Reference field-effect transistors

Kuroiwa S, Wang J, Satake D, Nomura S, Osaka T (2009) Effect of surface morphology of reference field effect transistor modified by octadecyltrimethoxysilane on ionic responses. J Electrochem Soc 156 167-172... 

ISFETs are used for pH measurement with a reference field-effect transistor. Sensing elements of the ISFETs were covered by the plasma polymerised styrene and were characterised [81]. An N02gas sensor was fabricated using plasma polymerisation of copper phthalocyanines. The materials processed were copper phthalocyanines without (CuPc) and with a chlorine substituent (CUPc-Cl), and with hydroxymethyl (CuPc-CH2OH) and phthalimidomethyl substituents [82]. It has also been reported that the tetramethyl plasma-polymerised films can be used for sensing propane gas [83]. 

Comte and Janata [39] were the first using a modified pH-ISFET as a reference field effect transistor (R(E)FET). A small compartment filled with buffered agarose gel is built on the membrane of the reference gate and is connected with the sample solution via a glass capillary (see Fig. 11.8). 

Microreference elements (reference electrodes or reference field-effect transistors— REFETs) micromachined in silicon were precursors of screen-printed reference electrodes (e.g., [4-7]). The major motivation to look for optimal layered reference system was usually to buUd a potentiometric chip containing disposable sensing and reference electrode, preferably made in the same or similar layer application technology, e.g., thin-film technology, which can be regarded as direct ancestor of nowadays screen-printed reference elements. 

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. 

D.S. Kim, H.J. Park, H.M. Jung, J.K. Shin, Y.T. Jeong, P. Choi, J.H. Lee, and G. Lim, Field-effect transistor-based biomolecular sensor employing a Pt reference electrode for the detection of deoxyribonucleic acid sequence. Jpn, J. Appl. Phys. 43, 3855-3859 (2004). 

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). 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Fig. 20 Charge carrier mobility in P3HT as a function of the charge carrier concentration. Squares refer to an experiment performed on a field effect transistor while circles refer to experiments done on an electrochemically doped sample. In the latter case the mobility is inferred from the steady state current at a given doping level. Solid and dashed lines have been fitted using the theory of [101]. The fit parameters are the site separation a, the prefactor Vq in the Miller-Abrahams-type hopping rate, the inverse wavefunction decay parameter y and the dielectric constant e. From [101] with permission. Copyright (2005) by the American Institute of Physics...

Ion-Selective Field Effect Transistors [22b,c,d] An ion-selective field effect transistor (ISFET) is a hybrid of an ion-selective electrode and a metal-oxide semiconductor field effect transistor (MOSFET), the metal gate of the MOSFET being replaced by or contacted with a thin film of a solid or liquid ion-sensitive material. The ISFET and a reference electrode are immersed in the solution containing ion i, to which the ISFET is sensitive, and electrically connected as in Fig. 5.37. A potential which varies with the activity of ion i, o(i), as in Eq. (5.38), is developed at the ion-sensitive film ... 

Fig. 5.37 An ion-selective field-effect transistor (ISFET). 1, drain 2, source 3, substrate 4, insulator 5, metal lead 6, reference electrode 7, solution 8, membrane 9, encapsulant [22b].

Figure 15-29 Operation of a chemicalsensing field effect transistor. The transistor is coated with an insulating Si02 layer and a second layer of Si3N4 (silicon nitride), which is impervious to ions and improves electrical stability. The circuit at the lower left adjusts the potential difference between the reference electrode and the source in response to changes in the analyte solution such that a constant drain-source current is maintained.

Volume has been the most significant limitation on the size and construction of microreference electrodes, a limitation that complements the small size of the microfabricated ion sensors (Section 6.23.2). There have been many attempts to prepare a liquid junction free microreference electrode that would be comparable in size with the integrated ion sensors, such as ion-sensitive field-effect transistors (Section 6.23.2). These attempts have followed broadly three tines of reasoning scaling down of a macroscopic reference electrode (Comte and Janata, 1978 Smith and Scott, 1986), elimination of the reference solution compartment while preserving the internal element structure (e.g., Ag/AgCl), and utilization of inert materials such as polyfluorinated hydrocarbons and the tike, particularly in the so-called reference FET configuration. 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

Fig. 6.26 Individual and differential response of the NaCI ion-sensitive field-effect transistor (ISFET) sensor (a) response of Na-ISFET and (b) Cl ISFET both measured against regular reference electrode (c) differential current measurement of concentration of NaClin 0.01MMgSO4 solution (adapted from Bezegh et al., 1987)...

The field known as organic or plastic electronics is centered on field effect transistor (FET)-based circuits mounted on large-area and/or flexible substrates. When the semiconductor is organic, the device is referred to as an organic field-effect transistor (OFET). Work on OFET has been extensively reviewed, most notably and comprehensively in Chemistry of Materials and Journal of Materials Research special issues, to which the one of us has contributed three articles [1-3]. 

Electrometer amplifier— An electronic amplifier with an extremely high -> input impedance (Rln > 1014 Q). The device allows measurements of electrical voltages (potentials) at practically zero current. Early devices employed specially designed and selected vacuum tubes (electrometer tubes) operated in a mode with very low grid current. The development of field effect transistors of various types allowed the application of solid-state devices. Electrometer amplifiers are employed in - pH meters (and generally in so-called pi meters, where I stands for ion), all types of instruments for po-tentiometric measurements and in the reference electrode input of -> potentiostats. Because of the high input impedance electrometer amplifiers are sensitive towards electric interferences, consequently some potentiostats have their -> reference electrode input circuitry (essentially an electrometer amplifier) mounted in a separate housing to be attached as close as possible to the reference electrode in order to minimize external interference. 

Ion-sensitive field effect transistor (ISFET) — In a semiconductor device based on the principle of the field effect transistor (FET) the current between two - semiconductor electrodes (designated source and drain) is controlled by a third electrode, the gate. In an ISFET this gate is modified on its surface in a way which makes the surface ion-responsive (-selective and -sensitive). Changes in the concentration of the species in the solution in contact with the gate surface thus control the current between source and drain. In order to establish proper working conditions a reference electrode (e.g., a -+ REFET) is needed. See also - CHEM-FET. 

Reference electrode field effect transistor REFET... 

A new development in the field of potentiometric enzyme sensors came in the 1980s from the work of Caras and Janata (72). They describe a penicillin-responsive device which consists of a pH-sensitive, ion-selective field effect transistor (ISFET) and an enzyme-immobilized ISFET (ENFET). Determining urea with ISFETs covered with immobilized urease is also possible (73). Current research is focused on the construction and characterization of ENFETs (27,73). Although ISFETs have several interesting features, the need to compensate for variations in the pH and buffering capacity of the sample is a serious hurdle for the rapid development of ENFETs. For detailed information on the principles and applications of ENFETs, the reader is referred to several recent reviews (27, 74) and Chapter 8. 

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