JPH0781973B2 - Oxide semiconductor gas sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas sensor using a metal oxide semiconductor such as tin oxide or zinc oxide, and more particularly to a contact resistance between an oxide semiconductor and a metal electrode layer. A few gas sensors.

[Prior Art] A gas sensor using a metal oxide semiconductor utilizes the property that an n-type semiconductor such as tin oxide or zinc oxide comes into contact with a combustible gas to reduce electric resistance. It is widely used for simple detection of Japanese Patent Publication 45-
Since the disclosure of this type of gas sensor in 38200 publication, the elucidation and practical application of the gas detection mechanism have been promoted, and various types of elements have been proposed so far, but the element structure is roughly classified into a sintered type. It can be divided into membrane type. As shown in FIG. 3, the sintering type is formed by adhering a paste prepared by kneading the oxide powder 31 with water or the like around the electrode coil 6 and drying it. As shown in FIG. 4, in the film type, metal electrode layers 11 and 12, a gas sensitive layer 2 and a coating layer 3 are sequentially laminated on the main surface of a substrate 1 such as alumina, and a heater is formed on the other main surface of the substrate 1. 4 are provided.

[Problems to be Solved by the Invention] The film-type device has an advantage that the manufacturing process is simple and mass producibility is high. However, compared with the sintered-type device, isobutane, methane,
There are problems that the sensitivity of resistance change to combustible gas such as hydrogen is not sufficient, selection performance that enhances sensitivity to a specific gas cannot be obtained, and long-term stability is lacking.

The present invention has been made in view of the above points, and an object thereof is to reduce the contact resistance between a gas-sensitive layer which is an oxide semiconductor and a metal electrode layer, and thereby an oxide semiconductor gas sensor having excellent reproducibility, sensitivity, selectivity, stability, and the like. To provide.

[Means for Solving the Problems]

The present inventor has conducted extensive studies on the electric resistance of the film-type device, and as a result, unlike the case of the sintered type, the device resistance is not only the resistance of the gas-sensitive layer which is an oxide semiconductor but also the gas-sensitive layer and the metal electrode layer. It was found that the contact resistance is a factor that impedes the device performance, and the present invention has been completed based on this finding.

According to the present invention, the above-mentioned object comprises a pair of metal electrode layers formed on a substrate and an n-type oxide semiconductor carrying a noble metal formed on the substrate so as to cover a part of the metal electrode layers. In an oxide semiconductor gas sensor including a gas-sensitive layer, an n-type oxidation for ohmic-contacting the metal electrode layer and the gas-sensitive layer between the metal electrode layer and the gas-sensitive layer formed on the metal electrode layer. This is achieved by providing a contact layer having a low resistance of the object semiconductor.

The contact layer is formed by reducing the resistance of the n-type oxide by, for example, doping the n-type oxide with another element. In this case for example the resistivity of SnO ₂ is the specific resistance of the SnO ₂ is 1 × 10 ^-3 Ω · is a 1~10Ω · cm was 1% doped with Sb
cm.

[Action]

When the n-type semiconductor layer comes into contact with the metal electrode layer having a larger work function, a Schottky barrier is formed to exhibit a large contact resistance without ohmic contact. However, an n-type semiconductor is formed between the gas sensitive layer made of the n-type semiconductor and the metal electrode layer. When the contact layer having a low resistance is provided, the work relationship between the gas-sensitive layer and the contact layer is similar to each other, so that these two layers make ohmic contact. On the other hand, a current flows between the contact layer and the metal electrode layer due to the tunnel effect, and practically an ohmic contact is formed. In this way, the gas-sensitive layer and the metal electrode layer make an ohmic contact, so that the contact resistance can be reduced, and the total resistance of the device is mainly controlled by the resistance of the gas-sensitive layer.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a membrane type sectional view of a gas sensor according to an embodiment of the present invention. This gas sensor is different from the conventional gas sensor in that a contact layer 5 is provided between the metal electrode layers 11 and 12 on the substrate 1 and the gas sensitive layer 2.

Such a gas sensor is prepared as follows.
Platinum paste is screen-printed on the substrate 1 in a predetermined pattern and baked at 1000 ° C. to form the metal electrode layers 11 and 12 and the heater 4. Next, antimony pentoxide (Sb
With ₂ O ₅₎ as a tin oxide (raw material SnO ₂₎ Peretsuto in a proportion of 1 wt%, the metal electrode layer by electron beam evaporation
A contact layer 5 is formed on 11,12. Contact layer thickness is 0.0
It is adjusted to 5 to 0.5 μm. Subsequently, tin oxide powder having an average particle diameter of 2 μm is impregnated with an aqueous solution of chloroplatinic acid so that the platinum content is 0.5% by weight, and after drying, heating is performed at 600 ° C. for 2 hours to support the fine particles of platinum on the surface of the tin oxide powder. A paste is prepared by adding water and silica sol to this powder, printed on a substrate 1 provided with a contact layer so as to have a thickness of 70 μm, dried, and then heat-treated at a temperature of 750 ° C. for 30 minutes to give a feel of tin oxide. The gas layer 2 is formed. Following formation of the gas-sensitive layer 2, tin oxide powder having an average particle size of 2 μm is impregnated with an aqueous solution of chloroplatinic acid so that the platinum content is 2% by weight, and after drying at 600 ° C.
By heating for a time, platinum fine particles are supported on the surface of the tin oxide powder. Water and silica sol are added to this platinum-supported tin oxide powder to prepare a paste, and 50 μm is applied so as to cover the gas-sensitive layer 2.
The coating layer 3 is formed by applying the coating layer 3 to a thickness of m, drying at room temperature, and then heating at 730 ° C. for 30 minutes.

Figure 2 shows the device resistance in 0.2% isobutane gas. The polygonal line A is the resistance value of the conventional element, and the polygonal line B is the resistance value of the element according to the embodiment of the present invention. The conventional element has a high resistance value of 12 to 27 KΩ and has a large variation. It can be seen that the elements according to the examples of the present invention have a low resistance value of 6.8 to 9 KΩ and little variation. The difference in element resistance between the conventional element and the element according to the embodiment of the present invention depends on the magnitude of the contact resistance. That is, the conventional element has a large contact resistance value because the gas-sensitive layer and the metal electrode layer make a non-ohmic contact. On the other hand, in the device according to the embodiment of the present invention, the contact resistance between the gas sensitive layer 2 and the metal electrode layers 11 and 12 is small, so that the contact resistance is small and the device resistance is determined by the resistance exhibited by the gas sensitive device. . It can be seen that the difference between the polygonal line A and the polygonal line B is the non-ohmic contact resistance indicated by the shutter barrier, which is about 10 KΩ. Therefore, in the conventional element, about 50% of the element resistance is occupied by the non-ohmic contact resistance. Since this contact resistance is controlled by various factors and is unstable, it impairs the reproducibility and long-term stability of the element resistance, and masks the resistance change of the gas-sensitive layer to deteriorate the sensitivity and selection performance of the element. Since the elements according to the examples of the present invention have a small contact resistance, they are not affected by the contact resistance, and the resistance change of only the gas-sensitive layer is taken out, and the reproducibility, stability, sensitivity, and selectivity are excellent. It becomes possible to prepare a gas sensor.

〔The invention's effect〕

According to this invention, a pair of metal electrode layers formed on the substrate, and a gas sensitive layer made of an n-type oxide semiconductor supporting a noble metal formed on the substrate so as to cover a part of the metal electrode layers. In the oxide semiconductor gas sensor including: an n-type oxide semiconductor for ohmic-contacting the metal electrode layer and the gas-sensitive layer between the metal electrode layer and the gas-sensitive layer formed on the metal electrode layer. The ohmic contact between the gas-sensitive layer and the metal electrode layer makes ohmic contact between the gas-sensitive layer and the metal electrode layer, and the contact resistance is reduced, and the element resistance is controlled by the resistance value of the gas-sensitive layer. As a result, an oxide semiconductor gas sensor having excellent reproducibility, stability, sensitivity, selectivity, etc. can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of a gas sensor according to an embodiment of the present invention, FIG. 2 is a diagram showing element resistances of a conventional element and an element according to an embodiment of the present invention, and FIG. FIG. 4 is a schematic sectional view showing a conventional sintering type gas sensor, and FIG. 4 is a schematic sectional view showing a conventional membrane type gas sensor. 1 ... Substrate, 2 ... Gas sensitive layer, 3 ... Covering layer, 5 ... Contact layer.

Claims (1)

[Claims] 1. A pair of metal electrode layers formed on a substrate,
An oxide semiconductor gas sensor comprising a gas sensitive layer made of an n-type oxide semiconductor supporting a noble metal formed on a substrate so as to cover a part of the metal electrode layer, the metal electrode layer and the metal electrode layer An oxide semiconductor gas sensor, characterized in that a contact layer having a low resistance of an n-type oxide semiconductor that makes ohmic contact between the metal electrode layer and the gas sensitive layer is provided between the gas sensitive layer formed above.