Gas sensors have very long history in our modern society. Semiconductor gas sensors using metal oxides were found by Prof. Seiyama and Dr. Taguchi, individually, in 1962. Prof. Seiyama researched ZnO sensor for the purpose of detector in gas chromatography. On the other hand, Dr. Taguchi investigated SnO2 sensor for gas leakage. In the late 1980’s, detection of the environmental related gas was tried for the importance, so detection limitation was decreased to sub-ppm from ppm level. In the late 1990’s, the sensor devices for compact and low power consumption were investigated. After that, the sensitivity is going to enter into ppb-level. In addition, the device size is going to be reduced in hundreds of micro meters. The applications of gas sensor to new field are tried widely and actively by using a characteristic to be compact and cheap. For these researches and developments, materials design are very important. In this lecture, I would like to present the update of materials design as well as its prospects for the future.
In the oxide semiconductor gas sensors, we have reported important three functions i.e. receptor function, transducer function and utility factor, as shown in Fig. 1. Receptor function concerns the ability of the oxide surface to interact with the target gas. In addition, when the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen. Transducer function concerns that the electron transport through the contact can thus be achieved by migration or tunneling of the surface electrons, indifferent to the bulk electrons inside. The device resistance is then inversely proportional to the surface density of electrons. For the utility factor, the target gas molecules diffuse the inside of a sensing body while reacting with the oxide surface. Based on the above, in order to establish high-performance gas sensors, the three important design factors should be combined as shown in Fig. 2.
Fig. 1. Important three factors for semiconductor gas sensors.
Fig. 2. Integration of three factors for high performance gas sensor.
Usual gas sensors are operated at high temperature, so the sensors system need much electric power. To overcome the problem, MEMS platform (Fig. 3) is proposed. The MEMS element is very useful for semiconductor gas sensors. We reported that the above materials design is very useful for an MEMS-type sensor. When such a small sensor elements are developed, the communalization of the gas sensing data through the network can be realized. For example, we can obtain much information by integrating a position sensor, a temperature sensor, a humidity sensor, plural gas sensors on one-tip. I think that such integrated sensors become the important device as an IoT society.
Fig. 3. MEMS gas sensor element.