PROPERTIES AND TYPES OF SEMICONDUCTORS

The temperature dependence is quite different for metals and semiconductors. At a temperature of 0 K, a semiconductor will behave as a perfect insulator. However, metals will exhibit electrical conductivity at absolute zero due to the delocalized electron density and lattices described in Chapter 3. However, as the temperature is increased, the respective conductivities of these materials will be reversed, with 

For direct bandgap materials, the electron-hole recombination may occur without any change in momentum, resulting in the emission of photons. We will describe some important applications for direct semiconductors later in this chapter. 

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This chapter introduces some properties and definitions of semiconductor materials used in heterogeneous photocatalysis. The comparison of heterogeneous photocatalytic systems and a brief description of the types of membranes that can be used is also reported. Some aspects of membrane operations such as fouling, separation of a photocatalyst and effectiveness of photodegradation on permeate quality are discussed. 

There are two very broad, general conclusions resulting from the above review. The first is that MoS2-type nanoparticles are very different than other types of semiconductor nanoparticles. Nanoparticles of several different types of semiconductors, such as CdSe, CdS, and InP, have been extensively studied. Experimental and theoretical studies have elucidated much of their spectroscopy, photophysics, and dynamics. The results reviewed above are, in many places, in sharp contrast with those obtained on other types of quantum dots. This does not come as a surprise. The properties of the bulk semiconductor are reflected in those of the nanoparticle, and properties of layered semiconductors are vastly different from those of semiconductors having three-dimensional crystal structures. Although the electronic and spectroscopic properties of nanoparticles are strongly influenced by quantum confinement effects, the differences in the semiconductors cause there to be few generalizations about semiconductor quantum dots that can be made. 

Semiconductors Germanium has the same type of structure and semiconducting properties as silicon. What type of semiconductor would you expect arsenic-doped germanium to be ... 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

Silicon as whisker material is entering the area of engineering application, but the dominant area of interest is the use of silicon and doped silicon as semiconductor materials. The use of hardness measurement as a probe of the properties of specially prepared silicon crystals for semiconductor device use has been attempted several times but as some of the data presented below show, doping at the levels used in the device industry does not show up as change in hardness even through etch-pit rosette studies in the area around indentations show some dependence of dislocation movement on concentration and type of dopant. 

In comparisons of muons with protons and of muonium with hydrogen atoms, pronounced quantum effects occur whenever dynamics are involved. In this way, muons have been utilized to probe a large variety of properties and materials insulators, semiconductors, metals, superconductors, insulators, gases, liquids, crystalline and amorphous solids, static and dynamic magnetic properties of all kinds, electron mobility, quantum diffusion, chemical reactivity and molecular structure and dynamics. The term adopted for the broad field of muon spin spectroscopy techniques, fiSR, emphasizes the analogy with other types of magnetic resonance for example EPR. juS represents muon spin , and R in a more general sense stands simultaneously for rotation , relaxation and resonance . 

The operation of a semiconductor photoelectrode is influenced by both intensive and extensive properties. Fundamentally, sustained photoelectrochemical conversion is a process that works not at equilibrium but rather under (ideally) steady-state conditions. Accordingly, intensity and type of illumination can greatly affect the behavior of a given semiconductor photoelectrode beyond just increasing or decreasing the total number of reaction turnovers of an electrochemical reaction. The principal loss mechanisms can change for the same semiconductor photoelectrode with changes to the steady-state condition. 

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