. As can be seen from Figure 7, a porous structure (sensing layer pores 5a) is formed on the surface of the gas sensing layer 5 due to the porous structure of the relief structure layer 4. This increases the surface area of the gas sensing layer 5, improving sensor sensitivity.

The thickness of the gas sensing layer 5 is not particularly limited, but is preferably 50 nm or more and 1000 nm or less. If the thickness of the gas sensing layer 5 is thinner than 50 nm, the gas sensing layer 5 will have high resistance and measurement sensitivity will decrease. Furthermore, if the thickness of the gas sensing layer 5 is thicker than 1000 nm, when the gas sensing layer 5 is formed by vapor deposition (e.g., reactive plasma vapor deposition), the time required to form the gas sensing layer 5 will be longer, resulting in reduced production efficiency.

In addition, by providing multiple gas sensor 1 regions on a single substrate 2, multiple gas sensors 1 can be formed simultaneously, and then each gas sensor 1 can be separated into individual pieces, thereby reducing manufacturing costs.

Next, we will explain some example measurements using gas sensor 1.

The gas sensor 1 used in this example had a gas sensing layer 5 made of tungsten oxide with 0.5 at% indium added, and was manufactured using the manufacturing process described with reference to Figure 4. The measurement gas used had an acetone concentration of 25 to 1,000 ppb (parts per billion). A conventional gas sensor was used as a comparative example. The gas sensor 1 was heated to, for example, 300°C, and the sensor resistance ( _Rair ) when the gas sensing layer 5 was exposed to air and the sensor resistance ( _Rgas ) when the gas sensing layer 5 was exposed to the measurement gas were measured to determine the sensitivity (= _Rair / _Rgas ).

Figure 8 is a graph showing the results of using a gas sensor of one embodiment and a conventional gas sensor to detect acetone. In Figure 8, the vertical axis represents sensitivity (arbitrary units), and the horizontal axis represents the acetone concentration of the measured gas. As can be seen from Figure 8, gas sensor 1 of the embodiment (see "Example") achieved measurement sensitivity approximately 10 times higher than the conventional gas sensor (see "Conventional Sensor") (see measurement results for an acetone concentration of 1,000 ppb). Furthermore, gas sensor 1 of the embodiment achieved a sensitivity of approximately 8 for an acetone concentration of 500 ppb, demonstrating high detection sensitivity for acetone. Furthermore, gas sensor 1 of Example 1 was also able to detect acetone at a concentration of 25 ppb, demonstrating its high detection sensitivity for acetone.

It is known that the breath of diabetic patients has elevated acetone concentrations, and attempts have been made to detect this elevated acetone concentration using a gas sensor, making it possible to diagnose diabetes. For this type of diabetes diagnosis, gas sensors are required to be able to detect acetone at low concentrations of around 500 to 1000 ppb, and to be selective for components other than acetone (especially ethanol) contained in breath.

The detection sensitivity of the gas sensor 1 of the above example to acetone and ethanol was verified. Figures 9 and 10 are graphs showing the response characteristics of the gas sensor 1 of the example to acetone and ethanol. The measurement gases used were an acetone concentration of 1000 ppb (1 ppm) and an ethanol concentration of 1000 ppb. In each of the graphs in Figures 9 and 10, the horizontal axis represents relative time (seconds) and the vertical axis represents sensitivity (arbitrary units).

As shown in Figure 9, the gas sensor 1 of the example had a detection peak of approximately 24.0 for acetone at a concentration of 1000 ppb, and a detection peak of approximately 25.0 for ethanol at a concentration of 1000 ppb. These results demonstrate that the gas sensor 1 of the example, which has a gas sensing layer 5 made of tungsten oxide doped with indium, exhibits high detection sensitivity for both acetone and ethanol.

Next, we will explain the results of verifying the effect of forming the gas detection layer 5 on the concave-convex structure layer 4. As a comparative gas sensor, a sensor was fabricated in the manufacturing process described with reference to Figure 4, in which (1) the electrode layer 3 was formed on the substrate 2, and then (2) the alumina layer 6 formation process and (3) the hot water treatment process were omitted, and (4) the gas detection layer 5 was formed on the substrate 2 and the electrode layer 3. The detection sensitivity of the comparative gas sensor to acetone and ethanol was verified.

Figures 11 and 12 are graphs showing the response characteristics of the gas sensor of the comparative example to acetone and ethanol. The measurement conditions were the same as those used to measure the response characteristics of gas sensor 1 of the example shown in Figures 9 and 10.

As shown in Figure 11, the gas sensor of the comparative example had a detection peak of approximately 12.5 for acetone at a concentration of 1000 ppb. Furthermore, as shown in Figure 12, the gas sensor of the comparative example had a detection peak of approximately 18.7 for ethanol at a concentration of 1000 ppb. Compared with the response characteristics of gas sensor 1 of the example shown in Figures 9 and 10, gas sensor 1 of the example was found to have higher detection sensitivity for both acetone and ethanol than the gas sensor of the comparative example.

Next, we will explain the results of examining the types of impurities added to tungsten oxide. For the gas sensors used for verification, three types of gas sensors, Reference Examples 1 to 3, were fabricated, each with a structure similar to that of the gas sensor in the comparative example described above, but with different impurities added to the gas sensing layer (tungsten oxide). The impurities added to the tungsten oxide were 0.5 at% nickel (Reference Example 1), 0.5 at% silicon (Reference Example 2), and 0.5 at% aluminum (Reference Example 3).

Figures 13 and 14 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 1, which has a gas sensing layer made of tungsten oxide with nickel added. Figures 15 and 16 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 2, which has a gas sensing layer made of tungsten oxide with silicon added. Figures 17 and 18 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 3, which has a gas sensing layer made of tungsten oxide with aluminum added. The measurement conditions were the same as those used to measure the response characteristics of Example Gas Sensor 1 shown in Figures 9 and 10. In each of the graphs in Figures 13 to 18, the horizontal axis represents relative time (seconds) and the vertical axis represents sensitivity (arbitrary units).

As shown in Figure 13, the gas sensor of Reference Example 1, which has a gas sensing layer made of tungsten oxide with added nickel, has a detection peak of approximately 3.9 for acetone at a concentration of 1000 ppb, demonstrating very high detection sensitivity for acetone. On the other hand, as shown in Figure 14, the gas sensor of Reference Example 1 has a detection peak of approximately 2.3 for ethanol at a concentration of 1000 ppb, roughly half the detection sensitivity for acetone, demonstrating a difference in detection sensitivity for acetone and ethanol.

These results indicate that a gas sensor 1 having nickel-added tungsten oxide as the gas sensing layer 5 formed on the concave-convex structure layer 4 is likely to have a difference in detection sensitivity for acetone and ethanol.

As shown in Figure 15, the gas sensor of Reference Example 2, which has a gas sensing layer 5 made of tungsten oxide with silicon added, had a detection peak of approximately 4.7 for acetone at a concentration of 1000 ppb. On the other hand, as shown in Figure 16, the gas sensor of Reference Example 2 had a detection peak of approximately 3.7 for ethanol at a concentration of 1000 ppb. The gas sensor of Reference Example 2 showed a difference in detection sensitivity for acetone and ethanol.

As shown in Figures 17 and 18, the gas sensor of Reference Example 3, which has a gas sensing layer made of tungsten oxide with aluminum added, had detection peaks of approximately 3.0 for both acetone at a concentration of 1000 ppb and ethanol at a concentration of 1000 ppb, and the detection sensitivity for acetone and ethanol was approximately the same.

As described above, it was found that gas sensors having a gas sensing layer of tungsten oxide (WO3) doped with nickel or silicon exhibit better detection sensitivity to acetone than gas sensors having tungsten oxide doped with aluminum. Furthermore, it was found that gas sensors having a gas sensing layer made of tungsten oxide doped with nickel exhibit a large difference in detection sensitivity to acetone and ethanol.

The present invention is not limited to the above-described embodiment and can be embodied in various forms. For example, the porous uneven structure layer 4 formed on the insulating substrate 2 may have a structure that results in a porous structure being formed in the gas detection layer 5 due to its porous structure. Such an uneven structure layer may be, for example, a porous film of titanium oxide, tin oxide, magnesium oxide, or the like; a porous metal oxide containing multiple metal elements; or a porous layer formed by the bonding of numerous microparticles made of an insulating metal oxide. The porous uneven structure layer 4 may be any layer that allows electrical connection between the gas detection layer 5 formed on the uneven structure layer 4 and the electrode layer 3 below the uneven structure layer 4 via pores.

Such a concave-convex structure layer 4 may be formed, for example, by subjecting a film (e.g., a film made of an insulating material) formed across the substrate 2 and the electrode layer 3 to a process (such as etching) to form numerous base pores 4a, resulting in a porous structure. Furthermore, the numerous base pores 4a in the concave-convex structure layer 4 may be formed irregularly or regularly. This also applies to the above-described embodiment described with reference to Figures 1 to 3. Note that not all of the base pores 4a opening on the upper surface of the concave-convex structure layer 4 need to reach the lower surface of the concave-convex structure layer 4. In other words, the concave-convex structure layer 4 may be configured such that at least some of the base pores 4a reach the lower surface of the concave-convex structure layer 4, enabling electrical connection between the upper gas detection layer 5 and the lower electrode layer 3.

1 Gas sensor 2 Substrate 3 Electrode layers 3a, 3b Interdigital electrode layer 4 Concave-convex structure layer 4a Base pore 5 Gas detection layer 5a Detection layer pore 6 Alumina layer

Claims (6)

an electrode layer formed on an insulating substrate;
a concave-convex structure layer formed across the substrate and the electrode layer;
a gas detection layer made of an oxide semiconductor formed on the concave-convex structure layer,
the concave-convex structure layer has a large number of base pores ,
the gas sensing layer has a large number of sensing layer pores formed due to the large number of base pores , and is electrically connected to the electrode layer via the base pores;
Semiconductor gas sensor. the concave-convex structure layer is formed of an insulating material;
2. The semiconductor gas sensor according to claim 1. the concave-convex structure layer is formed of an insulating metal oxide;
2. The semiconductor gas sensor according to claim 1. the concave-convex structure layer is formed of an insulating aluminum compound layer obtained by making an alumina layer porous by hot water treatment;
2. The semiconductor gas sensor according to claim 1. The gas sensing layer is formed of tungsten trioxide to which an impurity is added and which is biased to a negative charge.
5. The semiconductor gas sensor according to claim 1. The gas sensing layer is formed of nickel-added tungsten trioxide.
5. The semiconductor gas sensor according to claim 1.