– A new type of NO2 gas sensor has been made using the Floating Gate Field Effect Transistor (FG-FET) sensor system. 200 nm ZnO films were deposited on Si/Ti/Pt electrodes, which are mounted on FG-FET chips. SEM and EDX characterization methods were employed to study the surface of these films. The change in the work function of the film due to their interaction with NO2 has been measured at various temperatures and relative humidities. The sensor exhibits high sensitivity and selectivity to 1-20 ppm NO2 at a temperature range between room temperature and 165°C. With t50~10 s, the response time is quite fast. Index Term-Gas Sensor, Floating Gate Field Effect Transistor (FG-FET), Zinc Oxide, Nitrogen Dioxide.

Semiconductor gas sensor (10) on the basis of a field effect transistor, - having a separated by a gap of a channel region (50) of gas-sensitive control electrode (100) and is designed as a suspended gate FET (SGFET), or - the control electrode (100) is arranged as a first plate of a capacitor with a gap and a second plate (290) of the capacitor having a gate (270) formed as the Capacitve Controlled field effect transistor (CCFET) is connected, and the control electrode (100), a semiconductor base layer (130) or conductive metallic support layer with an overlying primer layer (120) and one on the adhesive layer (120) overlying the gas-sensitive layer (110), and - the surface of the gas-sensitive layer (110) faces the channel region (50) or the second plate (290), characterized, in that the control electrode (100) as the gas-sensitive layer (110) comprises a platinum / gold alloy with a gold content in a range of 1% to 20% and a polymer layer having a thickness below 100 nm is formed on the surface of the platinum / gold alloy, and the gap is filled with an oxygen free gas mixture or a gas mixture with an oxygen content below 0.1%.

Looking at the literature about chemical sensors, it is evident that numerous sensor effects are reported but only a very few sensor concepts remain, which are suitable for industrial applications. This is in contrast to the measurement of physical parameters such as pressure, temperature, acceleration etc. There are several reasons for this discrepancy. First of all, for chemical reactions usually there exist much more cross correlations, which have to be considered in order to determine a correct concentration of chemical species, and secondly, environmental influences cannot be neglected at all. Therefore, it becomes extremely difficult to fulfill all the requirements, which have been listed in the introduction of this paper. One interesting and successful gas sensor concept is based on the measurement of surface work function changes due to chemical reactions. Besides a Kelvin probe with a vibrating capacitance, field effect transistors (FET) are well suited to act as transducers for the determination of the corresponding potential variations. This chapter reviews the detection mechanisms, which lead to work function changes and gives an overview of the various transducer concepts. Beginning with the so-called Lundström FET, the historical development all the way to the hybrid mounted “floating gate FET (FG-FET)” is presented. This latest concept is extremely flexible and depending on the chemical-sensitive layer, it can be used for the detection of a large variety of gases. As an example the development of a hydrogen sensor for future automotive application is presented in more detail. Using platinum as chemical-sensitive layer it is shown that under harsh environmental conditions, it is not sufficient to consider exclusively the reaction of hydrogen with the platinum, but rather, it is necessary to also take into account oxygen, which is always present in air. As a result, a characteristic catalytic ignition point, i.e., the adsorbed hydrogen atoms are consumed by a water-forming reaction, occurs at around 60°C. Only if the complete reaction scheme is considered, it is possible to understand the complex and partially intriguing transducer signals. The development of two-layer systems for the chemical-sensitive layer solves these problems by using a phase transition that comes along with the catalytic ignition and allows a stable sensor operation in the required temperature regime between −40°C and +120°C. Based on the theoretical modeling the temperature-dependent phase transition has been evaluated in order to measure hydrogen concentration up to 4% with high accuracy and to fulfill the automotive requirements. Finally, in the last section, a new GasFET concept is presented, which extends the operating regime to temperatures as high as 400°C. In the future, this allows the incorporation of a variety of new gas-sensing materials, which need temperatures above 200°C in order to show a decent adsorption–desorption equilibrium. Altogether, it has been proven that GasFETs are a very promising candidate for future industrial applications.

Ultra thin titanium films in the range of a few nanometers have been deposited on monocrystalline lithiumtantalate (LiTaO3) followed by deposition of 400nm pure aluminum (Al). Texture measurements by means of electron backscatter diffraction show that the thickness of the intermediate titanium (Ti) layer significantly influences texture and grain structure of the overlying Al film. Increasing the thickness of the Ti layer from 0nm to 20nm leads to a change of aluminums texture from unoriented polycrystalline over highly oriented in single direction to highly oriented in twin structure.

A new concept for high temperature gas detection with a floating gate field effect transistor (FG-FET) will be presented. The function of the FG-FET is based on measuring work function changes due to an adsorption and desorption process of gas molecules on a sensing film. All existing concepts are working in a temperature range from room temperature up to 200degC. Higher temperature leads to significant leakage current in the transducer electronics and consequently to device breakdown. The new concept has two key benefits. Firstly, a full isolation achieved by using SOI-substrates and secondly a vertical MOSFET with high doping concentrations in the channel region. Both improvements lead to an extension of the temperature range up to 350degC. This increases the range of layers for chemical sensitive gasdetection and allows the operation in hot ambiance.

A new hydrogen sensor based on a heterogeneous polymer-platinum layer is presented. The sensor is based on the well-known floating gate field effect transistor (FG-FET [1]), which is shown in fig. 1. This type of sensor is capable of detecting various gas species [1,2,3] by measuring the work function change of a well chosen sensitive layer on which the target gas molecules are adsorbed. In the past platinum [1] or palladium [4] films as well as some heterogeneous metal oxide - platinum layers [5] have been used as sensitive materials for hydrogen detection. The main insufficiency of all these sensors is the low upper detection limit of about 2 % of hydrogen and the instabilities of the baseline in combination with its analogous detection behavior. To overcome these deficits we developed a new heterogeneous polymer - platinum system acting as sensitive layer. Using this new combination of materials, the FG-FET response to different concentrations is proportional only below a certain limit. Above this limit, the signal immediately steps to a maximum value. As this switching point is tunable, a new operation mode was developed, allowing precise hydrogen concentration measurements up to concentrations of at least 4 %.

A new type of hydrogen sensor based on a floating gate field effect transistor (FG-FET) with a n-octadecyltrichlorsilane (ODTS) modified platinum layer of 20 nm in thickness as a gas sensitive layer is presented. The modifications stabilize the sensor signal even at temperatures up to 125degC. The surface of polycrystalline platinum is well known to act as sensitive layer for hydrogen detection in a FG -FET at room temperature. In the presence of hydrogen containing air the work function of a platinum surface is reduced up to 0.5 eV by the adsorption of atomic hydrogen. Unfortunately at temperatures above 60degC a high coverage of atomic oxygen occurs at the platinum surface. This raises the work function again, immediately after the hydrogen exposure so that the hydrogen concentration is no longer exactly detectable. With the deposition of a thin layer of ODTS, we were able to modify the platinum surface preventing it from being covered by oxygen during hydrogen exposure. A detailed model of the reactions at the platinum surface, which leads to the shift in work function, is given. Changes of the reaction paths by the modification are explained.

For safety reasons, while handling fuel cells, hydrogen concentrations of 0.1 -3% and above need to be detected. Low power hydrogen sensors, based on a Field Effect Transistor (FET), have been in use for about 25 years. In the past platinum and palladium were often used as gas sensitive layers. Unfortunately in the required concentration range, the Pt based sensors have a poor selectivity at room temperature and were not stable at operating temperatures above 60degC. To solve this problem Pt with a porous tin oxide (SnO2) top layer is used as a chemically sensitive electrode in a Floating Gate Field Effect Transistor (FG -FET). The results show that the SnO2 film on Pt stabilizes the sensor signal response between room temperature and 135degC. Also the sensor response time with t50 < 10s is quite fast and the cross sensitivity to other gases compared to pure Pt is reduced.