Field effect transistor operational principle

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]... 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

Field-Effect Transistors (FETs) are part of all modern pH meters. With the introduction of ion-sensitive field-effect transistors, they have both been brought to the attention of chemists. In order to understand the principles of operation of these new electrochemical devices, it is necessary to include the FET in the overall discussion of the electrochemical cell. The outline of the operation of an insulated gate field-effect transistor is given in Appendix C. 

The ion sensitive field-effect transistor (ISFET) is a special member of the family of potentiometric chemical sensors [6,7. Like the other members of this family, it transduces information from the chemical into the electrical domain. Unlike the common potentiometric sensors, however, the principle of operation of the ISFET cannot be listed on the usual table of operation principles of potentiometric sensors. These principles, e.g., the determination of the redox potential at an inert electrode, or of the electrode potential of an electrode immersed in a solution of its own ions (electrode of the first kind), all have in common that a galvanic contact exists between the electrode and the solution, allowing a faradaic current to flow, even when this is only a very small measuring current. 

In addition to applications as functional materials in OLEDs and OSCs, semiconducting polymers are needed for other (opto)electronic devices as well. With regard to displays, sensors, and radio-frequency identification tags (RFIDs) for example, it is a challenge to create polymer-based organic transistors (thin-film transistors, OTFT field-effect transistors, OFETs). Figure 6.7 sketches an optional OFET design, and additionally shows schematically its principle of operation. 

The essential principle of an active matrix display is that each pixel has associated with it a semiconductor device that is used to control the operation of that pixel. It is this rectangular array of semiconductor devices (the active matrix) that is addressed by the drive circuitry. The devices, which are fabricated by thin-film techniques on the inner surface of a substrate (usually glass) forming one wall of the LCD cell, may be either two-terminal devices (Fig. 6) or three terminal devices (Fig. 7). Various two-terminal devices have been proposed ZnO varistors, MIM devices, and several structures involving one or more a-Si diodes. Much of the research effort, however, has concentrated on the three-terminal devices, namely thin-film, insulated-gate, field-effect transistors. The subject of thin-film transistors (TFTs) is considered elsewhere in this volume suffice it to say that of the various materials that have been suggested for the semiconductor, only a-Si and poly-Si appear to have serious prospects of commercial exploitation. 

There has recently been considerable interest in the detection and identification of air-borne volatile compounds in such diverse areas as quality control of perfume to detection of toxic gases. Miniaturization, the potential low cost of sensors and the variety of applications promise an enormous market (29, 30), Chemical sensors (29) for volatile compounds operate on varied principles and can be classified according to the method of functioning into basic groups such as electrical (field-effect transistors, metal oxide semiconductors and organic semiconductors), optical (spectrophotometric, luminescence, optothermal) and sensors that are sensitive to a change of mass (piezoelectric and acoustosurface). 

The sensitive component of an ISFET operates along the same principles as a potentiometric electrode. In both systems, there is no consumption of reaction product, and the interaction of ions with the sensitive component leads to a membrane potential. This potential is measured directly in the potentiometric electrode and indirectly in the field-effect transistor. The measurement of the activity of the ions is generally made at a constant current the measured output potential is thus a function of the ionic activity of the solution. Once the ISFET and the potentiometric electrode are covered with enzymatic membranes to give an ENFET and an enzyme electrode, respectively, the two biosensors differ only in their method of transduction. 

The control of charge flow by an electric quantity is a key issue of today s electronics. The concept to electrically specify the conductivity of a resistor by pure solid state effects was already proposed in 1928 by Julius Edgar Lilien-feld in Germany [1], The basic idea was to control the charge carrier density in a solid by an electric field, applied over a third electrode. However, there is no evidence for a practical realisation by Lilienfeld. The first report about a pure electrically controllable solid state device was the well know Germanium transistor from William Shockley, John Bardeen and Walter Brattain [2]. The new term transistor was later explained as a combination of the words transconductance and varistor . Meanwhile a broad variety of different transistor concepts exists, which, however, can be mainly subdivided in two basic operational principles ... 

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