Carrier of charge

In the last chapter we examined data for the yield strengths exhibited by materials. But what would we expect From our understanding of the structure of solids and the stiffness of the bonds between the atoms, can we estimate what the yield strength should be A simple calculation (given in the next section) overestimates it grossly. This is because real crystals contain defects, dislocations, which move easily. When they move, the crystal deforms the stress needed to move them is the yield strength. Dislocations are the carriers of deformation, much as electrons are the carriers of charge. 

Much of the rather slow information transfer in biological systems is achieved by the release and subsequent transport of messenger molecules. For a fast information transfer over large distances, however, a combination of electrical and chemical transport processes is involved (in part cited from Ref. 1). Since the body is an aqueous organization, rather hostile to free electrons, it comes as no surprise that the carriers of charge are predominantly ions.1 Among the inorganic ions, Na, K+, Ca2+, and Cl- play an essential role in nerve transmission. 

Partially filled bands of collective-electron states support metallic conductivity. The electrical conductivity is defined as the ratio of current density J = nev to electric field strength, E, where n is the number of carriers of charge e per unit volume and v is their average velocity. Since the average force on a charged particle is eE = m v/r, where r is the mean time between collisions and m is the effective mass, it follows that... 

Schmidt s data indicate that N4 is destroyed by energetic collisions near the wall. The decomposition product N2 is suggested to be observed instead. His calculation indicates that is the major carrier of charge to the wall. 

Potassium is accumulated within cells by the action of the Na. K -ATPase (sodium pump) in which it participates in exchange for sodium that is extruded from the cell during potassium uptake [10]. Potassium has a major function as a carrier of charge within cells [1]. It is extremely mobile and therefore if it is allowed to pass through membranes it may be used to regulate potentials across cells, especially excitable cells such as muscle and nerve [16]. The regulation of such metal ion flows, especially of potassium and sodium, is crucial to life and is most clearly exemplified by the ionic movements that occur in nerve cells during excitation and transmission of the action potential [17]. 

This definition is theoretically adequate, but may give rise to confusion when measurements are to be interpreted. In electrolyte solutions and the solution side of electrical double layers, the ions are the carriers of charge. However, in ions, charge is always linked to matter so that transport of the charge can only be achieved by simultaneously transporting matter. Consequently, the total work of ion transport includes an electrical and a chemical contribution. 

The principal of the dry photocopier is illustrated in Fig. 22.4. A cylinder is covered with a material such as arsenic selenide, which is photoconductive because of trapped carriers of charge (see Index if necessary). In the dark, a high voltage is used to spray electrons onto the cylinder, and they stay there in the form of a static electricity charge, because the material is an insulator in the dark (see the first diagram in the figure). 

Electrophotography is the only area in which the conductivity of sophisticated organic materials and polymers is exploited in a large scale industrial process today. Photoconductors are characterized by an increase of electrical conductivity upon irradiation. According to this definition photoconductive materials are insulators in the dark and become semiconductors if illuminated. In contrast to electrically conductive compounds photoconductors do not contain free carriers of charge. In photoconductors these carriers are generated by the action of light. 

Shallow donors (or acceptors) add new electrons to tire CB (or new holes to tire VB), resulting in a net increase in tire number of a particular type of charge carrier. The implantation of shallow donors or acceptors is perfonned for tliis purjDose. But tliis process can also occur unintentionally. For example, tire precipitation around 450°C of interstitial oxygen in Si generates a series of shallow double donors called tliennal donors. As-grown GaN crystal are always heavily n type, because of some intrinsic shallow-level defect. The presence and type of new charge carriers can be detected by Flail effect measurements. 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

The conductivity becomes complex if mote than one type of charge carrier is present and involved in the conduction process. The total conductivity is the sum of all the conduction associated with the net motion of electrons, holes, and ions, ie ... 

The equations generally developed include all forms of the conduction. Eor example, to determine the flux or conductivity of ions in a soHd electrolyte as compared to electrons in a semiconducting ceramic, two terms are of interest the number of charge carriers and the mobiUty. The effects of temperature, composition, and stmeture on each of these terms must also be considered. 

---