The present invention relates generally to a gas sensor and more particularly to a gas sensor operated at room temperature.
One conventional technique for the gas sensor utilizes the MIS (metal insulator semiconductor) switch diode structure. K. Kawamura and T. Yamamoto have prepared the MIS switch diode as a high sensitivity hydrogen gas sensor with catalytic metal Pd as the cathode electrode (i.e., with a Pd/SiO.sub.2 /n (Si)/p+(Si) structure). Due to a slow catalytic reaction under low temperature, the catalytic MIS sensor has a poor response time and poor sensitivity during room-temperature operation. Thus it is better to be used at an elevated temperature, i.e., above 100.degree. C. The catalytic MIS sensor under high temperature operation, however may be hazardous upon contact with a flammable gas such as hydrogen or ethanol. Furthermore, this catalytic MIS sensor is only sensitive to hydrogen.
As shown in FIG. 1, the structure of the conventional resistive gas sensor includes an SnO.sub.2 layer 2, an epitaxial silicon substrate 1, a palladium electrode 3, and an aluminum electrode 4. Because under an elevated temperature, palladium is an excellent catalytic metal, and also because the conducting electrons resulting from the surface defect of the SnO.sub.2 layer Z play an important role in sensing some poisonous or flammable gas, the palladium electrodes 3 can permit the poisonous or flammable gas to be absorbed by the SnO.sub.2 layer 2 whose conductivity will thus be changed. This absorption can in turn be used to locate the existence and the amount of the poisonous or flammable gas. Due to consideration of the sensitivity of the conductivity of SnO.sub.2 layer 2 in which the sensitivity S=(G-G.sub.0)/(G.sub.0). In this case Go represents a conductivity of SnO.sub.2 layer 2 in air. Therefore G, a conductivity of SnO.sub.2 layer 2 can absorb the poisonous or flammable gas. The resistance of SnO.sub.2 layer 2 is to be relatively high (in kilo ohm order). With this in mind, the operation current of the gas sensor is too small to drive an alarm or a security system.
The conventional resistive gas sensor in summary has the following disadvantages:
1) Because the resistance of SnO.sub.2 layer 2 is relatively high, the sensitivity thereof cannot meet demand and its gas sensing effect is not satisfactory.
2) Because palladium electrode 3 operates only under a relatively high temperature (e.g., over 100.degree. C.), it is very dangerous when the gas to be sensed is flammable.
3) Because the operation current is relatively small, an amplifying system is a must and the cost is therefore relatively higher.
4) The catalytic gas sensor has a poor response time. There is a time delay that occurs in the interval after the gas sensor detects the poisonous or flammable gas and before the alarm system sounds, which is normally hazardous.
It is therefore left to the applicant to deal with this unsatisfactory situation described above.