JPH0526834A - Hot-wire type semiconductor gas sensor - Google Patents

Abstract

PURPOSE:To obtain the title sensor capable of being miniaturized, reduced in power consumption and excellent in the measuring sensitivity of the concn. of gas. CONSTITUTION:A hot-wire type semiconductor gas sensor 1 consists of a two- terminal semiconductor device 10 and the metal oxide semiconductor 12 bonded to the surface of the device. By applying the switching action of the semiconductor device 10, the device 10 itself is allowed to cyclically function as a heater.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot wire type semiconductor gas sensor.

[0002]

2. Description of the Related Art A metal oxide semiconductor gas sensor detects a gas concentration by measuring a change in conductivity of a metal oxide semiconductor due to gas adsorption. In order to increase the gas adsorption / desorption rate, it is necessary to keep the metal oxide semiconductor at a constant high temperature. As the metal oxide semiconductor gas sensor, a hot-wire semiconductor type gas sensor, a sintered body element semiconductor gas sensor, etc. have been conventionally known. These gas sensors differ in the heating method and the measuring method of the resistance value of the metal oxide semiconductor (sensor element).

The hot wire semiconductor type gas sensor has a structure in which a noble metal wire coil 22 as a low resistance heater is embedded in the center of SnO ₂ ceramic as a sensor element 20, for example. The sensor element 20 is heated by the noble metal wire coil 22, and the gas concentration is measured by detecting a change in the resistance value of the sensor element 20 due to gas adsorption. An electric circuit of such a hot wire type semiconductor gas sensor is shown in FIG. The sensor element 20 and the noble metal wire coil 22 are connected in parallel.

In the sintered body element semiconductor gas sensor, as shown in FIG. 6A, a metal oxide semiconductor paste which is the sensor element 20 is attached to two parallel noble metal wire electrodes 24 and 24 '. It is applied and sintered. One of the noble metal wire electrodes 24 also functions as a heater. The electric circuit is shown in FIG.

[0005]

The hot wire semiconductor type gas sensor can be downsized, and the power consumption of the heater is about 150.
Although it has a merit of less than mW, since a low-resistance heater and a sensor element for measuring gas concentration are connected in parallel, when compared with a sintered body element semiconductor gas sensor, it is almost the same as that of a heater. It has a drawback that the resistance component gas concentration measurement sensitivity is lowered.

The sintered body element semiconductor gas sensor has high sensitivity because it can measure the change in the electrical resistance value of the sensor element due to the adsorption of gas on the surface of the sensor element as it is, but it is difficult to miniaturize it. In addition, since it is necessary to constantly supply current to the heater, the power consumption is about 1
There is a problem that it is difficult to drive the battery as large as 000 mW to 3000 mW.

Therefore, an object of the present invention is to provide a hot wire semiconductor gas sensor which can be miniaturized, consumes less power, and has excellent gas concentration measurement sensitivity.

[0008]

According to the present invention, the above object is to provide a two-terminal semiconductor device 10 and a metal oxide deposited on the surface of the device, as shown in the partially broken perspective view of FIG. It is achieved by the hot-wire semiconductor gas sensor 1 characterized by comprising a semiconductor 12.

The two-terminal semiconductor device 10 is a PN
A junction diode, a PIN diode, a Zener diode, a light emitting diode, a varistor, or a surge absorbing element can be exemplified. However, the present invention is not limited to these, and any two-terminal semiconductor device having a rectifying function can be used.
The surge absorbing element means an element that absorbs a surge, and is disclosed in, for example, JP-A-55-128283, JP-A-56-168379, and JP-A-55-146692.
Publication No. 55-163182, Japanese Utility Model Publication No. 55-161382
No. 161383.

The metal oxide semiconductor 12 to be used may be appropriately selected according to the type of gas to be detected. Examples of the metal oxide semiconductor 12 include SnO ₂ , ZnO, α
_{-Fe 2 O 3, γ-Fe} 2 O 3, In 2 O 3, or WO ₃ may be used. The metal oxide semiconductor may contain Pd, Pt, Au, Ag or the like as a sensitizer.

To attach the metal oxide semiconductor 12 to the surface of the two-terminal semiconductor device 10, the metal oxide semiconductor is molded into a predetermined shape on the surface of the two-terminal semiconductor device, and then the metal oxide semiconductor is sintered and fired. By sputtering a metal oxide semiconductor on the surface of a two-terminal semiconductor device,
This can be performed by vapor deposition, and further by printing and applying a metal oxide semiconductor on the surface of the two-terminal semiconductor device.

[0012]

In the hot-wire semiconductor gas sensor 1 of the present invention, as shown in FIG. 2, the two-terminal semiconductor device 10 and the metal oxide semiconductor 12 are connected in parallel. Usually, the hot wire semiconductor gas sensor 1 of the present invention is used at a high temperature of 300 ° C. or higher. In the prior art, a heater is used to heat the gas sensor. However, in the present invention, the switching action of the two-terminal semiconductor device is applied to cause the two-terminal semiconductor device to function as a heater. For example, the operation of the hot-wire semiconductor gas sensor 1 of the present invention will be described below, taking the case of using a junction diode as a two-terminal semiconductor device as an example.

Generally, the junction type diode has diode characteristics as shown in FIG. A forward voltage V _F is applied to the two-terminal semiconductor device in the forward direction. When the forward applied voltage V _F is equal to or higher than the voltage V _{TH at} which the forward current starts flowing,
The two-terminal semiconductor device becomes conductive and a current I _F flows, and the two-terminal semiconductor device generates heat (hereinafter, this state is referred to as a heating mode). In addition, V
_The heating mode is also activated when a reverse voltage less than _TH 'is applied. The heat generated by the two-terminal semiconductor device can heat the metal oxide semiconductor to a constant temperature. On the other hand, when a voltage of V _TH ′ or more and V _TH or less is applied to the two-terminal semiconductor device, no current flows in the two-terminal semiconductor device and the semiconductor device becomes non-conductive. That is, the two-terminal semiconductor device, which is connected in parallel with the metal oxide semiconductor (sensor element) and acts as a heater, is electrically separated from the metal oxide semiconductor. When the electric resistance value of the gas sensor is measured in this state, the electric resistance value of the metal oxide semiconductor is substantially measured (hereinafter, this state is referred to as a measurement mode), and the gas concentration should be measured with high accuracy. You can Therefore, the gas concentration can be measured with higher accuracy than the conventional hot-wire semiconductor gas sensor.

In summary, in order to switch the heating mode and the measurement mode using the two-terminal semiconductor device,
In the heating mode, a forward voltage of a certain voltage or more is applied to the diode or a reverse voltage of a certain voltage or less is applied to make the two-terminal semiconductor device conductive. In the measurement mode, a forward voltage of a certain voltage or less is applied. The two-terminal semiconductor device may be brought into a non-conducting state by applying a reverse voltage of a certain voltage or more.

It is preferable to alternately switch between the heating mode and the measurement mode. The time from one heating mode to the next heating mode is preferably 10 μsec to 100 msec. The ratio of time in heating mode to time in measurement mode is
It is preferably 6:94 to 94: 4.

By switching the heating mode and the measurement mode in a short time, the power consumption of the gas sensor can be reduced as compared with the sintered body element semiconductor gas sensor. For example, when a forward voltage of 1.2 V is applied to the hot wire semiconductor gas sensor, a forward current I _F of about 1.5 A flows in the forward direction. At this time, the power consumption of the gas sensor is about 1800 mW. If the time between the heating mode and the measurement mode is set to, for example, 1: 1, the average power consumption becomes about 900 mW, and the power consumption can be reduced. Furthermore, the size of the gas sensor is approximately the same as or slightly larger than the size of the two-terminal semiconductor device, and can be made smaller than the sintered body element semiconductor gas sensor.

[0017]

EXAMPLE A hot-wire semiconductor gas sensor 1 shown in a partially cutaway perspective view in FIG. 1 was produced. 2-terminal semiconductor device 1
A Zener diode was used as 0. As the metal oxide semiconductor 12, SnO ₂ containing 1% by weight of Pt was used. In the hot-wire semiconductor gas sensor 1, lead wires 14 are bonded to both ends of the two-terminal semiconductor device 10, and then the metal oxide semiconductor 12 is formed so as to cover the surface of the two-terminal semiconductor device 10 and also a part of the lead wire 14.
Was molded into a predetermined shape, and then the metal oxide semiconductor 12 was fired to manufacture.

In the steady state of being heated to about 300 ° C., the resistance value RS between the terminals of the metal oxide semiconductor 12 is about 5.5 kΩ when there is no gas adsorption. And 10
When 0 ppm of ethanol gas was adsorbed on the hot-wire semiconductor gas sensor 1, the resistance value between the terminals of the metal oxide semiconductor 12 was reduced to about 1.8 kΩ. Since the decrease in resistance value is proportional to the gas concentration, the gas concentration can be measured. Two-terminal semiconductor device 10 in conductive state, that is, in heating mode
The resistance value between terminals is several tens Ω, and the resistance value RDR between terminals in the non-conducting state, that is, the measurement mode is several hundreds kΩ or more. Two-terminal semiconductor device 10 and metal oxide semiconductor 12
Are connected in parallel as shown in FIG. Therefore,
2-terminal semiconductor device 10 is in non-conduction state (measurement mode)
In this case, R _DR >> R _S , and the resistance value R of the hot-wire semiconductor gas sensor 1 becomes substantially equal to the resistance value of the metal oxide semiconductor 12. That is,
In the measurement mode, the two-terminal semiconductor device 10
It can be considered that the metal oxide semiconductor 12 is electrically separated from the metal oxide semiconductor 12. The heating mode time was 20 μsec and the measurement mode time was 65 μsec. Also,
The voltage applied to the gas sensor 1 in the heating mode is set to 7.
It was set to 5V. At this time, the current flowing through the gas sensor 1 was 0.15A. Further, the voltage applied to the gas sensor 1 in the measurement mode was 3V. At this time, the current flowing through the gas sensor 1 was 150 μA. The hot-wire semiconductor gas sensor of the present invention is small, and since the heating mode and the measurement mode are switched in an extremely short time, the temperature of the gas sensor does not drop even in the measurement mode. Therefore, the gas concentration can be measured with high accuracy.

FIG. 4 shows a drive circuit for the hot-wire semiconductor gas sensor 1 of the present invention. A Zener diode is used as the two-terminal semiconductor device 10. Incidentally, not only a DC power supply but also an AC power supply can be used for measuring the resistance value of the metal oxide semiconductor 12 as the sensor element of the hot-wire semiconductor gas sensor.

In FIG. 1, the lead wire 14 is provided on the hot-wire semiconductor gas sensor 1, but the hot-wire semiconductor gas sensor 1 may be leadless.

[0021]

The hot wire semiconductor type gas sensor of the present invention is
Miniaturization can be achieved by using a two-terminal semiconductor device. Moreover, since the two-terminal semiconductor device functions as a heater in the heating mode and is electrically separated from the metal oxide semiconductor that is the sensor element in the measurement mode, the power consumption of the gas sensor can be reduced and the power source of the gas sensor can be reduced. A battery can be used, and a portable gas concentration measuring device can be manufactured. Also,
The gas concentration can be measured with high accuracy as compared with the conventional hot-wire semiconductor gas sensor.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of a hot-wire semiconductor gas sensor according to the present invention.

FIG. 2 is an electric circuit diagram of the gas sensor of the present invention shown in FIG.

FIG. 3 is a diagram showing an example of diode characteristics of a two-terminal semiconductor device.

FIG. 4 is a circuit diagram illustrating a drive circuit of the gas sensor of the present invention.

FIG. 5 is an electric circuit diagram of a conventional hot-wire semiconductor gas sensor.

FIG. 6A is a perspective view showing a conventional sintered body element semiconductor gas sensor, and FIG. 6B is an electric circuit diagram thereof.

[Explanation of symbols]

1 Hot wire semiconductor gas sensor 10 2-terminal semiconductor device 12 Metal oxide semiconductors 14 Lead wire 20 sensor elements 22 Noble metal wire coil 24, 24 'precious metal wire electrode

Claims (2)

[Claims] 1. A hot-wire semiconductor gas sensor, comprising a two-terminal semiconductor device and a metal oxide semiconductor attached to the surface of the device. 2. The hot-wire semiconductor gas sensor according to claim 1, wherein the two-terminal semiconductor device comprises a PN junction diode, a PIN diode, a Zener diode, a light emitting diode, a varistor, or a surge absorbing element.