Nonuniformities junctions

If a uniform metal/semiconductor junction exists and the transport is by thermionic emission, then the ideality parameter n will be nearly 1. There are two obvious possibilities that can cause deviations from 1. These are (1) recombination in the depletion region and (2) a nonuniform junction. In the first case, if the current transport is diffusion limited, then significant recombination may occur, and this will lead to a value of n that can be as high as 2. It should be noted, however, that even if the current is diffusion limited but no recombination occurs, then n will still approach I. 

For the case of a nonuniform junction, the barrier height may vary from place to place. The T- V measurement will be dominated by the lower barrier-height regions, but the characteristics will exhibit a hi ideality value (Freeouf et al, 1982). The aspect of interface formation that can lead to or minimize these effects are described in Part IV. 

MIECs may be made nonuniform to the extent that they become n-type on one side and p-type on the other side, thus forming pn or pin (/ = intrinsic) junctions. Zr02 + 10 mol % Y2O3 subject to an oxygen partial pressure, Pq, gradient at elevated temperatures becomes p-type near the high (P —l atm) side and n-type near the low P,... 

Anisotropy and nonuniformity are at least in part due to inhomogeneities in the distribution of gap junctions and the biophysical properties of the tissue are in fact influenced by the intercellular coupling. At least four features have to be considered. (1) Cardiac cells express different gap junction proteins (so-called connexins in the heart, connexin 40, connexin 43 and connexin 45 are most abundantly found for details see chapters 2 and 3). Channels formed by these connexins are different with regard to their biophysical properties. In various parts of the heart the content of each of these isoforms is different. 

What is the role of the gap junctions By coupling the myocardial cells in both directions (longitudinal and transverse) they are responsible for the biophysical properties of the tissue. A reduction in gap junction distribution or a closure of the gap junction channels causes nonuniformities and discontinuities which alter the biophysical properties of the tissue and make it more prone to nonuniform anisotropic reentry. According to the model proposed by Krinsky [1966], a reduction in gap junctions or a closure of gap junction channels will lead to local slowing of conduction, thereby allowing smaller perimeters of reentrant arrhythmia. In addition, slowing of conduction is generally believed to be a risk factor for initiation of reentry. Since in many... 

Furthermore, synchronization of contraction is facilitated by gap junctional communication as well as synchronization of electrical activation. The electrical coupling between cardiomyocytes mitigates differences in the membrane potential between these cells, for example in the course of an action potential if both cells repolarize at different timepoints. This results in smaller differences in the repolarization times thereby causing a reduction in the dispersion of the action potential duration. Since increased dispersion is known to make the heart more prone to reentrant arrhythmia, sufficient gap junctional communication can be considered as an endogenous arrhythmia-preventing mechanism. For a detailed discussion of the role of gap junctional communication in the biophysics of cardiac activation as related to anisotropy, nonuniformity and stochastic phenomena, see chapter 1 for a discussion of their role in arrhythmia, see chapter 6, and for a possible pharmacological intervention at the gap junctions for suppression of arrhythmia, refer to chapter 7. 

However, it is well known that with increasing age microfibrosis is observed which in turn will seperate the fibers from each other and thereby enhance the degree of nonuniformity as discussed in the first chapter of this book. This is accompanied by a reduction in side-to-side connections [Spach and Dolber, 1986]. Thus, with increasing age the intercellular communication can be expected to be reduced probably due to structural changes in the tissue with deposition of collagenous fibers. Concomitant changes in the gap junction distribution are probably secondary to cardiac diseases, although at present an effect of age per se cannot be excluded. 

In the present system nonuniformities in P and T are encountered only at the junction between the vessels accordingly, VP and VT may be replaced by the pressure and temperature differences at the junction, AP and AT, respectively the thickness of the connecting unit may be absorbed in the coefficients L. 

A set of generated nonuniform geometries by direct assembly of polystyrene (PS)/poly-acrylic acid (PAA) blends (a) 90° bends, (b) T-junctions, (c) square arrays, and (d) circle arrays. (Reprinted with permission from Wiley.)... 

Essentially there exist two sources of electric field nonuniformities at the reservoir-microchannel junction One is due to the reduction in cross-sectional area from reservoir to microchannel which gives rise to electric field gradients primarily parallel to the streamlines. 

When a trapped flux was present, the double peak structures were found to be deformed asymmetrically, which was attributed to a nonuniform magnetic field across the junction barrier. 

The cells observed in these membranes obviously have nonuniform curvature, i.e., the cells are not perfectly spherical. This nonuniform curvature leads to a flow process which results in the migration of material from cell walls to cell junctions. 

---