Field effect transducers for work function gas measurements: device improvements and comparison of performance

In recent years, a gas sensitive field effect transistor (FET) has been developed capable of detecting gas species in the ppm range by measuring the induced shift of work function DeltaPhi, at the surface of a sensitive film. Research and development ran in parallel: one direction was the evaluation and optimization of suitable sensitive films. Various classes of materials had been investigated: catalytic metals [Sens. Actuators B 80 (2001) 163; Sens. Actuators B 78 (2001) 138; Sens. Actuators B 80 (2001) 1741, metal compounds [Sens. Actuators B 85 (2002) 145; Sens. Actuators B 78 (2001) 69; Sens. Actuators B 49 (1998) 63; Sens. Actuators B 47 (1-3) (1998) 145; Sens. Actuators B 68 (1-3) (2000) 234; Sens. Actuators B 56 (1999) 65], hydrated salts [J. Vac. 55 (1999) 81; Sens. Actuators B 48 (1998) 2971, polymers, and other organic compounds [Thin Solid Films 132 (1996) 152; Polymer-oxide-silicon field effect transistor (POSFET) as sensor for gases and vapors, in: Proceedings of the Sixth IMCS, 1996, p. 179.]. High gas sensitivity S-G, high selectivity, high chemical stability, and good reversibility were the main criteria of quality. The second direction was design and optimization of the field effect device. The field effect transistor is used as a transducer which transforms the shift of work function DeltaPhi at the surface of the sensitive film into a corresponding electrical signal: a change of the drain-source current DeltaI(DS). A high transducer sensitivity S-T, high signal-to-noise ratio S/N, good signal stability and reduction of temperature and humidity influence were the main criteria in order to make the GasFET fit for applications with low power consumption at ambient and slightly increased temperatures up to 80 degreesC. The possibility to use the same type of transducer with different types of sensitive films makes this gas sensor flexible and versatile for a large range of applications. The scope of this article is to present the latest developments in transducer design. A new integrated device with a floating gate-designed and processed by Micronas GmbH will be presented and compared with the previous design of the hybrid suspended gate FET (HSGFET) [Sens. Actuators B 18-19 (1994) 632; Sens. Actuators B 78 (2001) 19; Sens. Actuators B 80 (3) (2001) 169]. A theory of operation will be given for both devices and the main influences to the transducers performance will be discussed. Special emphasis will be put on transducer sensitivity, signal-to-noise and temperature influence. (C) 2003 Elsevier B.V. All rights reserved.