The Field-Effect Device

The addition of an insulating layer between semiconductor and one of the metal layers of the diode structure discussed in section 5 gives the MIS (Metal Insulator Semiconductor) structure which is widely used in a range of silicon semiconductor 

The position of Ep in the extrinsically-doped p-type region must lie close to the energy threshold for accumulation, as shown in figure 22(a). Finally, in the charge depletion region, present in figure 22(b), there are no mid-gap soliton levels present. 

Electrical characterisation of the MIS device is restricted to measurement of the conq )lex impedance the equivalent circuit is shown in figure 23, and can be considered as a series circuit of the insulator (capacitance Q) and the semiconductor (capacitance Cs and conductance Gs). Where Gs is low, we have 

Both accumulation and inversion layo have very high differential capacitance, so that the measured capacitance is just the geometrical capacitance of the insulator layer, Q. However, in the depletion regime, Cs falls to a lower value, and if the width of the depletion layer is comparable to the thickness of the insulator layer, the fractional change of the measured c acitance can be large. The variation of the measured capacitance in this regime is given by 

The three-terminal field effect transistor allows measurement of the conductance of the surface charge layer, and we have used a variety of device structures including that shown in figure 3. The source and drain contacts on the top of this structure allow the setting of the electric field across the insulator by defining the voltage of the gate (Vgs) and also allow measurement of the conductance of the channel between source and drain. We defer to section 7 discussion of the standard conditions for the operation of the MISFET. 

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The gas response of the field-effect devices is determined by the catalytic properties of the contact material, which includes both the catalytic layer and the underlying material. The temperature plays a dominant role in the detection process because the origin of the gas response is found in the chemical reactions that take place on the sensor surface, and it is furthermore also influenced by the mass transport properties of the molecules in the gas phase. This permits arrays of sensors of a common design to be tailor-made for detection of a range of gases and for use in a range of applications... 

Thus, the Pd layer serves multiple purposes its surface catalyzes the dissociation of molecular hydrogen, it selectively forms palladium hydride, and it can be used as the metal gate of the field-effect devices. The scheme in Fig. 6.34 also shows the catalytic reaction involving oxygen. If both oxygen and hydrogen are present, the steady-state response of the Pd IGFET includes the surface-catalyzed oxidation. 

The capacitor-type gas sensors are one type of the field-effect devices. The field-effect devices can be classified into two types metal-oxide-semiconductor (MOS) capacitors and transistors (MOSFETs), as shown in Figures 1.2 and 1.3, respectively... 

Due to the symmetrical construction the resulting magnetic field between the two coils is zero in y-direction, if a conductive structure is symmetrically situated in the area a (see fig. 3) in the near of the probe. A resulting field is detectable by the Hall-effect device, if there are unsym-metrics in the structure in area a. The value of the Hall voltage is proportional to the detected magnetic field. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

Figure 14-23. Variation of the field-effect mobility, as deduced by differentiating the drain current at Vt,=-i V, as a function of the gale voltage, for the same device as in Figure 14-22.

Generally, there are a number of ways in which the adsorption and binding of charged macromolecules (in particular, DNA immobilization and hybridization) can affect the electrochemical properties of the analyte-FED interface. In the case of field-effect devices, two basic effects are usually considered ... 

Many active electronic devices can be operated at cryogenic temperatures [45], They are generally of the field-effect transistor (FET) type and are based on silicon (working down to 100K) or gallium arsenide (working even below 4K). 

Since the capacitor, Schottky diode, and transistor all contain an insulating layer under the catalytic metal, they are all referred to in this chapter as field-effect devices. In published literature, the capacitor and diode SiC devices are often referred to as MISiC devices, and the transistor as an MISiC-FET device. 

Figure 2.6 Schematic diagrams of the different field-effect devices described in this chapter. (From [19]. 2003 Springer-Verlag. Reprinted with permission.)...

Significant advances have occurred in microfabricated ion sensitive and Pd gated field effect devices and fiber optic, chemically rsnsitive elements. These elements are beginning to find their way into commercial development. Recent advances in these devices are discussed and compared. Pyroelectric sensor devices developed here are reviewed. A discussion of the utility of these devices is presented. 

Engstrom and Carlsson already introduced in 1983 an SLPT [119] for the characterisation of MIS structures, which was extended to chemical gas sensors by Lundstrom et al. [26]. Both SLPT and LAPS base upon the same technique and principle. However, due to the different fields of applications in history, one refers to LAPS for chemical sensors in electrolyte solutions and for biosensors, and the SLPT for gas sensors. A description of the development of a hydrogen sensor based on catalytic field-effect devices including the SLP technique can be found, e.g., in Refs. [120,121]. The SPLT consists of a metal surface as sensitive material which is heated by, for instance, underlying resistive heaters to a specific working-point temperature, and a prober tip replaces the reference electrode (see Fig. 5.10). 

Mode 2 devices which rely on a different detection principle are the Kelvin probe sensor and the CHEMFET. In the first case, a vibrating capacitor measures the change of the work function (see Figure 2), while in the second case the interaction is detected in the field-effect transistor mode.29 31... 

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