Field-Effect Transistors with Semiconductor Gate

3 Field-Effect Transistors with Semiconductor Gate 

The first report of a WF field-effect transistor with a gate material other than Pd apparently appeared in the late 1980s. A phthalocyanine film has been evaporated over the gate insulator and over the Al contacts (Burr et al., 1987). Since then, a variety of conducting polymers has been deposited over the transistor gate and evaluated as materials for gas sensing (Liess et al., 1996) (Fig. 6.31). The general 

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Shen, C. Li, M. F. Yu, H. Y Wang, X. P Yeo, Y. C. Chan, D. S. H. Kwong, D. L. 2005. Physical model for frequency-dependent dynamic charge trapping in metal-oxide-semiconductor field effect transistors with HfOj gate dielectric. Applied Physics Letters, 86 093510(1-3). 

Metal oxide semiconductor field-effect transistors (MOSFETs) are field effect transistors with a thin film of silicon dioxide between the gate electrode and the semiconductor. The charge on the silicon dioxide controls the size of the depletion zone in the polype semiconductor. MOSFETs are easier to mass produce and are used in integrated circuits and microprocessors for computers and in amplifiers for cassette players. Traditionally, transistors have been silicon based but a recent development is field-effect transistors based on organic materials. 

Fig. 5. Metal-insulated semiconductor field effect transistor with n-channel (n-MISFET). (A) the state with uncharged G-I-Su capacitor, (B) open transistor with charged G-I-Su capacitor. Abbreviations S, source electrode D, drain electrode G, gate electrode Su, substrate Al, aluminium contact I, insulator (dielectric) n-Si, p-Si, n- or p-type of silicone...

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. 

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

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 ... 

A field effect transistor (FET) measures the conductance of a semiconductor as a function of an electrical field perpendicular to the gate oxide surface (13). When the gate oxide contacts an aqueous solution a change of pH will change the SiOj surface potential p. A site-dissociation model describes the signal transduction, a function of the state of ionization of the amphoteric surface SiOH groups (14). Typical pH responses measured with SiO ISFETs are 37-40 mV/pH unit (15). 

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. 

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