Metal-N-type semiconductor ohmic contact with a shallow N+ surface layer

Most papers covering metal-semiconductor ohmic contact theory which have been published up to date consider systems with homogeneous impurity concentration in the semiconductor. However, there are techniques of ohmic contact formation on nondegenerate semiconductor where only a very shallow surface layer is impurity enriched. In this paper a model of such contacts is proposed and a simple approximate analytical expression for the specific resistivity is derived. If the impurity concentration in the surface layer is very high, the contact specific resistivity is essentially proportional to N_B^?1, N_B being the semiconductor substrate impurity concentration. To make a good ohmic contact, it is sufficient that the width of the heavily doped surface layer be equal to the equilibrium contact depletion region width. Any further enlargement of the enriched layer practically does not influence the total sample resistance due to the dominant share of the semiconductor body resistance. Experimental results confirm these conclusions qualitatively.     

Getreide,Getreideerzeugnisse u. dgl.

European Food Research and Technology -     

Stepwise decreasing current modes. I. Electrodeposition of metals from a bath with insoluble anode and limited volume of solution

A model of the stepwise decreasing current mode of electrodeposition of metals has been developed and checked with experiments. The electrodeposition of copper has been used for this purpose. It has been shown that the deposition time in the stepwise decreasing current mode, for sufficiently high step number, becomes close to that potentiotiostatic deposition.Nomenclature C concentration - C ₀ initial concentration - D diffusion coefficient - F Faraday constant - i integer - I current - I ₀ initial limiting diffusion current - k integer - m number - n number - Q quantity of electricity - S electrode surface area - t time - t _k ^* deposition time fork current steps - t ^* lim_k t _k ^* - t _m deposition time required to decrease concentration from (m + 0.1)C ₀ tomC ₀ byI=mI ₀ - t _n deposition time required to decrease concentration fromC ₀ tonC ₀ byI =nI ₀ - U cell voltage - V volume of solution - z number of electrons - thickness of the diffusion layer - defined by Equation 3     

Studies of the deposition of cadmium on foreign substrates

Cadmium deposition on nickel, lead, silver, gold and copper has been studied by the potential sweep and potentiostatic method with a simultaneous observation of the surface by optical microscopy. It was found that the substrates could be divided into two groups—those at which there is no significant underpotential deposition of cadmium and those at which the latter does take place. In the former case non-epitaxial growth predominates, once some nucleation overpotential (of about 16 mV) is overcome; the potential sweep diagrams are characterized by a loop exhibiting an inductive nature. Two maxima on the anodic branch of the voltammogram could be ascribed to the dissolution of non-epitaxial and epitaxial deposits. Microscopic observations confirmed those conclusions. Well developed granular deposits have been obtained with grains increasing size while remaining constant in number.     

Catecholamine uptake, accumulation, and release in acute porphyria

Hypertension and tachycardia are well known features of acute porphyria and have been shown to be related to increased circulating catecholamines. The mechanism by which circulating catecholamines are increased was studied using the isolated perfused rat heart and human platelets as a model of adrenergic neuronal function. It was found that neither delta-aminolevulinate (ALA) nor porphobilinogen (PBG) blocked uptake or caused release in the isolated perfused rat heart. Platelets from six patients with acute prophyria, three in remission and three latent, with matching normal controls were studied with regard to their uptake of [(3)H]norepinephrine in the presence of ALA or PBG. It was found that ALA and PBG significantly reduced uptake and accumulation of [(3)H]-norepinephrine in patients with acute porphyria; however, no similar reduction in uptake and accumulation was observed in the platelets of normal controls. Therefore, it appears that there is a latent defect in the catecholamine uptake and (or) accumulation of platelets of patients with acute prophyria which only manifests itself in the presence of ALA or PBG. If platelet uptake serves as a model of adrenergic neuron uptake, this suggests that elevated circulating catecholamine levels during acute attacks of acute porphyria are caused at least partially by blockade of re-uptake into the sympathetic neurons.     

Kinetics and mechanism of deposition of zinc from zincate in concentrated alkali hydroxide solutions

Galvanostatic investigation has been carried out of the kinetics of the reaction on zinc amalgam hanging mercury drop immersed in alkaline zincate solutions at KOH concentrations ranging from 1 to 14 normal. Three kinds of data have been extracted: (i) the pseudo-capacitance as a function of overpotential from the portion of the charging curves prior to plateaux, (ii) the Tafel plots from quasi-steady state values at the plateaux and (iii) the Sand's products as function of current density from transition times. In (i), the pseudo-capacitance was found to increase with overpotential, in (ii), two slopes (60 mV dec^?1 and 120 mV dec^?1) have been found in the cathodic direction and a limiting current in the anodic one; in (iii), the Sand's product was found to decrease with increasing (i). On the basis of the above findings the reaction mechanism has been suggested to be of the cece type, with the chemical dissociation of the intermediate univalent zinc complex in between the two elctrochemical steps as rate-determining. A modification of the method of evaluating the reaction orders was introduced to account for a simultaneous change of more than one activity of reactants in concentrated solutions. Using this, reaction order was found to be 2 with respect to both OH^? ions and water, suggesting that the electroactive species is not only reduced in the number of ligands but also dehydrated with respect to the prevailing species. The anodic limiting current appears to be due to the accumulation of Zn(OH) species to cover all the surface free of adsorbed Zn(OH)₂. Exchange cd and rate constants of all the four steps of the reaction have been estimated.     

Potential of proton-enhanced 13C n.m.r. for the classification of kerogens

High resolution n.m.r. spectroscopy, involving the technique of cross-polarization, along with magicangle spinning, was used in the structural characterization of eight kerogens of different origins, selected to represent the three types of kerogens (types I–III evolution paths). The influence of cross-polarization dynamics on the sensitivity of the method and the ratio of individual fractions in the spectrum was studied in more detail. It is suggested that an analysis of the influence of the mixing time is necessary prior to definitive characterization of any sample. Good separation of signals in aliphatic, aromatic, and carboxylic regions was achieved. The general correlation between the ¹³C n.m.r. structural characteristics and the classification based on ultimate analysis of the kerogens (types I–III, van Krevelen atomic H/C vs. O/C diagrams) was found to be satisfactory. The structural features of kerogens derived from ¹³C n.m.r. analysis agreed quite well with characteristics constituting the above mentioned classification. The ¹³C n.m.r. method used in this paper may be considered promising in the classification of kerogens.