Transport diffusion lengths

Global AMI.5 sun illumination of intensity 100 mW/cm ). The DOS (or defect) is found to be low with a dangling bond (DB) density, as measured by electron spin resonance (esr) of - 10 cm . The inherent disorder possessed by these materials manifests itself as band tails which emanate from the conduction and valence bands and are characterized by exponential tails with an energy of 25 and 45 meV, respectively the broader tail from the valence band provides for dispersive transport (shallow defect controlled) for holes with alow drift mobiUty of 10 cm /(s-V), whereas electrons exhibit nondispersive transport behavior with a higher mobiUty of - 1 cm /(s-V). Hence the material exhibits poor minority (hole) carrier transport with a diffusion length <0.5 //m, which puts a design limitation on electronic devices such as solar cells. 

Diffusion is characterized by a mass transfer coefficient U8 of 104 m/h, which can be regarded as a molecular diffusivity of 2 x 10 6 m2/h divided by a path length of 0.02 m. In practice, bioturbation may contribute substantially to this exchange process, and in shallow water current-induced turbulence may also increase the rate of transport. Diffusion in association with organic colloids is not included. The D value is thus given as Us AwZ2. 

To that end, an important idea contributed by Robertson and Michaels was that oxygen reduction on Pt could potentially be co-limited by adsorption and diffusion rather than by just one or the other. In modeling the system, they noted that it is not possible for adsorbed oxygen to be in chemical equilibrium with the gas at the gas-exposed Pt surface while at the same time being in electrochemical equilibrium with the applied potential at the three-phase boundary. To resolve this singularity, prior (and several subsequent) models for diffusion introduce an artificial fixed diffusion length governing transport from the gas-equilibrated surface to the TPb.56,57,59,64,65,70 coutrast, Robertsou and Michaels... 

A high gain transistor requires a nearly equal to 1. In the absence of collector junction breakdown, a is the product of the base transport factor and emitter efficiency. The base transport factor, aT, is the fraction of the minority current (electrons for an n-p—n transistor) that reaches the collector. ocT 1 — W2 /2L, where W is the base width, is the distance between emitter and collector junctions and Lg is the minority carrier diffusion length in the base. High gain transistors require a thin base as well as a long minority carrier lifetime for a large Lg. Because aT is >0.995 in modem transistors, there is little room for improvement. The emitter efficiency, the fraction of emitter current due to minority carriers injected into the base instead of the emitter,... 

Mass transport Diffusivity Diffusion length External surface area... 

Volume 21, Part C, is concerned with electronic and transport properties, including investigative techniques employing field effect, capacitance and deep level transient spectroscopy, nuclear and optically detected magnetic resonance, and electron spin resonance. Parameters and phenomena considered include electron densities, carrier mobilities and diffusion lengths, densities of states, surface effects, and the Staebler-Wronski effect. 

The treatment is greatly simplified, if the lateral diffusion into the bulk is absolutely negligible with respect to the boundary diffusion. The bulk transport then just takes place on a completely separate time scale the effective diffusion length of the ceramic is the grain size (X) rather than the sample size (L), but capacitive (oc lcs) and resistive terms (oc Me/) are unchanged compared to the bulk values (cf. Section VI.3.tv.). In the case of the chemical experiment,... 

The exhalation of Rn from material surfaces is controlled by the generation rate of Rn in the material, and the transport by diffusion through the material to the surface. The generation rate is determined by the Rn content of the material, and the emanation fraction. The transport through the material is controlled by the diffusion length through the material. The diffusion process is well described mathematically by one-dimensional diffusion theory, so that knowledge of these parameters will allow accurate calculation of the Rn exhalation rate from the material surface. 

Figure 1.20 shows the two extreme cases of light absorption, a < he and a > ho + h relation to the diffusion length and the width of the space charge layer. In the region x a , the holes generated in the depletion layer, x < x, are transported by the electric field to the surface where they are consumed in the electrode reactions. Outside the depletion layer at x > xj, the photogenerated holes are transported by... 

Diffusion length (or lifetime) is a key parameter for the performance of solar cells. It is usually admitted that diffusion length of minority carriers has to be four times the thickness of the film to assure good photovoltaic efficiencies. At 1,050°C, appropriated values of 136 and 120 pm were obtained with In and Sn, respectively. The lower performance of epilayer grown from Sn melt can be explained by its high solid solubility (5 x 1019 cm 3 at 1,050°C). Incorporation of Sn atoms within the Si crystal could create a large stress and affect carrier transport. It is clearly related to the defects density of epitaxial film (measured by SECCO etching). 

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