Electrode-oxide semiconductor contact

Electrodes of semiconductor gas sensors have a function of conductor to an external circuit as a contact material. Contact resistance that is formed in the electrode-oxide semiconductors may in some cases have significant contribution on the response of the sensors. Electrode materials are also utilized as catalyst activators in the sensor operation. In special cases, for electrode materials to operate at high temperature in automotive and aerospace industries, they have to endure at temperature up to 600°C. Therefore, the electrode materials of semiconductor gas sensors are responsible for their sensitivity and selectivity to specific gases. 

The physical understanding of the electrode-oxide semiconductor interfaces are described in this section. Interfaces of this type occur in oxide semiconductor gas sensors and metal-insulator-semiconductors (MIS) devices. When a metal is contacted with oxide, the potential barrier arises from the separation of changes of the metal-oxide interface as well as the metal-semiconductor contacts. 

The first significant step towards understanding the mechanism of the electrode-oxide semiconductor is given in an ideal case contact. Another advance in our understanding of the electrode-oxide semiconductor junction is concerned with contacts with surface states, and interfacial layer and... 

A novel development of the use of ion-selective electrodes is the incorporation of a very thin ion-selective membrane (C) into a modified metal oxide semiconductor field effect transistor (A) which is encased in a non-conducting shield (B) (Fig. 15.4). When the membrane is placed in contact with a test solution containing an appropriate ion, a potential is developed, and this potential affects the current flowing through the transistor between terminals Tt and T2. 

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 ... 

Figure 19.8—A selective electrode designed from a MOSFET (metal oxide semiconductor field effect transistor). A specific reaction can be monitored by putting an enzyme in contact with the electrodes. This schematic shows the three electrodes used for amperometric measurement.

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. 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

Let us consider an n-type semiconductor oxide in contact with a corroding metal in aqueous solution. Before contact, the electrode potential of the n-type oxide is set near its flat band potential... 

The resistance of a tin oxide gas sensor consists of bulk resistance, surface resistance and contact resistance. The reduction of contact resistance is useful for improving the properties of oxide semiconductor gas sensors. An ohmic contact between the electrode and sensing material can reduce the contact resistance. Zhou et al. compared conventional tin dioxide-gold electrode structures with devices in which an n+ layer was introduced between the sensor and electrode. The use of the metal-n+-n contact not only improved the sensitivity of the sensor to alcohol, but also the sensor selectivity to other gases did not change with the addition of an n+ layer. 

The metal-oxide-semiconductor field-effect transistor (MOSFET) is a transistor that uses a control electrode, the gate, to capacitively modulate the conductance of a surface channel joining two end contacts, the source and the drain. The gate is separated from the semiconductor body underlying the gate by a thin gate insulator, usually silicon dioxide. The surface channel is formed at the interface between the semiconductor body and the gate insulator, see Fig. 7.25. 

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