FET sensors

Figure 3.21 — (A) Integrated FET with two hydrogen ion-sensitive FET elements. (B) Structure of enzyme-modified FET sensor S plastic card FET enzyme-modified FET chip lUM, immobilized urease membrane. (C) Flowthrough cell Bl fixed sensor cell block B2 movable sensor cell block SC flowthrough cell EC electrical connector RP silicone rubber sheet AMP amplifier. (Reproduced from [151] with permission of Elsevier Science Publishers).

Figure 2.4 The ammonia response versus temperature for an MISiC-FET sensor with (a) 25-nm Pt and (b) 60-nm Iras the gate contact metai. NHj concentration 12.5, 25, 50,100, 200, and 250 ppm in 10% Oj/Nj. (From [23]. 2003 iEEE. Reprinted with permission.)...

Figure 2.9 A schematic drawing of the design of the MISiC-FET sensor. Cate and source contacts are connected together. The design allows the application of a voltage on the substrate, as indicated in the drawing. The location of the intrinsic gate is indicated in the drawing. (From [98]. 2003 IEEE. Reprinted with permission.)...

Due to the results mentioned in Section 2.4.1 by Tobias et al. [110] and Gosh [6] et ah, the authors tested the response (not using the MGO equipment) of the MISiG-FET sensors at 500°G for different constant current levels. A current of 65 juA showed the same fast speed of response as for normal operation at 100 juA. For a constant current of 500 /t A, the MISiG-FET showed the same size and speed of response but also a slow drift of the baseline, which was not sensitive to a change between oxygen and hydrogen. 

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

The MISiC and MISiC-FET sensors have been tested in a variety of applications, both in simulated environments in the laboratory [24,116] and in real applications in the field [51, 76, 77,128,133,134]. The authors have identified several areas in automotive and industrial applications where the excellent properties of SiC can be exploited. This section will review applications in both car exhaust gases and flue gases from boilers. 

Chemometric evaluation methods can be applied to the signal from a single sensor by feeding the whole data set into an evaluation program [133,135]. Both principle component analysis (PCA) and partial least square (PLS) models were used to evaluate the data. These are chemometric methods that may be used for extracting information from a multivariate data set (e.g., from sensor arrays) [135]. The PCA analysis shows that the MISiC-FET sensor differentiates very well between different lambda values in both lean gas mixtures (excess air) and rich gas mixtures (excess fuel). The MISiC-FET sensor is seen to behave as a linear lambda sensor [133]. It... 

The MISiC-FET sensor operated at 300°C has demonstrated very promising results in this application [52, 76]. In Figure 2.20, the sensor response to NH3 obtained from two MISiC-FET sensors with porous Pt gates is compared with the NH3 concentration as measured by an optical instrument [52]. It is seen that the MISiC-FET sensors follow the optical signal very closely. It can also be noted that... 

Figure 2.20 The sensor signal from two MISiC-FET sensors (upper curves) and the optical reference instrument (lower curve) during engine test rig measurements to simulate standard drive, (from [52]. 2004 IEEE. Reprinted with permission.)...

Carbon nanotubes, especially SWNTs, with their fascinating electrical properties, dimensional proximity to biomacromolecules (e.g., DNA of 1 nm in size), and high sensitivity to surrounding environments, are ideal components in biosensors not only as electrodes for signal transmission but also as detectors for sensing biomolecules and biospecies. In terms of configuration and detection mechanism, biosensors based on carbon nanotubes may be divided into two categories electrochemical sensors and field effect transistor (FET) sensors. Since a number of recent reviews on the former have been published,6,62,63 our focus here is mostly on FET sensors. 

For gas or ions, FET sensors response speed is not the main prerequisite because of the relatively long time constants associated with chemical measurements. However, the ON-OFF conductance ratio, as well as the transconductance IJ Vg should be maximized. As far as noise is concerned, attention shold be paid to reducing both the noise-equivalent input current zn and voltage vn generators of the sensor. In fact, the total rms input noise for unity bandwidth of such a device can be expressed in a first approximation as (Motchenbacher and Fitchen, 1973)... 

To better characterize the FET sensor, the noise-equivalent temperature... 

Gas sensors — (c) Metal oxide gas sensors — Figure 9. H2 FET sensor... 

A transistor-based chemical sensor used in aqueous environment must be electrically insulated. An ordinary FET sensor produced with a silicon wafer as starting material has four bare lateral edges, which are made when scribing a processed silicon wafer into FET chips. It has bonding pads on its surface for wires that are connected for the electrical operation of a FET (see Fig. la). It is necessary to insulate the bare lateral sides and bonding pad region, and this is usually done by polymer encapsulation. Because a FET chip is very small and the areas to be insulated are closely located to its ion-sensitive gates, which must... 

Fig. 5. Structure of FET sensor with two discrete FET chips 1, connectingwire 2, epoxy resin 3, platinum wire 4, FET chip 5, enzyme-immobilized membrane 6, epoxy laminate. (Reproduced from Nakako et al. (26), with permission.)...

Fig. 6. Response curves of a bifunctional FET sensor sensitive to urea and glucose. Response curves for (a) 8.3 mM urea solution, (i) 3.1 mM glucose solution, and (c) solution containing 8.3 mM urea and 3.1 mM glucose. (Reproduced from Hanazato et al. (8), with permission.)...

Fig. 11. Schematic diagram of continuous flow apparatus and structure of an enzymatically coupled FET. (a) Schematic diagram of continuous flow apparams S, enzymatically coupled FET sensor SC, sensor cell WB, water bath D, drtdnage P, peristaltic pump TV, three-way joint EV, electrical valve VC, valve controller WS, washing solution AS, analyte solution, (b) Detailed structure of flow-through cell OR, rubber O-ring. (c) Structure of enzymatically coupled FET (electrical insulation with epoxy resin is not shown here for simplicity) ISFET, ion-sensitive FET EM, enzyme membrane G, thin gold film TC, card edge connector. (Reproduced from Shiono et al. (9), with permission.)...

Fig. 16. Long-term stability of a urea-sensitive FET sensor. Phosphate buffer with (—) and without (—) EDTA is used for sample (1.7 mM urea) and wash solutions. (Reproduced from Shiono et al. (9), with permission.)...

Fig. 23. Relationship between glucose concentrations of human blood plasma samples measured by a glucose-sensitive FET sensor and by a conventional enzymatic method (F = 1.007X 14, r = 0.988, n = 101). (Re-...

The FET device has a high potential for the detection of ions and biomaterials. As described in some examples, the application of the FET as a sensor is expected to be useful for the next generation high-performance, on-chip sensing system. In addition, since the FET sensor enables the miniaturization of the sensor chip itself, it is especially expected to apply the advanced medical care and tailor-made medical diagnosis. Moreover, the combination between nanotechnology and biotechnology will accelerate the fusion of various iudustries such as the semicouductor industry and bioventures. 

Acknowledgements. This work was partially supported by the PRIN-06 Project -2006037708 - Plastic bio-FET sensors . 

Combined smart chemoresistive/FET sensor array, Proc. IEEE Sensors 2003 Conference, Canada, 22-24 October 2003. 

Adapt catalytic gate field effect transistor (FET) sensors to resolve and detect carbon monoxide (CO) contamination levels from 1-100 ppm in reformer produced hydrogen (H2) fuel for (proton exchange membrane (PEM) fuel cells... 

Use FET sensors on gallium nitride (GaN) for increased sensitivity and faster response in high temperature... 

Design catalytic gate FET sensor for high temperature applications... 

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