Field-effect transistor operation

Kreisl, E, Helwig, A., Muller, G, Obermeier, E. and Sotier, S. (2005), Detection of hydrocarbon species using silicon MOS field-effect transistors operated in a non-stationary temperature-pulse mode . Sens. Actuators B, 106,442-9. 

The invention of the germanium transistor in 1947 [I, 2] marked the birth of modem microelectronics, a revolution that has profoundly influenced our current way of life. This early device was actually a bipolar transistor, a structure that is mainly used nowadays in amplifiers. However, logical circuits, and particularly microprocessors, preferentially use field-effect transistors (FETs), the concept of which was first proposed by Lilicnficld in 1930 [3], but was not used as a practical application until 1960 [4]. In a FET, the current flowing between two electrodes is controlled by the voltage applied to a third electrode. This operating mode recalls that of the vacuum triode, which was the building block of earlier radio and TV sets, and of the first electronic computers. 

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]... 

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). 

Sanghyun, J. Lee, K. Janes, D. B. Yoon, M-H. Facchetti, A. Marks, T. J. 2005. Low operating voltage ZnO nanowire field-effect transistors enabled by self-assembled organic gate nanodielectrics. Nano Lett. 5 2281-2286. 

Operation of the Molecular Monolayer Field-Effect Transistor. 216... 

It is important to have a clear picture of the detection mechanism before we introduce the different types of field-effect transistor (FET) devices and their gas sensing properties. The sensing mechanism is largely independent of the device type, since the chemical reactions responsible for the gas response are defined by the type of catalytic material processed onto the device and the operation temperature [1,2, 20, 21]. Even at a temperature of 600°C, chemical reactions occur on the catalytic metal surface at a rate of a few milliseconds, which is slower than the response time of the devices. 

Figure 15-28 Operation of a field effect transistor, (a) Nearly random distribution of holes and electrons in the base in the absence of gate potential. ( >) Positive gate potential attracts electrons that form a conductive channel beneath the gate. Current can flow through this channel between source and drain.

Figure 15-29 Operation of a chemicalsensing field effect transistor. The transistor is coated with an insulating Si02 layer and a second layer of Si3N4 (silicon nitride), which is impervious to ions and improves electrical stability. The circuit at the lower left adjusts the potential difference between the reference electrode and the source in response to changes in the analyte solution such that a constant drain-source current is maintained.

Successful operation of potentiometric chemosensors opened up the possibility for the fabrication of chemical field-effect transistors (chemFETs) and ion-selective field-effect transistors (ISFETs). A sensing element in these devices, i.e. the MIP film loaded with the molecular, neutral or ionic, respectively, imprinted substance is used to modify surface of the transistor gate area. Apparently, any change in the potential of the film due to its interactions with the analyte alters the current flowing between the source and drain. 

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. 

Fig. 6.22 Analog circuits for operation of ion-sensitive field-effect transistor (ISFET) (a) in constant applied voltage mode ((6.62) and (6.63)) and (b) in (source-follower) constant current feedback mode ((6.65) and (6.69))...

Pd MOS STRUCTURES The Pd MOS device (capacitor and field effect transistor) has been extensively studied as a model chemical sensor system and as a practical element for the detection of hydrogen molecules in a gas. There have been two outstanding reviews of the status of the Pd MOS sensor with primary emphasis on the reactions at the surface (7,8). In this section, the use of the device as a model chemical sensor will be emphasized. As will be seen, the results are applicable not only to the Pd based devices, they also shed light on the operation of chemfet type systems as well. Because of its simplicity and the control that can be exercised in its fabrication, the discussion will focus on the study of the Pd-MOSCAP structure exclusively. The insights gained from these studies are immediately applicable to the more useful Pd-MOSFET. 

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