Two-Carrier Structures

A device model to describe two-carrier structures is basically similar to that used for one carrier structures except that continuity equations for both carrier types are solved. The additional process that must be considered is charge carrier recombination. The recombination is bimolecular, R = y np), where the recombination coefficient is given by [43] 

An accurate description of a single-layer LED should be obtained by using the injection and transport properties of electrons and holes, determined independently from the Schottky energy barrier and single-carrier device measurements, to describe the two carrier LED structure. To test this procedure consider structures fabricated from the conjugated oligomer 2-metooxy-5-(2 -etoylhexyloxy)- 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% at a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excitons. 

An efficient LED has two essential characteristics (1) all of the injected carriers recombine in the device and (2) the operating voltage is as low as possible. To produce light efficiently, all of the current should lead to recombination. This 

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In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

The central role of the thiolation domain is evident from their multiple interactions with adjacent and nonadjacent domains. The current multiple-carrier thiotemplate model predicts successive contacts of the first thiolation domain of ACV synthetase with the adjacent aminoadipate adenylate domain and the first condensation domain the second thiolation domain to interact with the cysteine adenylate domain, and both the adjacent and nonadjacent condensation domains the third thiolation domain then interacts with the valine adenylate domain, the nonadjacent second condensation domain, the adjacent epimerization domain, and the nonadjacent thioesterase domain (Fig. 2). Protein regions involved in these successive protein-protein interactions involve the highly conserved carrier domain structures of only about 80 amino acids. So far, only two detailed structures of the respective NRPS domains are available [61,108],... 

Flow switches using fluidics in small Reynolds number are fabricated. The principle of the flow switch is shown in Fig. 4 [21]. Mixing of the sample stream and carrier liquid is negligible when the contact area is small and the contact time within subsecond range. The width of the sample stream is controlled by two carrier flows. This structure can be applied for a valveless sample injection in FIA and for sorting of particles and living cells in flow cytometry. A flow switch having 5 outlets has also been obtained by this method. 

On Figure 8 the hole concentration is plotted against vacancy concentration. A theoretical curve calculated on the basis of two carriers per vacancy is also shown. There is a very poor agreement between the two curves. To get the theoretical curve to fit the experimental points, it is necessary to assume more than two carriers per vacancy. This is difficult to reconcile. It is apparent that SnTe is not a simple semiconductor like GeTe. At present single crystals of SnTe are being investigated at our laboratory, at the U. S. Naval Ordnance Laboratory, and at Lincoln Laboratory. Preliminary results indicate a complex band structure for this compound (1,2,5). 

For the majority of industrial catalysts, the sizes of supported metal particles arc less than the mean free path of the electrons analysed. All the metal in the particles is effectively analysed. For highly dispersed systems, XPS surface analysis and bulk X-ray nuorcsccncc analysis therefore give similar results. Comparing information from these two techniques can be used to show a change in the distribution of metals on the surface due, for example, to sintering or to the inclusion of one of the metals into the carrier structure. 

The self-consistent theoretical models based on the Boltzmann transport theory are used to characterize the microscale heat transfer mechanism by explaining mutual interactions among lattice temperature, and number density and temperature of carriers [12]. Especially, a new parameter related with non-equilibrium durability is introduced and its characteristics for various laser pulses and fluences are discussed. This study also investigates the temporal characteristics of carrier temperature distribution, such as the one- and two-peak structures, according to laser pulses and fluences, and establishes a regime criterion between one-peak and two-peak sttuctures for picosecond laser pulses. 

Criteria for immobilized liquid membrane (ILM) support selection can be divided into two categories structural properties and chemical properties. Structural properties include geometry, support thickness, porosity, pore size distribution and tortuosity. Chemical criteria consist of support surface properties and reactivity of the polymer support toward fluids in contact with it. The support thickness and tortuosity determine the diffusional path length, which should be minimized. Porosity determines the volume of the liquid membrane and therefore the quantity of carrier required. The mean pore size determines the maximum pressure difference the liquid membrane can support. The support must be chemically inert toward all components in the feed phase, membrane phase, and sweep or receiving phase. 

The two-domain, structural motif in FNR represents a common structural feature in a large class of enzymes that catalyze electron transfer between a nicotinamide dinucleotide molecule and a one-electron carrier. Beside the photosynthetic electron-transfer enzyme, others non-photosynthetic ones include flavodoxin reductase, sulfite reductase, nitrate reductase, cytochrome reductase, and NADPH-cyto-chrome P450 reductase. FNR belongs to the group of so-called dehydrogenases-electron transferases, i.e., flavoproteins that catalyze electron transfer from two, one-electron donor molecules to a single two-electron acceptor molecule. 

Figure 11-12. Comparison of measured and calculated current density as a function of bias for a two carrier Ca/MEH-PPV/Au structure. The main part of the figure is linear-linear plots and the inset is log-linear plots.

Fig. 2.5. A schematic diagram showing the formulation of the energy structure of butadiene as a particle in a box problem. Only the pi electrons are shown. In the free pi-electron model (which is a simplification) each state can be occupied by two carriers, each with opposite spin.

Not only is the structure of the sex pheromone identical in both moths and Asian elephants and its function similar, but the study of protein carriers promises to be equally fascinating. The delineation of postulated protein carriers for this acetate has the potential to provide similar evolutionary information about lipocalin-like urinary proteins and pheromone binding proteins. Insect pheromone binding proteins that bind Z7—12 Ac have a different amino acid composition and two-dimensional structure than vertebrate odorant binding proteins so far described, including major urinary proteins (Robertson, Cox, Gaskell, Evershed Beynon, 1996 Steinbrecht, 1996 Pelosi, 1994). 

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