A FET-ammonia sensor operating at room temperature is reported in this paper. The sensor employs a thin film of semiconducting Y:Ba:Cu:O (YBCO) compound, commonly known as 1-2-3 high temperature superconducting cuprate, as its sensing element. We observed that this material is highly and selectively sensitive to ammonia at and below room temperature. The measurements indicate that the non-amplified signal of the sensor is more than 10 mV for 5 ppm of ammonia. A typical rise time of 24 s and fall time of 250 s is measured at room temperature. The measurements on temperature dependence of sensitivity evince that the sensor signal increases if the operating temperature of the sensor is decreased. The sensor is selectively sensitive to ammonia only and exhibits a non-sensor like behaviour to hydrogen and hydrocarbon based gases. Although the exact mechanisms of gas-sensing properties of cuprates are not known, nevertheless, we tried to explain the observed sensor response to ammonia on the basis of physisorption. The demonstrative results reported here suggest that a room temperature FET ammonia sensor can be realized using semiconducting YBCO as a sensing material for the development of silicon-based integrated smart sensors.

s ili con based FET -sensors. However, an appropriate passivation is needed for their incorporation in s ilicon-CMOS technology. Utilization of thin gold films for thi s purpose is reported he re for th e gas sensing properties of gold-coated YBCO films. RF magnetron sputtering is used to deposit YBCO films on oxidized si li con substrates. These films are then coated with a thin gold layer and used to construct discrete structures of capacitively controlled field effect trans istor (CCFET) sensors to investigate their gas detection properties. The measurement technique and sensor response are discussed in detail. The result s evince th at gold-coated YBCO film s are sensitive to ammonia at room temperature and can be employed as sensitive layer in integrated silicon-FET sensors for detection of ammonia.

Cobalt oxide has been reported as a new material for room temperature FET gas sensor. Thin films of cobalt oxide have been prepared by DC magnetron sputtering on oxidized silicon substrates and used as gas sensitive layers in a capacitively controlled field effect transistor (CCFET) structures. CCFET is a MOSFET with an extended gate electrode. Gas sensing behaviour of these films has been investigated at room temperature for different gases of varying concentrations, ranging from 10 ppm to 10,000 ppm. Gases such as CH4, C3H8, NH3, CO, NO and H-2 have been used to study the sensor performance. Since the measured sensor signal is not amplified, it is a direct measure of sensitivity of the film to a gas to which it is exposed. The CCFET structure, preparation of sensitive films and measurements are described in this paper. The results indicate that cobalt oxide is selectively sensitive to ammonia and hydrocarbon gases only with a little or negligible response to other gases. Signals of 5 mV and 30 mV for 10 ppm of the hydrocarbons and ammonia, respectively have been observed.

An advanced recombination model based on the Shockley-Read-Hall-statistics with full trap dynamics is used for the simulation of irradiated power diodes. The model makes use of the rate equations which also take into account the dynamic effects in the space charge region of the power devices. The high-level lifetime calculated from DLTS (Deep Level Transient Spectroscopy) data does not agree well with the lifetime determined by lifetime measurements. An agreement is achieved by means of a temperature dependent capture coefficient of the dominating recombination center E(90K), calculated from the lifetime measurement results. Simulations and measurements are done to determine the dependencies of reverse recovery current maximum and reverse recovery charge on temperature. By use of the calculated capture coefficient, a sufficient agreement between simulation and measurement is achieved.

The responses of MIS gas sensors with Pd gate to the action of gas mixtures are experimentally studied. Computer modelling of the processes was undertaken, to disclose the peculiarities of co-adsorption of molecules on the real surface of Pd films.': To understand the processes responsible for aging the sensors with Pd films aged by different methods were considered. The effect of the promoted dissociative adsorption of hydrogen on aged surface in moist ambient has been-revealed. The pronounced selectivity of aged sensor with respect to hydrogen and moisture was also observed and explained. The modelling has allowed us to specify some scenarios for adsorption and surface reactions. In particular, the coexistence of both atomic and molecular transport through the aged Pd gate has been found. Besides, simultaneous two- and one-center H-2 room temperature adsorption on the external surface of the aged layer has been observed.

Room temperature gas sensitivity of Metal-Insulator-Silicon (MIS) structures with Pd and Pd/Cu composite metal layers have been compared. SiO2 and SiO2–Si3N4 dielectric films were used as insulator. The metal films were obtained by magnetron deposition in Ar plasma. Detailed investigations of surface morphology and chemical composition of the layers by SEM and AES depth profiling have been performed too. A shift of the flat band voltage (ΔVfb), obtained from the measurements of capacitance voltage characteristics was used as a response of a MIS structure on the supplying of gas pulses. Such gases as hydrogen, humidity, and air (oxygen) were tested by the sensors. The kinetics of the responses was studied experimentally and compared with the results of a computer simulation. To investigate the resistance of the MIS structures to electrode layer aging, they were subjected to an accelerated artificial aging by annealing in air at 100–250°C during 0.3–3 h. It was found that MIS sensors with Pd/Cu composite layer demonstrate higher sensitivity to hydrogen and higher resistance to aging than similar structures with Pd layer only. As a rule they also demonstrate quicker responses to the gas pulses. A model for explanation of the experimental results is proposed.

We have discovered an interesting property of high Tc cuprates, which are popularly known as high temperature superconductors, for gas sensing applications. Metallic films of YBCO and BSCCO compounds are examined for gas-sensing properties by incorporating them in a FET-sensor structure. These multi-component ceramic oxides exhibited room temperature gas detection capability in comparison to conventional metal-oxides, which are sensitive to gases only at elevated temperatures. The sensor responses to NH3, H2, CO, CO2, NO, NO2, and hydrocarbon gases are presented and discussed. The results evince that thin film of high Tc cuprates are highly sensitive to NH3 and NO2 at and around room temperature and less sensitive to other gases. These films, therefore, can be used to realize a room temperature low power FET-gas sensor.

We report a new FET sensor which can detect ammonia at room temperature. We discovered a new application of high T/sub c/ cuprates, commonly known as high temperature superconducting copper oxides. The gas sensing properties of thin films of these materials have been examined through work function measurements. Thin films of YBCO and BSCCO are prepared on oxidized silicon substrates. These are then used in a capacitively controlled field effect transistor. One plate of the capacitor is a gas sensitive film and the other one is a reference electrode which is connected to the FET gate electrode. The operating principle of this sensor is based on the change in work function of the sensitive film due to gas adsorption on its surface. This is measured as change in transistor gate voltage. Measurements show that the nonamplified sensor signal is more than 10 mV for only 5 ppm of ammonia. Typical rise time of 24 sec and fall time of 250 sec are measured at 18°C. The sensor is selectively sensitive to ammonia and shows little or no response to hydrogen and hydrocarbon based gases. Measurements at different temperatures reveal that the sensor performs optimally in the 15-25°C temperature range. Device sensitivity to carbon monoxide, carbon dioxide, hydrogen and hydrogen gases has been found to be negligibly low. Thus a new FET sensor capable of sensing ammonia with selectivity around room temperature is reported in this paper.

Models for gas chemosorption on metal-oxide layers and their implementation in a two-dimensional device simulation are the subject of this paper. First application examples and the simulation results like the work function change on the sensitive layer of a suspended-gate field-effect transistor and the current modulation in a ZnO layer are shown here. The chemosorption dependence on parameters such as the electrical field and sensitive layer doping is further discussed.

The implementation of models for the gas adsorption on metal-oxide-semiconductors in a two-dimensional device simulator is the subject of this paper. Further we give an application example for the simulation of the sensitive reactions and the electronic device in their exchange. For that we use a conductivity type gas sensor with a polycrystalline ZnO active layer.

The adsorption at the open insulator surface under the air gap of suspended-gate field-effect transistors (SGFETs), which leads to mobile charges, and their redistribution due to lateral electrical field strengths are two of the reasons for the threshold voltage instability of these gas sensors. A two-dimensional computer simulation has been carried out to determine the lateral electrical field in the device, especially in the gate insulator area. The evaluation of the modelling results has led to useful conclusions about the design optimization of the sensor and its operating conditions.

In this paper we consider models for some adsorption effects, the resulting change of the work function at metal-oxide-semiconductor sensitive layers of gas sensors and their implementation in a two dimensional device simulator, which solves the Poisson equation. An example for the application of this simulation program is given with a Suspended Gate Field Effect Transistor.