Charge carriers in metals

TABLE 7.1 Electrical Conductivity and Mobility of Charge Carriers in Metals, Band-like Semiconductors, and Hopping Semiconductors... 

How both the density and mobility of charge carriers in metals and band semiconductors (i.e. those in which electrons are not localized by disorder or correlation) are influenced by particular features of the electronic structure, namely band dispersion and band Ailing, will now be examined. Taking mobUity first, this book will briefly revisit the topic of band dispersion. Charge carriers in narrow bands have a lower mobility because they... 

Also, although Eqs. (1.69) and (1.70) resemble those for a Sehottky barrier, there are several important differences in the physical and chemical details (1) charge transfer between a semiconductor and a solution is a slow process, whereas that between a metal and a semiconductor is fast (2) the diffusion of redox speeies in the solution toward the electrode surface is slow whereas that of charge carriers in metal is fast (3) the reduced and oxidized species of the redox couple as donors and acceptors can change independently whereas the occupied and unoccupied states of the metal cannot be changed artificially (4) a Helmholtz layer is present between the semiconductor electrode and the solution whereas no such layer exists at the metal/semiconductor interface. 

Identify the charge carriers in metals and in ionic solutions. 

The charge carriers in metals are electrons that are free to migrate in the energy band called the conduction band (Figure 2.5). 

Electrons are usually the charge carriers in metals and semi-conductors while ions carry the charge in electrolytic solutions like battery acid or sea water where the ions are a minor species dissolved in the solvent (water) or like solid or liquid salts where the ions are the only constituents. Both ions and electrons may coexist for example in plasmas or ionized gases but the rapid movement of the small, light electron tends to dominate the conduction and a smaller proportion of the current is due to the larger, heavier ions. 

How can an electrical signal generated by an implanted device activate a nerve cell It is not as simple as two wires being connected. Part of the complexity arises from the difference in electrical charge carriers in metals, electrons carry charge, whereas in the body, ions carry charge. The conversion from electrons to... 

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal—insulator—semiconductor junction (MIS), a thin insulator layer, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

Degenerate semiconductors can be intrinsic or extrinsic semiconductors, but in these materials the band gap is similar to or less than the thermal energy. In such cases the number of charge carriers in each band becomes very high, as does the electronic conductivity. The compounds are said to show quasi-metallic behavior. 

Note that the above model is for a simple system in which there is only one defect and one type of mobile charge carrier. In semiconductors both holes and electrons contribute to the conductivity. In materials where this analysis applies, both holes and electrons contribute to the value of the Seebeck coefficient. If there are equal numbers of mobile electrons and holes, the value of the Seebeck coefficient will be zero (or close to it). Derivation of formulas for the Seebeck coefficient for band theory semiconductors such as Si and Ge, or metals, takes us beyond the scope of this book. 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

What is the situation inside the electrode That depends upon whether the electrode is a metal or a semiconductor. What is the most important difference between a metal and a semiconductor Operationally speaking, it is the order of magnitude of the conductivity. Metals have conductivities on the order of about 106 ohm-1 cm-1 and semiconductors, about 102-1(T9 ohm-1 cm"1. These tremendous differences in conductivity reflect predominantly the concentration of free charge carriers. In crystalline solids, the atomic nuclei are relatively fixed, and the charge carriers that drift in response to electric fields are the electrons. So the question is What determines the concentration of mobile electrons One has to take an inside look at electrons in crystalline solids. 

When one examines the value of n = p, it turns out that the density of charge carriers in an intrinsic semiconductor (Table 6.16) at room temperature is in the range of 10 to 10 cm, compared with about 10 cm in a metal. It is this relatively low concentration of charge carriers in intrinsic semiconductors that is responsible for the most important differences between semiconductor electrodes and metal electrodes. 

This current is conducted by electrons in a metal electrode, electrons and other charge carriers in a semiconductor, and by ions in the electrolyte. The conduction process provides an additional impediment, represented by the ohmic resistance Rn. Its effect is added to the interfacial potential difference, E, so that the total voltage will be... 

Semiconductor electrodes, which have much lower charge carrier densities (1013—1019 carriers/cm3), typically absorb in the infrared but exhibit much lower absorption by charge carriers than metals of comparable film thickness, and frequently show a transparency window in much of the visible spectrum due to a substantial band-gap energy, before absorbing again in the ultraviolet. For example, Sn02 and ZnO, like many common semiconductor electrode materi-... 

Other areas of specific interest include the role of the IA cations as charge carriers in the transmission of nerve impulses, as stabilizers of a variety of structures, and as activators of a large number of enzymes. The first topic is outside the scope of this chapter, although progress is being made in the study of metal-selective channels in nerve cells. 

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