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It is shown that low-frequency noise (LFN) in SiGe-base HBTs and SiGe-channel pMOSFETs can be made significantly lower than it is in their all-Si counterparts. The noise reduction in the HBTs results principally from the elimination of the emitter/base interfacial oxide, which, in the Si bipolar junction transistors, serves to boost the current gain. A reduction in the LFN in the SiGe channel pMOSFETs is also possible, but requires a careful design of the structure, as the power spectral density (PSD) of drain current fluctuations is often a non-monotonic function of their structure parameters. Optimisation of the LFN requires a simulation of the PSD. This can be done using an analytical model assuming (i) carrier-number fluctuation noise generation by trapping/release of charges at the SiO2/Si interface, resulting in correlated fluctuations in the retrograded SiGe channel and in the SiO2/Si interface channel, and (ii) mobility fluctuations in each channel. The quality of the SiO2/Si interface is crucial for the overall LFN properties of the MOSFETs, as it is responsible for the number fluctuation noise which is predominant in the range of currents commonly used.

Measurements of the source to drain current/voltage, Id(Vds), characteristics and low frequency (f < 100kHz) noise spectral density function, SV(f), were carried out at 77K < T < 300K on the vertical JFET (BiCMOS-compatible technology). We found that Id(Vds) characteristics do not significantly depend on T, down to 100K, thus low temperature applications of the device are plausible. At f < 10kHz, SV(f) was found to be dominated by the 1/f component. At low T, SV(f) shows features characteristic for the generation-recombination process, which become stronger at large and negative gate bias polarizations. This suggests that the recombination centers are localized at the interface between the channel and the depletion region near the gate.

It was recently shown that co‐expression of adenovirus type 3 (Ad3) penton base and fibre in the baculovirus system produces dodecahedral particles, as does the expression of the penton base alone. The structure of both of these dodecahedral particles, with and without fibre, has been determined by cryoelectron microscopy and 3‐dimensional reconstruction techniques to a resolution of 25 and 20 A, respectively. The general form of the penton base resembles that of the base protein in the recent reconstruction of adenovirus type 2. There is a remarkable difference in the penton base structure with and without the fibre. The five small protuberances on the outer surface of each base move away from the 5‐fold axis by approximately 15 A when the fibre is present. These protuberances are of relatively low density and most probably represent a flexible loop possibly containing the RGD site involved in integrin binding. The fibre is apparently bound to the outer surface of the penton base, rather than inserted into it. The fibre is flexible and the shaft contains two distinct globular regions 26 A in diameter. The volume of the inner cavity of the dodecahedron is 350 +/− 100 nm3. This small volume precludes the use of the inner cavity to house genetic information for gene therapy; however, the possibility remains of linking the gene to the dodecahedron surface in the hope that it will be internalized with the dodecahedron.

Conductivity, thermoelectric power, spin concentration and spin dynamics has been measured in polyaniline samples equilibrated at different pH. Data provide evidence for Metal-Insulator transition upon pH variation.

Formation of complex I between phage f2 RNA and coat protein, leading to repression of phage RNA polymerase synthesis, depends nonlinearly upon the concentration of the coat protein. Maximum formation of complex I was observed when six molecules of coat protein were bound to one molecule of RNA. RNase digestion of a glutaraldehyde-fixed complex left, as the products, coat protein oligomers. The heaviest, hexamers, predominated in the mixture. It was also shown that, in an ionic environment required for phage protein synthesis, coat protein at a concentration optimum for complex I formation exists in solution as a dimer. The results indicate that the translational repression of the RNA polymerase cistron is due to a cooperative attachment to phage template of three dimers of coat protein, forming a hexameric cluster on an RNA strand.

Formation of complexes between f2 RNA polymerase cistron was partially inhibited, some RNA and coat protein was studied using salt conditions which are optimum for phage protein synthesis. In this ionic environment, coat protein precipitation can be prevented by sulfhydryl group-protecting agents. Complexes formed at different protein-RNA input molar ratios were isolated and tested for template activity in an in vitro protein synthesizing system. Simultaneously, the number of protein molecules bound per RNA strand in such complexes was measured by the membrane (Millipore) filtration technique. Under conditions in which translation of the RNA strands were complexed with six molecules of coat protein, whereas some remained unbound. Strong inhibition of the translation of the RNA polymerase cistron was observed when each of the RNA strands present in the mixture was associated with six molecules of coat protein.

The resistivity of p-Ge with different acceptor concentrations NA has been measured at helium temperatures, at high stresses, and longitudinal and high magnetic fields. The ranges were: 15 K > T > 4.2 K, 1015 cm-3 < NA < 4 × 1016 cm-3, H < 8 T, tensile stresses up to 6 × 107 Pa and compressive stress up to 6 × 108 Pa. All the data are in the regime of hopping transport. The data is conveniently presented in terms of the activation energy ε3, and the pre-exponential ρ3. The main experimental features are as follows. The activation energy plotted against uniaxial stress X exhibits a peak for both compressive and tensile stress (although tensile stress was not large enough to observe the peak clearly). Around the stress where this peak occurs, the pre-exponential ρ3 begins to increase rather rapidly. Magnetic fields reduce the peak for compressive stress, and enhance the peak for tensile stress. At very high compressive stresses ρ3 decreases with stress. For high concentrations, ε3 also decreases with increasing stress at high stresses, but for low concentrations ε3 is rather flat there.We have developed a percolation theory for hopping transport through acceptor states which consist of an oblate and a prolate wavefunction, with an arbitrary energy splitting between these functions. Making this energy splitting depend on stress, and the size of the wavefunctions on magnetic field, we were able to reproduce most of the important experimental features, at least qualitatively. Some experimental features were not reproduced by the results of the percolation theory. These are mainly concentration-dependent features. Such features are in qualitative agreement with predictions for transport by correlated multi-electron hopping.The important conclusions of our experiments are (1) we have a correct understanding of many features of the acceptor wavefunctions, (2) percolation theory provides a good description of one electron hopping transport, and (3) multi-electron hopping processes make an important contribution to hopping transport in p-Ge for concentrations above 3 × 1015 cm-3.

The effects of uniaxial strees and strong magnetic fields on hopping conduction in lightly doped semiconductors are reviewed. The piezoresistance and magnetoresistance are interpreted in terms of modification of the shallow impurity states, making extensive use of the results of percolation theory. It is shown that our present understanding of the stress effects on shallow donor and acceptor states provides an adequate explanation of piezoresistance in both n- and p-type semiconductors. The effects of the magnetic field on the donor states are known sufficiently well to account for the magnetoresistance. However, the magnetoresistance in p-type semiconductors remain largely unexplained owing to the lack of detailed knowledge of magnetic field effects on shallow acceptor states.

Variational wavefunctions which show the correct asymptotic behaviour are constructed for the ground state of shallow acceptors in Ge and Si, utilizing the spherical tensor representation of the effective-mass Hamiltonian of Baldereschi and Lipari (1973), under a uniaxial stress X resulting from the application of a tensile or compressive force along the [001] orientation (respectively X < 0 or X > 0). The energies of the components of the ground state, computed variationally, account very well for the X-induced displacements of the binding energies and the stress splitting of shallow acceptors in both Ge and Si (at X > 0; no data for X < 0 are available). The wavefunctions were also used to compute the stress variation of resistivity ρ (X) for hopping conduction involving single transitions between the stress-split components of the ground acceptor state, within the framework of percolation theory. Graphic representations of the critical surfaces for percolation, and their evolution with X, are given. The calculations account adequately for ρ(X) in Ge samples with a low acceptor concentration for both X < 0 and X > 0. However, they account only qualitatively for the available ρ(X) data for Si (X > 0 only), probably because of a larger chemical shift of the acceptor ground state in Si and its possible variation with X. At larger acceptor concentrations ρ(X) decreases, at large X, much more strongly than predicted for either Ge or Si. This discrepancy is attributed to an increase in the contribution to electron transport of multiple-hopping transitions at large X values.

Impurity conduction, ρ-1, of Si is analysed through the parameters ρi and εi which appear in the expression ρ-1 = 3εi=1 ρ-1exp (- εi/kT) where the i=1 term designates conductivity associated with the impurity ionization, i = 2 the activation to the A+ impurity band, and i = 3 represents hopping conduction. In the absence of stress : (a) the dependence of ρ3 upon the concentration of majority impurities yields, by percolation theory arguments, the decay constants of the impurity wave- functions; these values are : 17 Å for Al, 21 Å for B, and 20 Å for the Sb impurity; (b) The function ε3(Nmaj, T) is calculated from the position of the Fermi level in the impurity band; for T→0 the calculated values of ε3 agree very well with experiment for a finite T; ε3 varies with T and from the form of the ε3(T) function the compensation for lightly doped and lightly compensated Si samples can be established.A similar analysis was also performed for Si samples subjected to a [001] uniaxial compression; the maximum value of the stress, X, was 1·6 × 109 Pa. Following former work on Ge, the samples were classified into three categories : low concentration region I (LCR I), LCR II and intermediate concentration region (ICR). It was observed that : (a) in LCR I, ε1 decreases sublinearly with × and is reduced by about 30% at the highest ×; (b) ρ3 in the LCR I exhibits a pronounced maximum at × ∼ 3 × 108 Pa, and ε3 remains practically stress-independent; (c) in the ICR both ε2 and ε3 exhibit non-monotonic behaviour, the maximum of ε2 occurs at about 4 × 108 Fa, and the maximum of ε3 is at values of X shifting toward higher values of × with decreasing impurity concentration, from X = 5 × 108 to 8 × 108 Pa. The behaviour of ρ1 with X in LCR I is found to be consistent with the behaviour of the ionization energy of acceptor impurities reported by other workers in the low-stress limit. The behaviour of ρ3 in LCR I is explained by the effects of stress on the acceptor wavefunction, in analogy to the former work on Ge. The initial depopulation of one of the ground-state components, followed by the expansion of the populated component, is responsible for the non-monotonic variation of ρ3(X). The stress variation of the exchange interaction between the A+ states is argued to be responsible for the behaviour of the ρ3(X) function. The nonmonotonic variation of ε3 in ICR is argued to follow from the effect of stress on the electron-electron interaction, yielding correlated hopping effects. The latter two effects are related to the stress variation of the overlap of the acceptor wave-functions in the ICR.The stress effects on the band states and on the ground states of acceptors in the effective-mass approximation are discussed in detail.

It was recently shown that co‐expression of adenovirus type 3 (Ad3) penton base and fibre in the baculovirus system produces dodecahedral particles, as does the expression of the penton base alone. The structure of both of these dodecahedral particles, with and without fibre, has been determined by cryoelectron microscopy and 3‐dimensional reconstruction techniques to a resolution of 25 and 20 A, respectively. The general form of the penton base resembles that of the base protein in the recent reconstruction of adenovirus type 2. There is a remarkable difference in the penton base structure with and without the fibre. The five small protuberances on the outer surface of each base move away from the 5‐fold axis by approximately 15 A when the fibre is present. These protuberances are of relatively low density and most probably represent a flexible loop possibly containing the RGD site involved in integrin binding. The fibre is apparently bound to the outer surface of the penton base, rather than inserted into it. The fibre is flexible and the shaft contains two distinct globular regions 26 A in diameter. The volume of the inner cavity of the dodecahedron is 350 +/− 100 nm3. This small volume precludes the use of the inner cavity to house genetic information for gene therapy; however, the possibility remains of linking the gene to the dodecahedron surface in the hope that it will be internalized with the dodecahedron.

Susceptibility of pancreatic islets to oxidant stress may affect islet viability and contribute to primary non function of allo- or xenogenic grafts. We investigated the influence of overexpression of catalase (CAT) on the viability of human, porcine and rat islets, as well as INS-1 beta-cell line. Islets were transfected with a replication-deficient adenovirus vector containing human CAT cDNA under the control of the adenovirus major late promoter (AdCAT) or a vector containing no foreign gene (AdNull) and used as a control. Oxidant stress was induced 48 h later by xanthine oxidase-hypoxanthine (XO 25 mU/ml, HX 0.5 mmol/l) or hydrogen peroxide (100 or 250 μmol/l). Islet cell viability was assessed 72 h after CAT transfer by 4-[3-(4-Idophenyl)-2-(4 nitrophenyl)-2H-5-tetrazolio]-1,2,benzene disulphonate (WST-1) test. Baseline catalase activity was three to fourfold lower in porcine than in human islets. CAT activity was reproducibly increased 2.5- to 7-fold in AdCAT infected islets, at least for 13 days. Overall, AdCAT conferred on human and pig islets a protection of 26.1 ± 6.1 and 21.2 ± 9.8 % on XOHX injury and 35.4 ± 4.2 and 57.9 ± 10.5 % on H2O2stress. Similarly, rat islet cells and INS-1 cells were protected on XOHX stress by 17.8 ± 2.3 and 30.8 ± 8.7 %, respectively. AdNull had no effect. Basal and stimulated insulin secretion was preserved in AdCAT-transfected human islets despite a XOHX challenge. This study validates adenovirus-mediated catalase gene transfer as a realistic approach to reduce non specific inflammation effects on human or porcine islet grafts. Moreover the relevance of defense mechanisms, previously suggested in human islets, is here illustrated in porcine islets. [Diabetologia (1998) 41: 1093–1100]Susceptibility of pancreatic islets to oxidant stress may affect islet viability and contribute to primary non function of allo- or xenogenic grafts. We investigated the influence of overexpression of catalase (CAT) on the viability of human, porcine and rat islets, as well as INS-1 beta-cell line. Islets were transfected with a replication-deficient adenovirus vector containing human CAT cDNA under the control of the adenovirus major late promoter (AdCAT) or a vector containing no foreign gene (AdNull) and used as a control. Oxidant stress was induced 48 h later by xanthine oxidase-hypoxanthine (XO 25 mU/ml, HX 0.5 mmol/l) or hydrogen peroxide (100 or 250 μmol/l). Islet cell viability was assessed 72 h after CAT transfer by 4-[3-(4-Idophenyl)-2-(4 nitrophenyl)-2H-5-tetrazolio]-1,2,benzene disulphonate (WST-1) test. Baseline catalase activity was three to fourfold lower in porcine than in human islets. CAT activity was reproducibly increased 2.5- to 7-fold in AdCAT infected islets, at least for 13 days. Overall, AdCAT conferred on human and pig islets a protection of 26.1 ± 6.1 and 21.2 ± 9.8 % on XOHX injury and 35.4 ± 4.2 and 57.9 ± 10.5 % on H2O2stress. Similarly, rat islet cells and INS-1 cells were protected on XOHX stress by 17.8 ± 2.3 and 30.8 ± 8.7 %, respectively. AdNull had no effect. Basal and stimulated insulin secretion was preserved in AdCAT-transfected human islets despite a XOHX challenge. This study validates adenovirus-mediated catalase gene transfer as a realistic approach to reduce non specific inflammation effects on human or porcine islet grafts. Moreover the relevance of defense mechanisms, previously suggested in human islets, is here illustrated in porcine islets. [Diabetologia (1998) 41: 1093–1100]

The stress-by-substrate bending technique has been used to evaluate the effect of stress on the superconducting critical temperature Tcof cobalt disilicide (CoSi2) thin films. The effect is small and does not explain the rapid decrease in Tcas the film thickness is reduced. Results at zero stress show that Tcis a sensitive probe of the homogeneity of CoSi2thin films.

Tyrosyl-tRNA synthetase (TyrRS) was purified 5,000-fold from wheat germ extract by ultracentrifugation, precipitation with ammonium acetate, and column chromatography. Under denaturing conditions the enzyme ran as a single band on SDS-polyacrylamide electrophoresis with an apparent Mr of 55,000. The native molecular weight determined by gel filtration was 110,000, suggesting a quaternary structure of an alpha 2 type for native TyrRS. Purified enzyme activity, based on the aminoacylation reaction, was studied in terms of Mg2+, ATP, pH, and KCl dependence. Optimum concentrations were 6 mM Mg2+, 4 mM ATP, and 200 mM KCl at pH 8. The Km values for ATP, tyrosine, and tRNA were 40, 3.3, and 1.5 microM, respectively. The instability of the TyrRS activity and the methods used for stabilizing it are discussed. In wheat germ extract we found a second tyrosylating activity that works with Escherichia coli tRNA, but not with wheat germ tRNA. We believe that this enzyme is the mitochondrial tyrosyl-tRNA synthetase of wheat germ.

The trimeric fiber of adenovirus type 2 (Ad2) mediates the first stage of virus–cell attachment, and the distal head region of the fiber has been implicated as the receptor-binding domain. To locate regions on the primary polypeptide sequence of the fiber which may be involved in virus–cell interaction, peptide-based epitope mapping was performed using (1) polyclonal antibodies prepared against both native Ad2 fiber and Ad2 head protein expressed inEscherichia coliand (2) 18 monoclonal antibodies prepared against trimeric Ad2 head protein expressed in baculovirus. The approach using polyclonal antibodies revealed eight domains on the primary sequence of the head which contain one or more continuous epitopes. At least two of these regions were also recognized by monoclonal antibodies reacting against both monomeric and trimeric fiber head protein. The majority of monoclonal antibodies which did not recognize Ad2 head-specific peptides in ELISA were also nonreactive against the monomeric form of protein in Western blot, suggesting that their recognition of trimer is due to the existence of as yet undefined discontinuous epitopes or to alterations in monomer configuration. Our results correspond well with the recently published X-ray crystallographic model of Ad5 fiber head (D. Xia, L. J. Henry, R. D. Gerard, and J. Deisenhofer,Structure2, 1259–1270, 1994), since most antigenic determinants containing linear epitopes mapped to the outer loops or uppermost β-sheets in this structure. Four of five neutralizing monoclonal antibodies recognized trimer only and none recognized linear peptides. This might suggest that the trimeric form of fiber is necessary for making contact with the receptor(s) and that discontinuous epitopes on the head domain may be involved in fiber–cell interaction.

An analytical WKB approximation of the numerical results of Rösner et al. (1984) for the hydrogen ground-state wavefunctions at megatesla fields is used to solve the percolation problem for transport involving donors in direct-gap semiconductors. We show that the transport coefficients thus obtained account for the magnetoresistance data in GaAs, to an accuracy comparable with previous theoretical work.

We were able to isolate viral fiber and penton from Ad3-infected KB cells using for their detection antibodies obtained against recombinant Ad3 fiber. The native material was examined by electron microscopy and the characteristic fiber shape of a shaft terminated by a globular head was observed. The native fiber was compared with two recombinant fibers synthesized in Escherichia coli cells. One, the Ad3 fiber protein expressed in E. coli with a 14-amino acid NH2-terminal fusion peptide, under the control of the T7 promoter has been described previously. The second is a recombinant Ad3 fiber without the fusion peptide (recAd3fib), expressed in the same system. As with the fusion protein recAd3fib was found to be insoluble upon expression. It was solubilized in 6 M urea and the gradual removal of urea during the purification cycle led to a soluble preparation. Biochemical and biophysical studies show that, similarly to fusion fiber, recAd3fib self-assembles as trimers in prokaryotic cells. Electron microscopy shows that, whereas the fusion fiber consists of a population of heterogeneous particles, recAd3fib has the characteristic morphology and size of the Ad3 trimeric native fiber. Small angle neutron scattering gives a molecular weight consistent with a trimeric fiber and a radius of gyration consistent with the dimensions derived from electron microscopy. These results suggest that the fusion peptide at the NH2 terminus prevents correct protein folding. They also indicate that after solubilization with urea and subsequent renaturation a correctly folded eukaryotic oligomeric protein can be produced in E. coli.

The adenovirus fiber appears as a long, thin projection terminated by a knob (head). The fiber consists of a trimeric protein whose head domain is thought to interact with cell receptors. The head part (amino acids 388 to 582) of adenovirus type 2 fiber was produced in a baculovirus expression system. The purified protein was shown to cross-link into trimers. It was very resistant to proteolytic attack and seemed to attain a high degree of compactness. The head domain efficiently inhibited attachment of adenovirus to receptors on the surface of HeLa cells, thereby confirming the hypothesis that the head domain interacts with viral receptors.