Inverse Bias Research Articles

Radon measurements with a PIN photodiode

Silicon photodiodes are well suited to detect alphas coming from different sources as neutron reactions or radon daughters. In this work a radon in air detecting device, using an 18×18 mm silicon PIN photodiode is studied. The ionized airborne decay products formed during radon diffusion were focused by an accelerating high voltage to the PIN surface. Several conducting rings were disposed inside a cylindrical PVC vessel in such a way that they reproduced the electric field created by a punctual charge located behind PIN position. Alpha spectra coming from the neutral and ionized species deposited on the PIN surface, dominated by 218Po and 214Po progeny peaks, were recorded for varying conditions. Those include radon concentration from a Pylon source, high voltage (thousands of volts) and PIN inverse bias voltage. Different parameters such as temperature and humidity were also registered during data acquisition. The increase in the particle collection efficiency with respect to zero electric field was compared with the corresponding to a parallel plates configuration. A discussion is made in terms of the most appropriate voltages for different radon concentrations.

δ-Doped MOS Ge/Si quantum dot/well infrared photodetector

Metal-oxide-semiconductor tunneling diodes can be used as photodetectors at the inversion bias. To increase the responsivity, δ-doping is used in the quantum dot infrared photodetectors (QDIPs) and quantum well infrared detectors (QWIPs). The peak responsivity of the quantum dot and quantum well infrared photodetector at 15 K is found to be 0.03 mA/W and 1.3 mA/W, respectively, at a gate voltage of 1 V. The higher responsivity of the QWIP is probably due to the continuous two-dimensional density of states and smaller transition energy as compared to the QDIP. The QD photoresponse in the peak wavelength range 3.5–5 μm can be measured up to 100 K, while that for QW 3–7 μm can only be detected up to 60 K. A higher dark current due to the lower transition energy in the QWIP limits its operating temperature as compared to the QDIP.

Fermi gas response to time-dependent perturbations

We describe the Riemann-Hilbert (RH) approach to computing the long-time response of a Fermi gas to a time-dependent perturbation. The approach maps the problem onto a noncommuting RH problem. The method is nonperturbative, quite general, and can be used to compute the Fermi gas response in driven (out of equilibrium) as well as equilibrium systems. It has the appealing feature of working directly with scattering amplitudes defined at the Fermi surface rather than with the bare Hamiltonian. We illustrate the power of the method by rederiving standard results for the core-hole and open-line Green's functions for the equilibrium Fermi edge singularity (FES) problem. We then show that the case of the nonseparable potential can be solved nonperturbatively with no more effort than for the separable case. We compute the corresponding results for a biased (nonequilibrium) model tunneling device, similar to those used in single-photon detectors, in which a photon absorption process can significantly change the conductance of the barrier. For times much larger than the inverse bias across the device, the response of the Fermi gases in the two electrodes shows that the equilibrium Fermi edge singularity is smoothed, shifted in frequency, and becomes polarity dependent. These results have a simple interpretation in terms of known results for the equilibrium case but with (in general complex-valued) combinations of elements of the scattering matrix replacing the equilibrium phase shifts. We also consider the shot noise spectrum of a tunnel junction subject to a time-dependent bias and demonstrate that the calculation is essentially the same as that for the FES problem. For the case of a periodically driven device we show that the noise spectrum for coherent states of alternating current can be easily obtained using this approach.

Electrical Characteristics of Thin HfO2 Gate Dielectrics Prepared Using Different Pre-Deposition Surface Treatments

We have investigated the effects that various pre-deposition surface treatments, such as HF dipping (HF-dipped), NH3 surface nitridation (NH3-annealed), and rapid thermal oxidation (RTO-treated), have on the electrical properties of HfO2 gate dielectrics. The NH3-annealed technique is superior to the others because the dielectric subject to NH3 surface nitridation possesses a tremendously reduced leakage current, the lowest equivalent oxide thickness (EOT), and a moderate hysteresis width. In contrast, the RTO-treated preparation can only effectively reduce the leakage current by its resultant increased physical thickness and displays considerably severe hysteresis. We have also studied the dependence of hysteresis on the initial inversion bias (Vinv), temperature, and frequency for all splits. The hysteresis width increases upon increasing the initial inversion bias and decreasing the temperature, but it is rather insensitive to the measuring frequency. In addition, our experimental results indicate that the hysteresis width depends exponentially on both the initial inversion bias and the temperature, and it can be described well by a general empirical relationship that has the form C(T)·exp (RvVinv). Finally, the conduction currents through the dielectrics are probably dominated by trap-assisted tunneling (TAT) because the current densities display stronger temperature dependence at low voltage than they do at higher voltages. Based on the trap-assisted tunneling model, the corresponding parameters have been extracted and are presented.

Semiconductor lasers with an inversely biased active region for generation of millimeter wave modulated signals

A semiconductor laser structure is proposed to introduce field effects in its active region. It has five layers of p-n-i-p-n with four electrical contacts to the doped layers, where the current injection into an inversely biased active region is based on the operation of two complementary bipolar transistors with their base-collector junction common. The inverse bias is useful to incorporate carrier transfer phenomena like Gunn effect in semiconductor lasers. Our analysis on the proposed lasers shows that its lasing operation is possible while applying a high electric field more than 106 V/m in the carrier transfer domain.

Clarifying the origin of near-infrared electroluminescence peaksfor nanocrystalline germanium in metal-insulator-silicon structures

The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10nm exhibit strong luminescence at 1350nm. The 1350nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350nm peak. The 1350nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.

Pulse shape analysis for electron mobility study in cadmium zinc telluride gamma-ray detectors

A pulse shape analysis technique was implemented to determine the electron mobility /spl mu//sub e/ and the electron mobility lifetime product /spl mu//sub e//spl tau//sub e/ in cadmium zinc telluride detectors (CZT). The digital gamma finder (DGF-4C) a single-width CAMAC module produced by X-Ray Instrumentation Associates (XIA), was used to extract pulse height, pulse shape, and signal rise time information. Data analyses using the extracted information allowed measuring the /spl mu//sub e/ and /spl mu//sub e//spl tau//sub e/ in selected CZT samples. An almost linear relationship was observed for the signal rise time as a function of the inverse bias. This observed linear relationship was the basis for determination of /spl mu//sub e/ using a simple linear fit. The measured signal amplitude was also used to determine /spl mu//sub e//spl tau//sub e/ using the Hecht formulation. Repeated measurements confirmed the consistency of the method in determining /spl mu//sub e/ and /spl mu//sub e//spl tau//sub e/.

Laser Doppler velocity bias in separated turbulent flows

Velocity bias effects on data obtained with a coincident two channel laser Doppler velocimeter in a highly turbulent separated supersonic flow are presented. Probability distributions of the fluctuating velocities were distorted by velocity bias in a manner consistent with theory and a two-dimensional velocity inverse weighting function bias correction produced reasonable appearing velocity probability distributions. The addition of an approximate correction term to account for the effects of the unmeasured third velocity component improved these results but had little effect on the velocity statistics. Experimental factors that could partially compensate or falsely add to the velocity bias, conditions for the bias to occur, and conditions for which the bias may also be observed and corrected for are discussed.

Growth and Electrical Characteristics of Liquid-Phase Deposited SiO[sub 2] on Ge

The liquid-phase deposition (LPD) of SiO 2 at 50°C on Ge substrates is investigated. Silicic acid (SiO 2 .xH 2 O) is used to saturate hydrofluorosilicic acid at 30°C and this shortens the time required for solution preparation to 3 h. The growth rate of LPD oxide on Ge is much slower than that of LPD oxide on Si at the beginning of the deposition process, while surface roughness of LPD oxide on Ge is larger than that of LPD oxide on Si. A metal-oxide-semiconductor tunneling diode on Ge is fabricated using the LPD oxide. The tunneling current of the Ge diodes at inversion bias increases as LPD oxide thickness increases, indicating that the trap density in the LPD oxide increases with increasing oxide thickness, and the current transport is dominated by the trap assistant tunneling.

Using high temporal frequency data for CO2 inversions

Almost all previous inversions for CO2 sources and sinks from atmospheric measurements have used monthly or annual mean data. Here, we assess the potential benefits and challenges of using higher time resolution CO2 concentration data. Inversions are performed with synthetic data since there are currently only limited continuous CO2 measurements available. The use of synthetic data also enables the inversion quality to be evaluated in terms of both systematic biases in the estimated sources and the magnitude of the source uncertainties. When estimating sources for 22 large regions, we find that using high time resolution data gives large reductions in uncertainty compared to using monthly mean data. Greater reductions are achieved (around 80%) for smaller rather than larger networks (around 45%), although larger networks give lower uncertainties overall (0.35 Gt C yr−1 on average compared to 1.2 Gt C yr−1). In most cases, inversion biases are larger than the estimated uncertainties (by up to 6 Gt C yr−1) due to the assumptions made in the inversion about the spatial distribution of sources within regions. These biases can be significantly reduced by estimating the sources for smaller regions. This is demonstrated using a case study that subdivides Australia into the model grid cells. Sources are successfully estimated from high time resolution data for the individual grid cells and for the total Australian source.

Investigation of hole-tunneling current through ultrathin oxynitride/oxide stack gate dielectrics in p-MOSFETs

The systematic investigation of hole tunneling current through ultrathin oxide, oxynitride, oxynitride/oxide (N/O) and oxide/oxynitride/oxide (ONO) gate dielectrics in p-MOSFETs using a physical model is reported for the first time. The validity of the model is corroborated by the good agreement between the simulated and experimental results. Under typical inversion biases (|VG|<2 V), hole tunneling current is lower through oxynitride and oxynitride/oxide with about 33 at.% N than through pure oxide and nitride gate dielectrics. This is attributed to the competitive effects of the increase in the dielectric constant, and hence dielectric thickness, and decrease in the hole barrier height at the dielectric/Si interface with increasing with N concentration for a given electrical oxide thickness (EOT). For a N/O stack film with the same N concentration in the oxynitride, the hole tunneling current decreases monotonically with oxynitride thickness under the typical inversion biases. For minimum gate leakage current and maintaining an acceptable dielectric/Si interfacial quality, an N/O stack structure consisting of an oxynitride layer with 33 at.% N and a 3 /spl Aring/ oxide layer is proposed. For a p-MOSFET at an operating voltage of -0.9 V, which is applicable to the 0.7 /spl mu/m technology node, this structure could be scaled to EOT=12 /spl Aring/ if the maximum allowed gate leakage current is 1 A/cm/sup 2/ and EOT=9 /spl Aring/ if the maximum allowed gate leakage current is 100 A/cm/sup 2/.

Spiral phyllotaxis of needle fascicles on branches and scales on cones in Pinus contorta var. latifolia: Are they influenced by wood-grain spiral?

Neither the relationship between the chirality of spiral phyllotaxis and spiral wood grain nor the cause or ontogeny of such a relationship has been examined previously. To this end, chirality of the spiral in phyllotaxis of needle fascicles and cone scales were contrasted with wood-grain spiral in seedlings, young, and mature trees of Rocky Mountain lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) in central British Columbia. To assess chirality of phyllotaxis, the relationship between numbers of contact parastichies and chirality of phyllotaxis in scales on cones and needle fascicles on branchlets was determined. Three or 8 clockwise acropetal contact parastichies were indicative of a clockwise generative spiral, while totals of 2, 5, or 13 clockwise acropetal parastichies were indicative of a counter-clockwise generative spiral. Lodgepole pine trees were nearly always chimeric, i.e., having clockwise and counter-clockwise phyllotaxis on the same individual, but there was a high overall correspondence between the chirality of phyllotaxis in cone scales and subtending needle fascicles. Seedlings (&lt;1.5 years old) had no measurable wood-grain angle and clockwise and counter-clockwise phyllotaxis occurred in equal proportions. However, young trees (13–15 years since planting) had a pronounced clockwise bias to their wood-grain spiral in contrast with a counter-clockwise bias in phyllotaxis. In contrast, mature trees ([Formula: see text]100 years old) had the reverse trend and exhibited a counter-clockwise bias in wood-grain spiral but a clockwise bias in phyllotaxis. A model is proposed to explain how chirality of spiral wood grain could generate an inverse bias in the chirality of phyllotaxis in lodgepole pine.Key words: lodgepole pine, Pinus contorta var. latifolia, phyllotaxis, generative spiral, Fibonacci numbers, spiral wood grain.

Solution of inverse heat conduction problems using maximum entropy method

A solution scheme based on the maximum entropy method (MEM) for the solution of one-dimensional inverse heat conduction problem is proposed. The present work introduces MEM in order to build a robust formulation of the inverse problem. MEM finds the solution which maximizes the entropy functional under the given temperature measurements. In order to seek the most likely inverse solution, the present method converts the inverse problem to a non-linear constrained optimization problem. The constraint of the problem is the statistical consistency between the measured temperature and the estimated temperature. Successive quadratic programming (SQP) facilitates the maximum entropy estimation. The characteristic feature of the method is discussed with the sample numerical results. The presented results show considerable enhancement in the resolution of the inverse problem and bias reduction in comparison with the conventional methods.

A comprehensive study of inversion current in MOS tunneling diodes

The gate current of MOS tunneling diodes biased at inversion region with different substrate doping is investigated. For p-type substrate (1-5 /spl Omega/-cm) devices, the tunneling diode works in the deep depletion region and the inversion current is dominated by the thermal generation rate of minority electrons via traps at Si/SiO/sub 2/ interface and in the deep depletion region. The activation energy is approximately equal to half of the silicon bandgap independent of gate voltage. For devices on p/sup +/ substrate (0.01-0.05 /spl Omega/-cm), the band-to-traps tunneling and band-to-band tunneling are the dominating current components at inversion bias, and reveal a strong field dependence and a weak temperature dependence. The band-to-traps and band-to-band current components are even more significant in the devices on the p/sup ++/ substrate (0.001-0.0025 /spl Omega/-cm). Finally, the effects of temperature and light illumination on inversion current of MOS tunneling diodes will be also discussed.

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS dark matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (−6 V instead of −2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event day −1 kg −1 keV recoil −1 at 90% confidence level in the 15–45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.

Polypurine.polypyrimidine sequences in complete bacterial genomes: preference for polypurines in protein-coding regions

The genomes of Methanococcus jannaschii, Mycoplasma genitalium, Haemophilus influenzae, Archaeoglobus fulgidus, Helicobacter pylori, Treponema pallidum, Borrelia burgdorferri, Rickettsia prowazekeii, Mycobacterium tuberculosis, Methanobacterium thermoautotrophicum, Synechocystis sp. PCC6803, Bacillus subtilis, Chlamydia trachomatis, Pyrococcus horikoshii, Aquifex aeolicus, Mycoplasma pneumoniae and Escherichia coli have been analysed for the presence of polypurine.polypyrimidine tracts, in order to understand their distribution in these genomes. We observed a variation in abundance of such sequences in these bacteria, with the archaeal genomes forming a high-abundance group and the canonical eubacteria forming a low-abundance group. The genomes of M. tuberculosis and A. aeolicus are unique among the organisms analysed here in the abnormal underrepresentation and overrepresentation of polypurine.polypyrimidine, respectively. We also observe a strand bias, i.e., a preferential occurrence of polypurines in coding strands. It varies widely among the bacteria, from the very high bias in M. jannaschii to the slightly inverse bias in the parasitic genomes of T. pallidum and C. trachomatis. The extent of strand bias, however, cannot be explained on the basis of the GC-content of the genome, use of all-purine codons or an excess in the amino acids that are encoded by such codons. The probable causes and effects of this phenomenon are discussed.

Temperature dependency of the intrinsic carrier density of hydrogenated amorphous silicon in MOS structures

The admittance versus frequency of a hydrogenated amorphous silicon metal oxide semiconductor capacitor is measured at a fixed bias in inversion and for temperatures in the range of 20–50 °C. The data are fitted to theoretical capacitance and conductance curves where the time constant of inversion is the result of the fit. In turn, the time constant can be converted to the (minority) carrier lifetime so that a lifetime value for each measurement temperature is available. The conversion from the time constant to the minority carrier lifetime requires the knowledge of the temperature-dependent intrinsic carrier density or rather its activation energy. The criterion for the correct choice is a temperature-independent carrier lifetime. Three published room temperature values of the intrinsic carrier density have been tested. The carrier lifetime activation energy is E a = 0.70 ± 0.03 eV.

Evidence of hole direct tunneling through ultrathin gate oxide using P/sup +/ poly-SiGe gate

P/sup +/ poly-Si and poly-Si/sub 0.75/Ge/sub 0.25/-gated PMOS transistors with ultrathin gate oxides of 25 and 29 /spl Aring/ were used for this study. The difference in the gate work function was used to determine the mechanisms of gate tunneling current in such thin gate oxides, Under negative gate bias (inversion bias), it was found that the source/drain terminal serves as a source of holes for small V/sub g/ value, and as gate bias increases (more negative), it becomes a hole sink. These observations can be interpreted in terms of two competing mechanisms. For the first time, hole direct tunneling is reported, Hole direct tunneling is the dominant mechanism for -2 V<V/sub g/<0 V. For Vg<-2 V, electron direct tunneling is dominant. Electron-hole pair generation by the tunneling electrons starts to dominate over hole direct tunneling only for V/sub g/<-4 V.

Recall and recognition of threatening, pleasant, and neutral words in repressors

Low accessibility of threatening information is a crucial element in the theory of repression. Despite many attempts, no empirical evidence has been found for such an inverse memory bias in normal subjects. An alternative approach is to study repressive processes as an individual difference. Adopting such a trait approach, several studies have shown impaired memory for threatening memories of real-life episodes in so-called repressors. Several other studies have yielded evidence of avoidant processing of mildly threatening information in the early stages of information processing in repressors. Thus far, little evidence for a similar avoidant bias in memory has been offered for repressors. To demonstrate the presence of memory deficits for mildly threatening information, we tested recall and cued recognition after a 30 min delay, for (i) threatening, (ii) pleasant, and (iii) neutral words in repressors and non-repressors. No proof was found for poorer recall and recognition of threatening words in repressors, or for lower numbers of threatening semantic intrusions in these individuals, while the power of the study was adequate. While repression-related avoidant processing may be present for even mildly threatening stimuli at earlier stages of information processing, the results of the present study indicate that repressed memory may only show up for relatively intense emotional events. Copyright © 1999 John Wiley & Sons, Ltd.

Status of the EDELWEISS experiment

The status of the EDELWEISS experiment, installed in the Fréjus tunnel, is presented. In its first stage, the experiment uses a 70 g high purity Ge bolometer with heat-ionization discrimination. Based on physics data, gamma and neutron calibrations, the influence of inverse bias voltage (−2 and −6 V) on incomplete surface charge collection, which limits at present the performances of these detectors, is presented. Analysing runs with a total exposure of 1.17 kg×day after cuts, an upper limit of 0.6 event day kg keV at 90% confidence level in the 12–70 keV recoil energy interval is obtained. Planned upgrades are summarized.