Transport carrier lifetimes

N. K. Dutta, Radiative Transitions in GaAs and Other III-V Compounds R. K. Ahrenkiel, Minority-Carrier Lifetime in III-V Semiconductors T. Furuta, High Field Minority Electron Transport in p-GaAs M. S. Lundstrom, Minority-Carrier Transport in III-V Semiconductors R A. Abram, Elfects of Heavy Doping and High Excitation on the Band Structure of GaAs D. Yevick and W. Bardyszewski, An Introduction to Non-Equilibrium Many-Body Analyses of Optical Processes in III-V Semiconductors... 

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,... 

Annealing in Gallium Arsenide. Gallium arsenide has a greater variety of defect interactions than silicon. Also, most gallium arsenide devices are based on majority carrier transport. This decreases the importance of the minority carrier lifetime. Therefore carrier activation is the primary purpose of the annealing process. 

Comparison of dye-sensitized ZnO and Ti02 solar cells studies of charge transport and carrier lifetime. J. Phys. Chem. C, 111, 1035-1041. 

As new metal oxide semiconductors are identified and explored for water photoelectrolysis, a persistent problem that arises is the low drift mobility of electrons and holes and the short lifetimes of photogenerated carriers. Many materials that seem promising on the basis of bandgap and stability are found inadequate for photoelectrolysis because the low mobility of minority carriers prevents rapid charge transport to the aqueous interface. Metal oxides are also prone to nonstoichiometry, which can result in traps that promote recombination and shorten carrier lifetimes. When short lifetimes and low mobility arise in the same material, device efficiency can drop rapidly and photocurrents are far below those that might be otherwise expected. 

The photoconductivity in polyacetylene, the simplest conjugated polymer, has been the subject of intensive investigations. Carrier mobilities in the range 1 to 100 cm A s have been observed [353-356]. Transient photoconductivity measurements on a picosecond time scale have been carried out [356-362]. These ultrafast methods form a powerful tool to investigate the transport properties as well as the recombination kinetics of charged excitations. It was found [357] that the photocurrent in /raw -polyacetylene consists of two components a fast component which relaxes on a picosecond time scale and for which a carrier mobility of about 1 cm A s was reported [360,361] and a slow component with carrier lifetimes of up to seconds. 

The utility and importance of multi-layer device structures was demonstrated in the first report of oiganic molecular LEDs [7]. Since then, their use has been widespread in both organic molecular and polymer LEDs [45, 46], The details of the operating principles of many multi-layer structures continue to be investigated [47—49], The relative importance of charge carrier blocking versus improved carrier transport of the additional, non-luminescent layers is often unclear. The dramatic improvements in diode performance and, in many cases, device lifetime make a detailed understanding of multi-layer device physics essential. 

Figure 13. Numerically calculated PMC potential curves from transport equations (14)—(17) without simplifications for different interfacial reaction rate constants for minority carriers (holes in n-type semiconductor) (a) PMC peak in depletion region. Bulk lifetime 10" s, combined interfacial rate constants (sr = sr + kr) inserted in drawing. Dark points, calculation from analytical formula (18). (b) PMC peak in accumulation region. Bulk lifetime 10 5s. The combined interfacial charge-transfer and recombination rate ranges from 10 (1), 100 (2), 103 (3), 3 x 103 (4), 104 (5), 3 x 104 (6) to 106 (7) cm s"1. The flatband potential is indicated.

The mechanism of facilitated transport involves using the metal ion only in its reduced state in the oxidized state the oxygen-carrying capacity is virtually nil. It is thus natural that electrochemical processes should be attempted to improve both the flux and selectivity obtained with the membranes described above by exploiting this 02 capacity difference. For example, the best of the ultra-thin membranes developed by Johnson et al. [24] delivered oxygen at a rate equivalent to a current density of only 3 mA/cm2, at least an order lower than that achievable electrochemically. Further, the purity was but 85% and the lifetime of the carrier less than a year. 

Significant curvature may be observed in the case of lifetime- (and intensity-) based sensors, mainly when the relation knri [Parameter]) is not linear. Figure 9.4 shows this type of nonlinear behavior for a fiberoptic oxygen sensor. The figure shows Stern-Volmer-type plots (r l versus [02]) at four different temperatures. The curvature is caused by the inability of the carrier to transport oxygen proportionally to the equilibrium partial pressure of oxygen. 

When electrons are injected as minority carriers into a/>-type semiconductor they may diffuse, drift, or disappear. That is, their electrical behavior is determined by diffusion in concentration gradients, drift in electric fields (potential gradients), or disappearance through recombination with majority carrier holes. Thus, the transport behavior of minority carriers can be described by a continuity equation. To derive the p—n junction equation, steady-state is assumed, so that dnp/dt = 0, and a neutral region outside the depletion region is assumed, so that the electric field is zero. Under these circumstances, the continuity equation reduces to a diffusion equation (eq. 10), where is the lifetime of minority... 

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... 

For reasonable functioning of these low cost, low mobility semiconductor solar cells, a considerable amount of the photogenerated chemical potential epc — fv of the electron hole ensemble must be used for carrier transport. An acceptable charge collection may be achieved if the extraction times for electrons and/or holes are smaller than their recombination lifetimes, i.e.,... 

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