Sense FETs

A microphotograph of an actual CFT is shown in Figure 5. The overall chip size is 2mm z 2mm, and the spacing between fingers of the interdigitated electrodes is 12 pm. Note the additional FET on the chip, called the reference FET. Its function is to compensate for process variations in the FET and for the temperature dependence of the transistor properties. A feedback circuit in which the reference FET is used in conjunction with the sensing FET is shown in Figure 6. Because the circuit requires the two transistor cur-... 

Photolithographic methods, which play a key part in the fabrication of semiconductors, are potential candidates for the photo-patterning of small enzyme-immobilized membranes on a FET at its wafer stage. Ion-sensing FET devices with neutral carrier membranes sensitive to alkaline and alkaline earth metal... 

Current flowing in the motor can be monitored using a number of different ICs or power FETs available from any number of suppliers. These devices include current shunt ICs (Maxim, Texas Instruments Zetex), sense FETs (Fairchild Semiconductor), HEX sense power FETs (International Rectifier), and fault-protected switches (fancy MOSFETs, Micrel) to name but a few. Current-shunt ICs are common because they find use in dc-to-dc converters, consequently, a large number of different manufacturers make them in many different forms and flavors (see Fig. 32.17). 

Typical nanopore devices require only two electrodes, one on either side of the nanopore. On the other hand, integration of one or more electrodes into a nanopore offers interesting prospects towards smart nanopore devices, e.g. enhanced capabilities for surface functionalisation improved control of pore transport (gating) fabrication of metallic nanopores via local electrodeposition or sensing (FET or tunnelling devices). Fig. 16 illustrates a few designs that we will return to in the text. 

The bottom line is—to measure Fet current, insert the current probe into its Drain, never in its Source. Put in a small sense resistor in the Source if you must, but nothing inductive please. 

These measure the potential difference between the transducing electrode and a reference electrode under conditions of zero current. Three types of potentiometric detectors are commonly employed ion-selective electrodes (ISE), gas-sensing electrodes and field effect transistors (FET). 

Polythiophene derivatives are being used in FETs. Polypyrrole is being used as microwaveabsorbing stealth screen coatings and in sensing devices. PPV derivatives are being used in the production of electroluminescent displays. 

SAMFETs have also been used in chemical sensing. The a-substituted quincpiethiophene SAMFETs were covered with a 10-nm pinhole-riddled iron tetraphenylporphyrin chloride layer, that acts as a receptor to nitric oxide (NO), an important biomarker [74]. The threshold voltage, measured by the FET transfer characteristics with the porphyrin receptor shifts upon increased exposure to NO. Annealing the monolayer FET in vacuum restores the initial FET behavior. Also, in the single monolayer HBC assembled FETs between metallic SWCNT source and drain electrodes increased current levels were measured in /d-Fds and Aj-Fg characteristics (Fig. 9) upon exposure to solutions of the electron acceptor TCNQ [68]. While the mechanism of response is not known, TCNQ has an affinity for coronene, and likely gives rise to charge transfer between electron-deficient TCNQ... 

We recently published a chapter in the book Silicon Carbide Recent Major Advances by Choyke et al. [19] that describes SiC gas sensor applications in detail. In this book, we emphasize device properties applications are only briefly reviewed at the end. The device and gas sensing properties of various field-effect chemical gas sensing devices based on SiC are described, and other wide bandgap material devices are reviewed. The detection principle and gas response is explained, and the buried channel SiC-FET device is described in detail. Some special phenomena related to the high-temperature influence of hydrogen at high temperature are also reported. 

It is important to have a clear picture of the detection mechanism before we introduce the different types of field-effect transistor (FET) devices and their gas sensing properties. The sensing mechanism is largely independent of the device type, since the chemical reactions responsible for the gas response are defined by the type of catalytic material processed onto the device and the operation temperature [1,2, 20, 21]. Even at a temperature of 600°C, chemical reactions occur on the catalytic metal surface at a rate of a few milliseconds, which is slower than the response time of the devices. 

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. 

Generally, a carbon nanotube FET device is constructed by a substrate (gate), two microelectrodes (source and drain), and bridging material between the electrodes, which is typically an individual SWNT or a SWNT network. A SWNT FET is usually fabricated by casting a dispersion of bulk SWNTs or directly growing nanotubes on the substrate by chemical vapor deposition (C VD) either before or after the electrodes are patterned.64 Due to the diffusive electron transport properties of semiconducting SWNTs, the current flow in SWNT FET is extremely sensitive to the substance adsorption or other related events on which the sensing is based. 

---