MOSFET sensors

In a MOSFET, the substrate and the source are both connected to the earth, and the gate-source and the drain-source potentials are given by Vg and Vd, respectively. When the transistor is not in a saturated mode, the drain current Id is given by the expression [186]  

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MOSFET sensors were divided into two arrays of five sensors each, one array operating at 140°C and the other at 170°C ... 

A variety of chemical gas sensors are or could be used in electronic nose instruments. So far, successful results have been reached with conductive polymer (CP) sensors, metal oxide semiconductor (MOS) sensors, metal oxide semiconductor field effect transistor (MOSFET) sensors, quartz crystal microbalance (QCM) sensors, and infrared sensors. 

A typical sensor array system interfaced to a bioreactor representative of the studies described in this paper is shown in Fig. 3. The bioreactor off-gas is conducted to a container by its overpressure. At sample injection the gas in the container is withdrawn by a suction pump placed in the electronic nose instrument. The sample gas passes the sensor array, which is distributed over three serially coupled units. In the example in Fig. 3 the first unit contains ten MOSFET sensors, the second six MOS sensors and the third unit an infrared sensor. After injection, a valve is switched to a reference carrier gas, taken from the ambient air or from a gas flask with a controlled composition. 

The ethanol concentration in the medium of a Saccharomyces cerevisiae cultivation can be monitored from its content in the gas phase by directly recording the current from a chemical MOS sensor [28]. The accuracy of such a measurement was significantly improved by using an electronic nose with five sensors in the array and recognizing the response pattern with ANN [29, 30]. The sensors were a combination of MOS and MOSFET sensors selected from a PCA loading plot. Data sets from three cultivations were used to train the ANN. When the trained net was applied on new cultivations the ethanol was predicted with a mean square error (RMSE) of 4.6% compared to the off-line determined ethanol (Fig. 6). With only one sensor the RMSE was 18%. 

The amplitude density function of Surface Acoustic Wave (SAW) resonators or MOSFET sensor signals contains information about the number of adsorbed molecules. By measuring the amplitude density function and comparing it with the theoretical form, the concentration of the detected gas can be estimated. If sensor size is in the nanometer range, then the exact number of adsorbed molecules can be determined. Fignre 2 shows the result of computer simulations of the output signal of a nanoscale MOSFET sensor. 

The fundamental requirement for the e-noses is to produce a pattern of discem-ibly different responses for different samples. In other words, each sensor composing the array is not required to be highly specific, but it should respond to a broad range of analytes. Moreover, the correlation between responses of different sensors into the array system should be avoided because it decreases the information content of the pattern produced by the array [37], The use of arrays of gas sensors to quantify the concentration of gasses in mixtures was first proposed in the 1980s, and since then different types of sensors, such as amperometric, piezoelectric, metal-oxide resistors, and MOSFET sensors, have been employed in this technology [38 1],... 

Schottky barrier, metal oxide semiconductor (MOS), and held-effect transistors (MOSFETs) sensors are other types of gas sensor which involve metal films. These types of sensors were pioneered by Lundstrom et al. (1975). Besides silicon, compound semiconductors such as GaAs, InP, GaN, Ga Oj, and SiC have been alternatively employed as substrate materials for the above-mentioned... 

The ISFET is an electrochemical sensor based on a modification of the metal oxide semiconductor field effect transistor (MOSFET). The metal gate of the MOSFET is replaced by a reference electrode and the gate insulator is exposed to the analyte solution or is coated with an ion-selective membrane as illustrated in Fig. 

It is not obvious that the sensor technology takes advantage of the integrated transistor shrink (ITRS) trend. In fact actual MOSFET dimen-... 

Therefore the flicker noise is expected to grow with 7 as the device size is scaled down. In deep submicron MOSFETs the corner frequency at which thermal noise equals flicker noise may be as large as 100 MHz, indicating that, at low frequency, 1/f noise is the most severe noise source which affects sensor performance. 

Fig.4.22. Sensor resistance changes of a MOSFET-heated microhotplate upon exposure to CO concentrations of 5 to 50 ppm at 40% r.h.

D. Briand, B. van der School, N.E de Rooij, H. Sundgren, and I. Lundstrom. Alow-power micromachined MOSFET gas sensor , lournal ofMicroelectromechanical Systems 9 (2000), 303-308. 

Liu et al. have reported the development and applications of the commonly used local anaesthetic sensitive field-effect transistor(FET) [56]. The ion-pair complexes of procaine with silicotungstate, tetraphenylborate, or reineckate were prepared as electroactive materials for a drug sensor. These active materials were coated onto the platinum draw wire of a MOSFET to make a local anaesthetic-sensitive FET that... 

Apart from the aforementioned most frequently used sensor technologies, also selective electrochemical sensor combinations have been commercialised for use in dedicated applications. The combination of electrochemical CO, H2S, SO2 and NH3 sensors was used for quality and freshness control of foods like fish [98] and meat [99]. Combinations of MOSs and MOSFETs supplemented with a selective IR absorption sensor for carbon dioxide and a humidity sensor for measuring relative humidity were also described [100]. 

A great disadvantage of MOS, MOSFET, conducting polymer and QMB sensor arrays is that system-to-system matching is not possible in practice, as can... 

MOS metal oxide sensor, MOSFET metal oxide semiconductor field-effect transistor, IR infrared, CP conducting polymer, QMS quartz crystal microbalance, IMS ion mobility spectrometry, BAW bulk acoustic wave, MS mass spectrometry, SAW siuface acoustic wave, REMPI-TOFMS resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry... 

Pd MOS STRUCTURES The Pd MOS device (capacitor and field effect transistor) has been extensively studied as a model chemical sensor system and as a practical element for the detection of hydrogen molecules in a gas. There have been two outstanding reviews of the status of the Pd MOS sensor with primary emphasis on the reactions at the surface (7,8). In this section, the use of the device as a model chemical sensor will be emphasized. As will be seen, the results are applicable not only to the Pd based devices, they also shed light on the operation of chemfet type systems as well. Because of its simplicity and the control that can be exercised in its fabrication, the discussion will focus on the study of the Pd-MOSCAP structure exclusively. The insights gained from these studies are immediately applicable to the more useful Pd-MOSFET. 

In particular, just 10 variables were selected for ripened Asiago cheese total acids, total alcohols and total ketones the sensor MOS 3, sensor MOSFET 4 and sensor MOSFET 8 hardness, acid and bitterness for the sensory analysis and the casein. 

Also for the fresh Asiago cheese (Fig. 19.4), it is possible to individualize the important indexes for characterization total alcohols, total ketones, total volatile acids salty, bitterness and hardness for the sensory analysis the sensor MOS 2, sensor MOSFET 3 and sensor MOSFET 8 the humidity and the casein. 

The QCM sensors [5] have gained popularity because they are operated at room temperature and allow combinations of selective layers different to those of MOS, MOSFET and PC sensors. The QCM measures physical mass of the analyte by recording the change in frequency of a quartz crystal when the analyte binds to it. Layers of gas chromatographic stationary phases and natural or synthetic lipids can discriminate between alcoholic drinks, perfume and flavor odorants. 

The sensors of the electronic nose are assembled in an array. The array is normally a small electronic unit that integrates the different sensors into a practical circuit card or another appropriate system that is easy to insert into the electronic nose instrument. If the array is to be used in a flow injection setup the unit also comprises a flow cell compartment with minimal volume. The system depicted in Fig. 2 shows how MOS and MOSFET arrays are integrated in a flow injection system [11]. Larger arrays can be integrated into silicon chips, as described for CP sensors where, for example an ASIC chip with 32 sensors has been fabricated with BiCMOS technology and having an area of 7 x 7 mm [18]. If the array is be inserted in the headspace volume of a bioreactor, the technical solution is a remote array probe that can be placed in a gas sample container [19]. 

Fig. 8 a, b. a PCA classification of lignocellulose hydrolysates from pine, spruce, aspen and birch using a combination of MOS, MOSFET and CP sensors, b Prediction of the ferment-ability of the same hydrolysates expressed as specific ethanol production rate using ANNs with topologies adapted to the sensor array (from [34] with permission of ACS)... 

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