Metal oxide semiconductor reference electrode

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. 

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 ... 

One of the most widely used materials for the fabrication of modern VLSI circuits is polycrystalline silicon, commonly referred to as polysilicon. It is used for the gate electrode in metal oxide semiconductor (MOS) devices, for the fabrication of high value resistors, for diffusion sources to form shallow junctions, for conduction lines, and for ensuring ohmic contact between crystalline silicon substrates and overlying metallization structures. 

Fig. 1. (a) Structure of eui ion-sensitive FET transducer with single FET element, (b) Structure of an ion-sensitive FET with metal oxide semiconductor FET. D, drain S, source IG, ion-sensitive gate CP, contact pad G, gate RE, reference electrode AS, analyte solution MOSFET, metal oxide semiconductor FET ISFET, ion-sensitive FET. 

Figure 6.1. Structure of a metal-oxide-semiconductor field-effect transistor (MOSFET) and an ion-sensitive field-effect transistor (ISFET). (a) Cross section of an n-type MOSFET (b) An ISFET is created by replacing the metal gate of the MOSFET by an electrolyte and a reference electrode.

With the advanced development of complementary metal-oxide-semiconductors (CMOS) technology, large-scale integration (LSI) makes the parallel multi-point bio-sensing possible. The working electrodes and reference electrodes of amperometric sensor are able to be integrated on a tiny chip (e.g., 10.4 x 10.4 mm) on... 

For the construction of the miniaturized pH device, the complementary metal oxide semiconductor (CMOS) approach was utilized. The Si substrate (p-doped) with a CMOS material was covered with a Sb film. This layer was converted to NW by exposing it to the ion beam. An Ag pad atop of the oxide was also deposited which was afterwards converted to a Ag/AgCI reference electrode by applying a ferric chloride solution. Thus, the final microscale pH sensor contained Sb-NWs as the working electrode and Ag/AgCl as the reference electrode. These electrodes were separated by a Si02 insulation layer and could measure the electromotive force developed between these two electrodes while exposing them to an analyte solution. 

Another explanation is that metal oxide electrodes behave like glass pH electrodes. The analogy, however, really does not work Glass membranes in pH electrodes connect a sample solution with a second, reference solution of known pH. The semiconductor electrode, on the other hand, is in contact with only a single solution. 

What follows is intended as a review of our own work, which is a small part of a rapidly growing area of chemistry. Note, for example, reference 1. Our emphasis has been on redox events at chemically fabricated metal and semiconductor interfaces. Given the chemical sites used and the configuration of the resulting structures, the presence of the electrode automatically provides a method of analysis and a means for monitoring interfacial events. The electrode also serves as a controlled potential source of oxidizing or reducing equivalents for the interface. 

As already discussed in Section 3.2 the potential across a single solid-liquid interface cannot be measured. One can only measure the potential of an electrode vs. a reference electrode. It has already been shown in Section 3.2 that a certain potential is produced at a metal or semiconductor electrode upon the addition of a redox system, because the redox system equilibriates with the electrons in the electrode, i.e. the Fermi level on both sides of the interface must be equal under equilibrium. It should be emphasized here that the potential caused upon addition of a redox couple to the solution occurs in addition to that already formed by the specific adsorption of, for instance, hydroxyl ions. A variation in the relative concentrations of the oxidized and reduced species of the redox system leads to a corresponding change of the potential across the outer Helmholtz layer, as required by Nernst s law (see Eq. 3.47), which can be detected by measuring the electrode potential vs, a reference electrode. However, there still exists a potential across the inner Helmholtz layer which remains unknown. 

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