Suspended Gate Field Effect Transistor Research Articles

Iridium Oxide as Low Temperature<tex>$hbox NO_2$</tex>-Sensitive Material for Work Function-Based Gas Sensors

This paper presents the results on work function-based NO/sub 2/-sensing properties of iridium-oxide thin films at 130/spl deg/C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO/sub 2/, H/sub 2/, CO, Cl/sub 2/, and NH/sub 3/ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by /spl sim/100 mV on exposure to 5-ppm NO/sub 2/ (German safety limit). Cross sensitivity to other gases (except NH/sub 3/) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO/sub 2/ sensor.

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 ΔΦ, 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) 174], 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) 297], 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 SG, 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 ΔΦ at the surface of the sensitive film into a corresponding electrical signal: a change of the drain–source current ΔIDS. A high transducer sensitivity ST, 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°C. 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.

A room temperature HSGFET ammonia sensor based on iridium oxide thin film

Gas-sensing properties of hybrid suspended gate FET (HSGFET) sensor containing iridium oxide as sensitive layer are reported in this paper. The sensor response to NH 3 and H 2 was measured. We obtained a non-amplified signal of 70 mV for 50 ppm of ammonia and no signal to hydrogen even for 10,000 ppm. Further, the gas-sensing properties of thin film of iridium oxide has been studied using Kelvin probe (KP) setup. Sensitivity of these films to gases like CO, H 2, SO 2, Cl 2, NO 2 under dry and humid conditions were examined by measuring contact potential difference (CPD) resulting from work function change of the film due to exposure to test gases. The results suggest that iridium oxide-based HSGFET is selectively sensitive to ammonia with negligible signal to all other gases. Iridium oxide films of 20 nm thickness used in this study were prepared by dc sputter deposition at 300 °C followed by sintering at 600 °C in oxygen. The films were characterized using Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and atomic force microscopy (AFM) to determine their composition, phase and surface morphology.

Low-power gas sensors based on work-function measurement in low-cost hybrid flip–chip technology

To implement low-power gas sensors with low component costs, the principle of work-function read out via a hybrid suspended gate FET (SGFET) is being pursued, whereby a freely selectable sensor film undergoes a reversible work-function change corresponding to the build-up of a potential difference on the surface in response to gas adsorption/reaction. This is read out via an ISFET structure. An innovative design which allows cheap manufacturing will be described for the principle that has already been successfully demonstrated. The starting point of the design is a ceramic Al 2O 3 substrate coated with conductor patterns and sensitive materials onto which the FET is mounted in flip–chip technology. By means of the freely selectable sensor film and its preparation method, a wide range of applications can be opened up.

Gold and platinum as ozone sensitive layer in work-function gas sensors

Hybrid suspended gate FET (HSGFET) devices with a sensitive layer of platinum or gold were proven to be well-suited as reproducible ozone detection from the low ppb region up to 500 ppb and higher. Operating at 130°C, gold shows a nearly linear dependence on ozone concentration with high signals and a small humidity influence. Platinum at 130°C convinces with a fast response time and low cross sensitivities to other gases.

Low temperature hydrogen detection at high concentrations: comparison of platinum and iridium

A comparison of platinum and iridium in work-function measurements with regard to hydrogen sensitivity is presented. The measurements were performed by means of a Kelvin probe and an ISFET-based measurement setup. They are concentrated towards high concentrations of hydrogen and low temperature in order to develop room temperature operated FET-based sensors which can withstand H 2-concentrations of some percent. With both materials the detection of H 2 up to 2% is possible. However, iridium shows very-slow reactions with adsorption as well as with desorption times of some hours. In contrast, the response times of platinum could be determined to be below 10 s in the case of 0.1% H 2. Therefore, platinum is the most promising candidate with air-gap configurations of FET like hybrid suspended gate FET (HSGFET).

Enhanced room temperature gas sensing with metal oxides by means of the electroadsorptive effect in hybrid suspended gate FET

A thin semiconducting PtO 2 film is exposed to several gases at room temperature. This exposure causes a work function change at the surface of the film due to adsorption of the molecules. It is found that there is a strong dependence of the adsorptivity, i.e., the amount of work function change per unit time, on the magnitude of an electrical field that is applied perpendicular to the film surface. This induced adsorptivity change is known as the electroadsorptive effect. In order to modulate the adsorptivity significantly, the electrical field strength must exceed 10 4 V/cm. This requirement can be achieved by using hybrid suspended gate FET (HSGFET) with an air gap height smaller than 1 μm. Thus metal oxides may be used as sensitive layers even at room temperature.

In situ control of the electrochemical gap height modification of a suspended gate field-effect transistor by capacitance–voltage measurement technique

Abstract A technique is described to monitor in situ the variation of the gap height, a mechanical parameter of a microelectronic integrated circuit known as suspended gate field-effect transistor (SGFET), by electrochemical deposition of a metal layer on the gate electrode. During the deposition process, the gap height was measured simultaneously starting from 800 nm down to 200 nm using the capacitance–voltage (CV) measurement technique. As a result of this modification, the threshold voltage has been shifted from over 10 V down to 1 V leading to a decrease of the noise level of the SGFET as well. The modified chips were electrochemically coated with a polymer layer and used as a gas sensor for the lower ppm range.

Room temperature ozone sensing with KI layers integrated in HSGFET gas sensors

Hybrid suspended gate FET (HSGFET) with a thin potassium iodide (KI) sensitive layer were proven to be well suited as a reproducible ozone sensor operating at room temperature with an excellent resolution in the low ppb region. The impact of humidity limits the applicability of the sensor to moderate ambient conditions.

Thin (NiO)1−x(Al2O3)x, Al doped and Al coated NiO layers for gas detection with HSGFET

Abstract Work function changes of (NiO)1−x(Al2O3)x, Al doped and Al coated NiO layers upon NO2, CO2, SO2, Cl2, CO, NH3 and H2 at 30°C and 130°C were measured by means of a Kelvin probe. Compared to the `pure' oxides NiO and Al2O3, a significant impact of the stoichiometry x, the Al dopant concentration and the Al surface modification not only on the sensitivity, but also on the selectivity and the response behaviour of the samples was found. The material screening was carried out with respect to the incorporation of the most sensitive and selective layers into hybrid suspended gate FET (HSGFET) devices. The sensor measurements were found to be in good agreement with those carried out by the Kelvin probe.

Two-dimensional numerical simulation of semiconductor gas sensors

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.

Suspended gate field‐effect transistor sensitive to gaseous hydrogen cyanide

AbstractThe optimization, characterization, and analytical performance of a suspended gate field‐effect transistor (SGFET), with a selective polyaniline/mercury layer, for monitoring low levels of hydrogen cyanide in air are described. Reversible and reproducible detection is reported.

Optimization of sputtered SnO 2 films as gas-sensitive layers for suspended-gate FETs

Tin oxide films have been prepared by reactive sputter deposition. Characterization with respect to stoichiometry and homogeneity has been carried out. Optimized films are incorporated as gas-sensitive layers in suspended-gate field-effect transistors. These sensors have been measured with respect to oxidizing and reducing gases.

Silicon condenser microphone with integrated field-effect transistor

A silicon condenser microphone is described, which works with an integrated field-effect transistor (FET). The gate of the transistor corresponds to the membrane of the microphone. Between the membrane and the gate oxide is a small air gap. The drain current of the transistor is controlled by the deflections of the membrane. The structure, which carries the FET and which is placed beyond the membrane, can have very small lateral dimensions. This results in small values of air-gap streaming losses and high air-gap compliances, thus yielding a good acoustic behaviour. The design of a silicon microphone with suspended-gate FET is described and experimental results of frequency response and noise are presented. The measured sensitivities are in the range 0.1–1 mV/Pa, which is about 15 dB lower than the calculated values. The reduction in sensitivity is caused by the silicon fabrication process of the microphones and can be eliminated. The frequency response is smooth up to 30 kHz. The noise measurement shows a 1/ ▪f slope, which is typical for the noise behaviour of the FET.

New suspended gate FET technology for physical deposition of chemically sensitive layers

Abstract The chemically sensitive layer that is located underneath the suspended gate metal of the field-effect transistor has until now been deposited exclusively by electrochemical means. A new method for the physical deposition of the sensitive layer within the suspended gate structure has been developed. By introducing the single-layer lift-off process to the fabrication sequence of the device, it is now possible to deposit selective materials before the suspended gate is made. Therefore general thin-film techniques can be used. This offers the advantage that materials of great interest, such as non-soluble metal oxides, can be deposited. Also all chips on a wafer can be processed at the same time.

Recognition of hydrogen and ammonia by modified gate metallization of the suspended-gate FET

Suspended-gate field-effect transistors (SGFETs) are investigated for a possible independent detection of hydrogen and ammonia in synthetic air. The suspended gate is modified by electrochemical deposition in order to obtain Pd and SnO x active layers. For SnO x -coated SGFETs, a sensitivity to NH 3 even at room temperature is observed without any response to H 2. The Pd system is stable at temperatures above 140°C and sensitive to H 2 as well as NH 3. The combination of both sensors allows the two gaseous species to be recognized independently.

Hydrogen response of palladium coated suspended gate field effect transistor

Abstract : A suspended metal gate field effect transistor was studied as a hydrogen sensor and the surface processes at an electrochemically deposited Pd layer on the gate were examined. It has been found that the time constant as well as the magnitude of the response depends on the operating conditions particularly on the presence of oxygen. If the device is tested in air the dynamic range spans logarithmically five decades of partial pressure of hydrogen.