J. Hendrikse

Theory and application of the material work function for chemical sensors based on the field effect principle

This paper gives first of all the definition of the work function of an electronic as well as an ionic conductor. FET-type gas sensors known from the literature are considered in view of this basic theory and the parameter of the work function which is responsible for the sensing properties is noticed. This appears not always to be possible for the various types of gas sensors, in this case the gas FET, the SGFET and the IGFET. In contrast to this ambiguity, the sensing parameters of the recently developed MOSFET, meant for application in an electrolyte, can clearly be identified. As an example an oxygen sensor based on the developed MOSFET is described.

A method of reducing oxygen induced drift in iridium oxide pH sensors

A novel way of reducing drift problems of the electrode potential of metal oxide pH sensors is presented. The method employs a FET-structure that uses the conducting metal oxide electrode as a gate contact. From the threshold voltage of this structure a signal can be derived which is independent of the electrode potential and can be used to compensate for the drift of it. In addition to the reduced drift, the compensation leads to an almost ideal nernstian response. First a theoretical explanation is given, which is then confirmed by measurement results.

THE EMOSFET AS AN OXYGEN SENSOR : CONSTANT CURRENT POTENTIOMETRY

In a previous paper, a novel type of potentiometric dissolved oxygen sensor was introduced. The transduction principle of the sensor is based on the modulation of the work function of an iridium oxide film by the ratio of IrIII/IrIV oxide in the film. This ratio depends on the oxygen concentration in the solution, so that the work function of the iridium oxide is a measure of the amount of dissolved oxygen. The work function changes are determined by using the iridium oxide film as the gate contact of a MOSFET. Because the threshold voltage of a MOSFET depends on the work function of the gate contact, it can be used as a sensor signal. In the present paper, a reducing current of constant magnitude is in addition externally applied to the iridium oxide film, so that an overpotential is generated. The influence of this overpotential on the sensor signal is studied and it is shown that the sensitivity towards oxygen increases compared to the equilibrium potential. Measurement results are shown and compared to the theory. The applied current leads to sensitivities to oxygen and pH that correspond well to the values that may be expected from the literature.

Gas-Dependent Field Effect Transistor with an Electrodeposited Conducting Polymer Gate Contact

Field effect transistor structures with a conducting polymer gate contact have been realized. An electrodeposition technique, suitable for a wide range of conducting polymers, was used. This is illustrated with the deposition of two different types of polypyrroles. The sensors show responses to organic vapors due to partial charge transfer between the sorbed vapor and the conducting polymer, and can be used for electronic nose applications. Due to the measuring principle, based on work function instead of resistance changes, this sensor is also believed to be useful for fundamental research on gas-polymer interactions. ©1999 The Electrochemical Society

The EMOSFET as a potentiometric transducer in an oxygen sensor

Oxygen concentrations have been measured using the so called ‘Clark cell’ for a long time. In this paper, first some drawbacks of the Clark cell are discussed, with special attention towards miniaturization of the sensor. Subsequently, the use of potentiometric sensors is discussed. In both the miniaturized Clark cell and in the potentiometric sensor a reference electrode is needed, which is hard to fabricate using thin film techniques. For this reason a novel way of carrying out potentiometric measurements, using a specially developed type of potentiometric oxygen sensor, is described. As the measurand the threshold voltage of a FET-structure having an iridium oxide gate is used, rather than the electrode potential. The potential advantages of such a sensor compared with the Clark cell are examined and the theoretical response of the sensor is derived. In addition some relevant experiments are described.

Characterization of the EMOSFET, a novel one-electrode chemical transducer for redox measurements

A sensor device consisting of a MOSFET with an iridium oxide gate contact and denoted an EMOSFET is presented. When the gate of this device is in contact with an electrolyte, the iridium oxide can take part in a redox reaction, enabling thermodynamic equilibrium between the electrons in the iridium oxide and protons in the solution. The chemical potential of the electrons in the bulk of the iridium oxide can be changed by oxidation or reduction of the material and is related closely to the iridium oxide work function. Since the threshold voltage, VT, of the MOSFET depends on the work function difference between the gate contact and the silicon bulk, it is influenced also by the redox reaction. So if this sensor is connected to the appropriate amplifier circuit, the redox reaction induces changes in the output signal of the system due to changes in the threshold voltage, VT, of the MOSFET. It is shown on a theoretical basis how the output signal is influenced by the redox reaction and this description of the device is supported by measurement results.

A drift free nernstian iridium oxide pH sensor

A novel way of eliminating drift problems in metal oxide pH sensors is presented. The method employs a FET-structure under the electrode that uses the metal oxide as a gate contact. In addition to the enhanced drift properties, the new sensor has an almost ideal nernstian response. First a theoretical explanation is given, which is then confirmed by measurements.

A One-Spot Electrochemical Transducer for Use in μTAS

A novel one spot electrochemical transducer consisting of a FET-structure having a redox polymer as a gate contact is presented. The usefulness of the combination of this transducer with so called wired enzyme electrodes is illustrated and the advantage of one spot electrochemical measurements in μTAS is demonstrated.