Mobility, charge carrier drift

Thus when an electric field is appHed to a soHd material the mobile charge carriers are accelerated to an average drift velocity v, which, under steady-state conditions, is proportional to the field strength. The proportionality factor is defined as the mobility, = v/E. An absolute mobility defined as the velocity pet unit driving force acting on the particle, is given as ... 

Li Y, Nakano C, Imaeda K, Inokuchi H, Maruyama Y, Iwasawa N, Saito G (1990) Charge-carrier drift mobilities and phase transition in tetrakis(octylthio)tetrathiafulvalene, TTCg-TTF. Bull Chem Soc Jpn 63 1857-1859... 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

V is the applied voltage L is the sample thickness M is the charge-carrier drift mobility e is the static dielectric constant in the solid... 

In most cases an external electric field is applied across the material with the result that the mobile carrier distribution will experience drift in the field toward a new position. Even in the absence of an applied field, the nonuniform distribution of the mobile charge carriers created will lead to their relocation due to diffusion. Although the free carriers are generated where the optical intensity is high, their recombination with counterions (in the case of hole transport these are anions) may occur anywhere in the medium. This includes recombination where the intensity is low, resulting in the separation of the charge distributions. Subsequent optical excitation is unlikely in these darker regions. We know that the counterions exist in... 

Charge carrier drift mobilities of a number of amorphous molecular materials have been determined by a time-of-fhght method, and their electric-field and temperature dependencies have been analyzed in terms of the disorder formalism [56, 57] ... 

The second approach deals with charge transport macroscopically. An amorphous organic semiconductor can be treated as an ensemble of disordered hopping sites through which injected carriers drift xmder the influence of an external applied field. Historically, the Poole-Frenkel (PF) model was one of the first models to explain the electric field dependence of charge carrier drift mobilities [21,22], The field-dependent mobility can be written as... 

The Hall coefficient is related in a simple way to the number of mobile charge carriers. Suppose that the current is made up of electrons flowing parallel to X with a drift velocity Vx- The magnetic field present will exert a force, F, on an electron ... 

Commonly, holes are the mobile charge carriers in photorefractive polymers. Since the migration of holes by diffusion is a rather slow process, a drift is enforced by the application of an external electric field. The latter not only promotes hole migration, but also provides essential assistance during the photo-... 

Mozer, A.J., and N.S. Sariciftci. 2004. Negative electric field dependence of charge carrier drift mobility in conjugated, semiconducting polymers. Chem Phys Lett 389 438. 

The time dependence of the dielectric properties of a material (expressed by e or CT ) under study can have different molecular origins. Resonance phenomena are due to atomic or molecular vibrations and can be analyzed by optical spectroscopy. The discussion of these processes is out of the scope of this chapter. Relaxation phenomena are related to molecular fluctuations of dipoles due to molecules or parts of them in a potential landscape. Moreover, drift motion of mobile charge carriers (electrons, ions, or charged defects) causes conductive contributions to the dielectric response. Moreover, the blocking of carriers at internal and external interfaces introduces further time-dependent processes which are known as Maxwell/Wagner/Sillars (Wagner 1914 Sillars 1937) or electrode polarization (see, for instance, Serghei et al. 2009). 

The key parameters in characterizing a FET are its field-effect mobility and on/o f ratio. Field-effect mobility quantifies the average charge carrier drift velocity per unit electric field, whereas on/off ratio is defined as the drain-source... 

Charge carriers in a semiconductor are always in random thermal motion with an average thermal speed, given by the equipartion relation of classical thermodynamics as m v /2 = 3KT/2. As a result of this random thermal motion, carriers diffuse from regions of higher concentration. Applying an electric field superposes a drift of carriers on this random thermal motion. Carriers are accelerated by the electric field but lose momentum to collisions with impurities or phonons, ie, quantized lattice vibrations. This results in a drift speed, which is proportional to the electric field = p E where E is the electric field in volts per cm and is the electron s mobility in units of cm /Vs. 

In addition to the number of electrons, the other factor in Eq. (6.9) that affects conductivity is the electron mobility, pe. The mobility is the average charge carrier velocity, or drift velocity, v, divided by the electric field strength, ... 

As a consequence of random variations in the propagation characteristics of individual charge carriers, an initially discrete packet of carriers will necessary broaden out in profile as it drifts across a specimen. For the carriers that move exclusively in extended states, this dispersion results from statistical variations in scattering processes and may be described in terms of a diffusion coefficient D, which is related to the carrier mobility via Einstein relation... 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

Traditionally, charge-carrier transport in pure and doped a-Se is considered within the framework of the multiple-trapping model [17], and the density-of-state distribution in this material was determined from the temperature dependence of the drift mobility and from xerographic residual measurements [18] and posttransient photocurrent analysis. 

The assignment of ti, t2, and t- (see inflection points in Fig. 4.17) to transit times in the top, middle, and bottom layers is supported by the fact that the drift mobility of charge carriers for the three layers were calculated to be similar to the corresponding single layers. The general features of current waveforms described earlier are common to both hole and electron response. 

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. 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

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