Charge transport impurity

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

The previous discussion did not consider trapping effects, which result in a loss of charge to the collection process and consequent distortion of the shape of the peak as observed with a multichannel analyzer. Trapping of a charge carrier in a semiconductor occurs when the carrier is captured by an impurity or imperfection centre and is temporarily lost to any charge transport process. In semiconductor detectors, it is useful to introduce the quantity (mean free drift time) ... 

History-free, reproducible, transient currents are reported in as-received and Na doped PI films between 150 and 330°C. From a calculation of the total charge transported, purely ionic mechanisms can be ruled out, and an electronic conduction mechanism must be invoked. The electronic conduction is, however, modulated by the presence mobile ionic impurities. The current and total charge transported vary in proportion to the amount of Na ions in the film. Thus an ion/electron interaction in PI is postulated. 

The marked effects of disorder in pseudo-ID systems have been clarified by both experiment and theory. These include (a) transport in the absence of collective effects (INV 1, INV 13), (b) the role of impurities in pinning incommensurate CDW s and the effect on charge transport (F3), and (c) the relative effect of impurities on the Peierls and superconducting transition temperature. 

The simplest model of charge transport in delocalized bands is the Drude model, which assumes the carriers are free to move under the inhuence of an applied electric held, but subject to collisional damping forces. Note that the scattering centers are not the nuclei of the background material, but rather phonons (lattice vibrahons) or impurities. A statistical equahon for estimahng the mean drift velocity of the carriers in the direction of the electric held may be written as... 

In real organic transistors, charge transport is most of the time limited by localized states induced by defects and unwanted impurities. Clear evidence for such a process is given by the fact that the performance of the devices is strongly sample dependent. 

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