There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

/pdf/new-materials-for-micro-scale-sensors-and-actuators-an-21qnaonn0l.pdf

New materials for micro-scale sensors and actuators An engineering review

Principle Component Analysis

There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H2. Of particular interest is detection of ethylene (C2H4), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.

https://iopscience.iop.org/article/10.1088/0953-8984/16/29/R02/pdf

GaN-based diodes and transistors for chemical, gas, biological and pressure sensing

This paper presents a comparative study of the gas-sensing behavior of MoO3 thin films prepared by ion beam deposition and sol–gel techniques, respectively. The sensing response of these films to ammonia in the presence of interfering gases (such as NO2) is assessed. The microstructural characteristics of the sensing materials prepared by these two techniques are determined using electron microscopy and microanalysis techniques. MoO3 is shown to be highly sensitive to ammonia, and the sensitivity depends on processing. The sensing properties of MoO3 thin films and the origin of the p–n-type transition occurring in sol–gel processed films are correlated with the type of the MoO3 polymorph(s) used for sensing.

Comparison of sol–gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection

Field effect chemical sensors, utilising silicon carbide as semiconductor, can be operated at high temperature and in rough environments. Gas sensitive field effect transistors, MISiCFET, are now d ...

High temperature catalytic metal field effect transistors for industrial applications

The development and field-testing of high-temperature sensors based on silicon carbide devices have shown promising results in several application areas. Silicon carbide based field-effect sensors can be operated over a large temperature range, 100-600 °C, and since silicon carbide is a chemically very inert material these sensors can be used in environments like exhaust gases and flue gases from boilers. The sensors respond to reducing gases like hydrogen, hydrocarbons and carbon monoxide. The use of different temperatures, different catalytic metals and different structures of the gate metal gives selectivity to different gases and arrays of sensors can be used to identify and monitor several components in gas mixtures. MOSFET sensors based on SiC combine the advantage of simple circuitry with a thicker insulator, which increases the long term stability of the devices. In this paper we describe silicon carbide MOSFET sensors and their performance and give examples of industrial applications such as monitoring of car exhausts and flue gases. Chemometric methods have been used for the evaluation of the data.

SiC Based Field Effect Gas Sensors for Industrial Applications

Metal insulator silicon carbide field-effect transistor sensors, metal-oxide sensors, and a linear Lambda sensor in an electronic nose was used to measure on-line in hot flue gases from a boiler. Flue gas from a 100-MW pellets-fuelled boiler has been used to feed the experimental setup. Several reference instruments, which measure the flue gases in parallel to the sensor array, are connected to the electronic nose. Data was collected during six weeks and then evaluated. Using principal component analysis as the data evaluation method, different operating modes for the boiler have been identified in the data set. The different modes could be described in terms of high or low O/sub 2/ and CO concentration. Furthermore, we have shown that it seems possible to use a sensor array to determine the operating mode of the boiler and, by partial least-squares models, measure the CO concentration when the boiler operates in its optimum mode.

Evaluation of on-line flue gas measurements by MISiCFET and metal-oxide sensors in boilers

Selective catalytic reduction (SCR) is a method in which ammonia reacts with nitric oxides in a catalytic converter to form water and nitrogen. We show that catalytic Metal Insulator Silicon Carbid ...

Schottky diodes with thin catalytic gate metals for potential use as ammonia sensors for exhaust gases

We report the results of the effect of three different stacks of ohmic contacts such as TiW (180 nm)/Ti (30 nm)/Pt (300 nm), Ni (100 nm)/TaSi x (200 nm)/Pt (400 nm), and TaSi x (200 nm)/Pt (400 nm) on a highly doped n-type 4H-silicon carbide for high temperature measurements in oxidizing ambient. The ohmic contacts in the transmission line method (TLM) structures were cut to 2 × 2 mm 2 chips, and glued on heater, which has a built-in Pt-100 element, for the temperature measurement. Finally the contacts on the heater were mounted on a 16-pin socket, which was placed in an Al-block with a gas flow channel. For this measurement, we used 20% O 2 in N 2 flowing at a rate of 80 ml/min at temperatures of 500 °C and 600 °C. The long-term reliability test of each ohmic contact in an oxidizing environment up to 520 h showed that TiW/Ti/Pt had the best stability and the lowest contact resistivity, while other contacts (Ni/TaSi x /Pt and TaSi x /Pt) had a severe degradation of contacts due to the oxidation. Therefore, we believe that our TiW based metallization schemes can be applied in harsh environment such as the automobile and aerospace exhaust systems.

Lars Unéus

Papers

High temperature catalytic metal field effect transistors for industrial applications

SiC Based Field Effect Gas Sensors for Industrial Applications

Evaluation of on-line flue gas measurements by MISiCFET and metal-oxide sensors in boilers

Schottky diodes with thin catalytic gate metals for potential use as ammonia sensors for exhaust gases

Comparison study of ohmic contacts to 4H-silicon carbide in oxidizing ambient for harsh environment gas sensor applications