Semiconducting device principle

A thermistor is a semiconducting device which has a negative coefficient of resistance with temperature, e.g. its resistance decreases with increasing temperature. The principles behind its operation follows. 

The prime objective of this chapter is to explore the electrical properties of materials— that is, their responses to an applied electric field. We begin with the phenomenon of electrical conduction the parameters by which it is expressed, the mechanism of conduction by electrons, and how the electron energy band structure of a material influences its ability to conduct. These principles are extended to metals, semiconductors, and insulators. Particular attention is given to the characteristics of semiconductors and then to semiconducting devices. The dielectric characteristics of insulating materials are also treated. The final sections are devoted to the phenomena of ferroelectricity and piezoelectricity. 

All systems presented in this section show lasing only in the optical pumping mode. There is much interest in electrically pumped devices, but for molecular glasses the difficulties in achieving high excitation densities and low absorption due to charge carriers and electrodes have yet to be overcome. This problem and the related semiconducting polymer lasers that are based on the same principles will not be covered here, but are treated in recent reviews [214-216]. 

If we place n- and p-type semiconducting crystals in contact (a p-n junction), we create a device that conducts electricity preferentially in one direction this is the basis of action of the semiconductor diodes used in the electronics industry, although specially refined silicon (Section 17.8.2) is usually employed rather than Ge. Transistors and electronic chips are designed using similar basic principles—typically with n-p-n or p-n-p junctions. We consider chemical aspects of electronic devices in more detail in Chapter 19. 

Fig. 6 Principles of device function for organic semiconducting layers sandwiched between two metallic electrodes a short circuit condition, b flat band condition, c reverse bias, and d forward bias. Band bending effects at the ohmic contacts are neglected...

The fundamental principles of the photo-electrochemical cell for light-driven electrolysis of water are well known. Its future as a device for the production of hydrogen from water rests solely on the ability to find the correct semiconducting material for the photoactive electrode. 

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. 

For the purposes of biochemistry, electrical conductivity in the liquid phase is also measured. In addition to the classical principle based on the monitoring of ionic mobilities in the tested solution, the changes in conductivity of a semiconductive polymer film, when exposed to an analyte, can also be utilized. The technique of enzyme entrapment in conductive polymers has been used for the construction of microcon-ductometric devices [140] and a device based on a dual measurement principle (amperometric and conductometric) [141]. 

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