Transit Time Effects Research Articles

Oscillation frequency control of 60 GHz-band TUNNETT diodes

The effects of impedance matching on the oscillation characteristics of the tunnel injection transit time effect (TUNNETT) diode are investigated. It is shown that the most significant factor determining oscillation frequency is a DC bias current and that for the oscillation output power is impedance matching.

On the optimum design of the front-end PIN-heterojunction bipolar transistor optoelectronic integrated circuit photoreceiver

In this article, a detailed analysis of the performance of an integrated front-end PIN-heterojunction bipolar transistor photoreceiver has been given and optimum designs are suggested for maximum gain-bandwidth product (GB) and gain-bandwidth-sensitivity measure (GBS) of the device. The present SPICE-based analysis requires the equivalent circuit model of the receiver components. An expression for the impulse response of a PIN photodiode in the frequency domain has been derived to include the effect of transit time on the overall frequency response of the receiver. The models are verified with experimental data. Device parameters are tailored to obtain designs corresponding to maximum GB and GBS. These designs show much improvement in the performance when compared to the published results on unoptimized structures. Bandwidths greater than 40 GHz can be achieved using a simple three-stage amplifier circuit consisting of a common emitter and two emitter–follower circuits. Noise performance is calculated by minimum detectable power at a bit-error rate of 10−9 and the suggested optimum designs show improved noise performance.

Time-delay circuit model of high-speed p-i-n photodiodes

We have proposed an equivalent electric circuit model of p-i-n photodiode including the transit-time effect for performance evaluation of optical systems through the aid of a SPICE-like circuit simulator. No model-dependent close-form expression is necessary in our model. Its validity is shown for the analysis of frequency response in simplified p-i-n photodiodes with different lengths of i-region under uniform or nonuniform illumination. Its application to eye-diagram analysis is demonstrated also.

Dynamic Negative Differential Conductivity due to Electron Transit Time Effects

Dynamic Negative Differential Conductivity due to Electron Transit Time Effects

Collisionless electron heating by capacitive rf sheaths.

High-frequency plasma discharges are often sustained by collisionless heating of electrons; the nature of these mechanisms is a central problem in the theory of such discharges. In capacitive discharges, collisionless heating occurs near boundaries, and is usually attributed to inelastic collisions of electrons with oscillating plasma sheaths, regarded as moving rigid barriers. We show that, when current conservation is required, such heating necessarily vanishes, and we conclude that this model of the heating process is not correct. We develop an alternative view that associates the heating with acoustic disturbances in the electron fluid. An analytic model, based on moments of the Vlasov equation, gives results in good agreement with particle-in-cell simulations. In terms of individual particle dynamics, this acoustic heating may be interpreted as a transit-time effect.

Velocity-modulation and transit-time effects in InP/InGaAs HBTs

The base-collector capacitance C/sub bc/ and the collector transit time delay /spl tau/c govern the high-speed performance of modern heterojunction bipolar transistors (HBTs). Both are shown to be strongly modified by velocity modulation effects in InP/InGaAs HBTs: The carrier velocity in the collector depends on V/sub cb/ and I/sub c/, causing a reduction of C/sub bc/ and /spl tau/c respectively. The current induced transit time modulation is shown to be conveniently expressed by a minor but important modification of the conventional transit time delay expression. Particle simulations are performed to assess the relevance of these effects.

Dynamics of ion-ion plasmas under radio frequency bias

A time-dependent one-dimensional fluid model was developed to study the dynamics of a positive ion-negative ion (ion-ion) plasma under the influence of a rf bias voltage. The full ion momentum and continuity equations were coupled to the Poisson equation for the electrostatic field. Special emphasis was placed on the effect of applied bias frequency. Due to the lower temperature and greater mass of negative ions compared to electrons, the sheath structure in ion-ion plasmas differs significantly from that of conventional electron-ion plasmas, and shows profound structure changes as the bias frequency is varied. For low bias frequencies (100 kHz), the charge distribution in the sheath is monotonic (switching from positive to negative) during each half cycle. For intermediate frequencies (10 MHz), when the bias period approaches the ion transit time through the sheath, double layers form with both positive and negative charges coexisting in the sheath. For high frequencies (60 MHz), beyond the plasma frequency, plasma waves are launched from the sheath edge, and the sheath consists of multiple peaks of positive and negative charge (multiple double layers). For a relatively large range of bias frequencies (up to the plasma frequency), each electrode is bombarded alternately by high energy positive and negative ions during a rf bias cycle. For bias frequencies greater than the plasma frequency, however, the electrode is bombarded simultaneously by low energy positive and negative ions with ion energies approaching the thermal value. The ion energy was found to increase with the applied bias potential. Also, at relatively high pressures (20 mTorr), the ion energy at low frequencies (100 kHz) is limited by collisions. The peak ion energy may then be increased by using an intermediate bias frequency (10 MHz). At lower pressures, however, the effect of collisions is mitigated while the effect of ion transit time becomes significant as the bias frequency increases. In this case, a low bias frequency (100 s of kHz) is favorable for extracting high energy ions from the plasma.

Charge Transport Generated by Electron Beam in Molecularly Doped Polymers

Charge carrier transport in a typical molecularly doped polymer has been studied by the time-of-flight and radiation induced conductivity methods. In the time range bracketing the conventional transit time a strong disagreement between two sets of data has been found. Unlike time-of-flight signal that rather closely reproduces all salient features reported in literature, the radiation induced conductivity transient lacks the plateau. In fact, it exhibits a strict power law decay t−0.5 changing at long times to t−1 or t−1.5 patterns due to bimolecular recombination or true transit time effects, respectively. At short times, both transients are described by the same t−0.5 dependence. These facts suggest the dispersive rather than the Gaussian charge transport.

The Transit Time Effect of a Modulated Electron Beam

Energy change of an electron beam is studied numerically when it transits a rf field gap of width L. For a premodulated beam, the transit time L/ for maximum energy extraction is shortened to 0.475 T (T=2π/ω, is the average velocity of the electrons) when the modulation frequency ωm is matched to the rf field frequency ω. Enhancement of the extraction efficiency ηo is dependent on the modulation coefficient h1 and h2. For a uniform beam with V=500 kV, ηo is less than 2%. ηo reaches 11% for a prebunched beam with h1=0.7 and h2=0.1 and will surpass 20% for an ideally modulated beam.

Transit-time limitations inp-i-n photodiodes

A novel derivation for predicting transit-time effects in p-i-n photodiodes is described in which the diode current is found by simple integration. Experimental results obtained from two wide-area photodiodes compare well with theoretical predictions. It is also shown that previously published theory does not agree with these experimental observations.

Effect of transit times on quantification of cerebral blood flow by the FAIR T(1)-difference approach.

The effect of finite transit times for the tagging bolus is known to be a significant error source for perfusion quantification using the flow-sensitive alternating inversion recovery (FAIR) technique. It is shown that, in the presence of transit times, both the slice-selective (SS) and nonselective (NS) inversion recovery experiments actually consist of an NS period followed by an SS period. This mixed process can be described using a newly defined time constant called the "switching time," which separates the two periods. Calculations predict that finite transit times always lead to decreased flow values in the signal-intensity-difference approach, but that the measured flows in the T(1)-difference approach may be decreased or increased. This theory well explains our recent experimental flow results on cat brain as a function of predelay. The results show the signal-intensity-difference method is superior over the T(1)-difference approach in terms of convenience and ease of quantification. Magn Reson Med 42:890-894, 1999.

Effect of transit times on quantification of cerebral blood flow by the FAIR T1‐difference approach

Effect of transit times on quantification of cerebral blood flow by the FAIR T1‐difference approach

Effect of transit times on quantification of cerebral blood flow by the FAIR T1‐difference approach

Effect of transit times on quantification of cerebral blood flow by the FAIR T1‐difference approach

Exciton resonance induced dips in photocurrent spectra of weakly coupled GaAs [formula omitted]AlAs superlattice p–i–n diodes: barrier thickness dependence

Low-temperature photocurrent (PC) spectra of 100-period GaAs /AlAs superlattice p–i–n diodes with narrow electron miniband widths show negative PC peaks (dips) at the exciton resonance wavelengths under low field conditions. These dips observed at 17–100 K in the positive PC background are caused by the resonance-induced changes of the photogenerated carriers distribution within the thick intrinsic region and by the transit time effects on the vertical tunneling transport which is reduced with increasing the barrier thickness. Enhancement of the dip appearance at higher temperatures (up to 60 K) consistently explains the causes of the PC dips in terms of slowing down of the tunneling transport due to the thermal saturation of the drift velocity.

Description of a dynamic in vitro model of the dog gastrointestinal tract and an evaluation of various transit times for protein and calcium.

In order to manufacture complete and balanced dog diets, it is important to know the nutrient requirements of dogs and the availability of these nutrients from food. As pet food manufacturers are restricted in their options for (invasive) animal studies, due to ethical constraints, it is important to have alternative methods for researching the effects of various dog diets. To simulate the gastrointestinal tract of the dog, the dynamic gastrointestinal tract model developed by Minekus et al. was further developed and modified in this study. The model consists of four compartments which simulate the stomach and small intestine (duodenum, jejunum and ileum). Each compartment is made of glass, with a flexible inner wall. This wall can be compressed by increasing the pressure of the surrounding water, mimicking the peristaltic movements and mixing seen in vivo. The model is computer-controlled to simulate physiological parameters such as pH, transit time and secretion of digestive juices, as derived from the literature. Gastric meal delivery and the effects of intestinal transit time on protein digestibility and availability for absorption of calcium from dog food were studied to evaluate the model. The gastric meal delivery of dry dog food was identical to a preset curve, which was based on in vivo data from healthy dogs. The emptying time for canned dog food was somewhat slower than the preset values, probably due to the viscosity of the meal. The differences between the preset values and the measured delivery were not significant. The digestibility of protein and the availability of calcium for absorption increased with a longer transit time. A significant difference was found between medium and slow transit times for the nitrogen content in the ileal delivery effluent and the jejunal dialysates (p < 0.05). The same trend was seen for calcium (not significant). The overall conclusion is that the model is a useful tool for mimicking the gastrointestinal tract of dogs. Parameters such as pH, transit time and enzyme activity can be mimicked and can be kept within a physiological range.

Direct extraction and numerical simulation of the base and collector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, /spl tau//sub B/ and /spl tau//sub C/ in heterojunction bipolar transistors (BBT's) from the phase and magnitude of the common-base current gain, /spl alpha/(/spl omega/), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to /spl tau//sub B/ and /spl tau//sub C/ due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications.

Perfusion magnetic resonance imaging with continuous arterial spin labeling: methods and clinical applications in the central nervous system

Several methods are now available for measuring cerebral perfusion and related hemodynamic parameters using magnetic resonance imaging (MRI). One class of techniques utilizes electromagnetically labeled arterial blood water as a noninvasive diffusible tracer for blood flow measurements. The electromagnetically labeled tracer has a decay rate of T1, which is sufficiently long to allow perfusion of the tissue and microvasculature to be detected. Alternatively, electromagnetic arterial spin labeling (ASL) may be used to obtain qualitative perfusion contrast for detecting changes in blood flow, similar to the use of susceptibility contrast in blood oxygenation level dependent functional MRI (BOLD fMRI) to detect functional activation in the brain. The ability to obtain blood flow maps using a non-invasive and widely available modality such as MRI should greatly enhance the utility of blood flow measurement as a means of gaining further insight into the broad range of hemodynamically related physiology and pathophysiology. This article describes the biophysical considerations pertaining to the generation of quantitative blood flow maps using a particular form of ASL in which arterial blood water is continuously labeled, termed continuous arterial spin labeling (CASL). Technical advances permit multislice perfusion imaging using CASL with reduced sensitivity to motion and transit time effects. Interpretable cerebral perfusion images can now be reliably obtained in a variety of clinical settings including acute stroke, chronic cerebrovascular disease, degenerative diseases and epilepsy. Over the past several years, the technical and theoretical foundations of CASL perfusion MRI techniques have evolved from feasibility studies into practical usage. Currently existing methodologies are sufficient to make reliable and clinically relevant observations which complement structural assessment using MRI. Future technical improvements should further reduce the acquisition times for CASL perfusion MRI, while increasing the slice coverage, resolution and stability of the images. These techniques have a broad range of potential applications in clinical and basic research of brain physiology, as well as in other organs.

Ion energy distributions and sheath voltages in a radio-frequency-biased, inductively coupled, high-density plasma reactor

Ion energy distributions were measured at a grounded surface in an inductively coupled, high-density plasma reactor for pure argon, argon–helium, and argon–xenon discharges at 1.33 Pa (10 mTorr), as a function of radio-frequency (rf) bias amplitude, rf bias frequency, radial position, inductive source power, and ion mass. The ground sheath voltage which accelerates the ions was also determined using capacitive probe measurements and Langmuir probe data. Together, the measurements provide a complete characterization of ion dynamics in the sheath, allowing ion transit time effects to be distinguished from sheath impedance effects. Models are presented which describe both effects and explain why they are observed in the same range of rf bias frequency.

Characterisation of high pressure quadrupole collision cells possessing direct current axial fields

An investigation into the performance characteristics of various designs of novel quadrupolar collision cells possessing axial fields has been undertaken. Among the advantages of such devices possessing conical, tilted or segmented rod geometries is their ability to enable the cell to be operated at high pressure without the deleterious effects of long ion transit times. The collisional focusing benefits of operating at such pressures can therefore be maximised, with increased transmission and resolution of fragment ion spectra. The present work compares the relative merits of the three main designs of axial field cells. The ion optics software package SIMION 6.0 has been utilised to simulate the various designs of cell with user-written programs to model each quadrupolar device, including ion-neutral collisional phenomena. The effect on ion transmission of applying a direct current (DC) offset voltage to the conical and tilted rod cells to generate the axial field is shown. Where possible the simulations and theory are compared to experimental data. Crown Copyright © 1999 National Research Council of Canada.

The Effect of Intestinal Transit Time on Bacterial Translocation

SummaryIncrease in intraluminal bacterial count, disruption of the mucosal integrity, changes in intestinal immunity and transit time are the factors involved in bacterial translocation. The relationship between intestinal transit time, intra luminal bacterial count and translocation rate were investigated in 40 Wistar-albino rats. The study was conducted in 4 groups with 10 animals in each. Group I (controls): saline + laboratory chow, Group II: saline + oral total parenteral nutrition (TPN) solution, Group III: morphine sulfate (MS) + oral TPN solution, Group IV: neostigmine bromide (NB) + oral TPN solution.Intestinal transit time was measured by using Indium111-labeled diethylene triamine pentaacetic acid (DTPA). It was prolonged in the MS-treated group and shortened in the NB-treated group (p < 0.01). The frequency of bacterial translocation was 60% in the oral TPN solution group, 100% in the MS-treated group, 20% in the NB-treated group and 10% in controls. Bacterial counts in duodenum, jejunum, ileum and caecum were significantly increased (p < 0.001) in the MS-treated group and decreased (p < 0.05) in the NB-treated group in comparison with the control group. In conclusion, the prolongation of intestinal transit time increased the intraluminal bacterial count and augmented bacterial translocation. The decrease in intestinal transit time had a converse effect.