Transit Time Effects Research Articles

The Deformable-Channel Model--A New Approach to High-Frequency MESFET Modeling

High-frequency small-signal circuit parameters are evaluated for a saturated-channel MESFET by including transit-time effects in a rigorous way through a study of induced shape changes in the saturated-channel region. Results agree well with empirical equivalent-circuit parameters for a physical MESFET.

Microfields in stroboscopic voltage measurements via electron emisson. II. Effects on electron dynamics

In this article, the transit-time and cross-talk effects occurring in voltage measurement via electron emission are analyzed as a function of geometry, extraction fields, electron start energies, and rise times of the actual input signals. The investigation was performed to obtain information about the maximum achievable time resolution and the disturbance of measured signals by cross-talk effects in corresponding contactless measurement techniques like photoemissive sampling and electron-beam probing. The field distribution above the sample was assumed as being two-dimensional, and the influence of space-charge effects neglected. A strong correspondence to the purely geometry-dependent response function of the potential energy, analyzed in Part I [J. Appl. Phys. 62, 1553 (1987)], is established.

Effect of the Atom Transit Time on the Frequency Stability of Cesium Beam Frequency Standards

The effect of the transit time of the atoms in cesium beam tubes is analyzed in the case of fast sine-wave and square-wave fre- quency modulations and in the case of square-wave phase modulation. Analytical expressions are given for the slope of the error signal and for the frequency stability. The effect of the modulation frequency and of the separation between the second interaction region and the atom detection zone is illustrated for the interaction time distributions en- countered when optical methods are implemented for the state selec- tion and the detection of the cesium atoms.

Microfields in stroboscopic voltage measurements via electron emission. I. Response function of the potential energy

It is shown that the response of the electrostatic potential on voltage changes at a conducting line in a microscopic structure is fully described by a pure geometry function. This function is determined by the special microscopic character of the conducting line and by the screening due to neighboring conductors. It already allows a qualitative description of transit-time and cross-talk effects in voltage measurements via electron emission or electro-optical sampling, and provides a basis for the quantitative calculation of these effects. The geometry dependence of this function will be analyzed in detail.

Effect of changing transit time on colonic microbial metabolism in man.

An investigation was made of the effect of changing mean transit time (MTT) by administration of drugs which affect colonic motility on faecal microbial mass in man. Senokot was used to accelerate and codeine and/or loperamide to prolong transit in subjects maintained on a constant high fibre diet. Doses of Senokot or codeine/loperamide were adjusted to halve or double transit time measured during a three week control period on diet alone. Stools were collected throughout and analysed for bacterial mass by a gravimetric procedure. Transit was measured by a continuous marker method. Senokot decreased mean transit time from 63.9 to 25.0 hours (n = 6), with increased stool weight from 148 to 285 g/day. Bacterial mass increased in all subjects from a mean of 16.5 to 20.3 g/day (dry weight) (p less than 0.025). Codeine/loperamide increased mean transit time from 47.1 to 87.6 hours (n = 5), with decreased stool weight from 182 to 119 g/day. Bacterial mass decreased in all but one subject from a mean of 18.9 to 16.1 g/day (NS). There was a significant correlation between transit time and bacterial mass in all three periods (r = 0.77, p less than 0.001). Changes in transit time are shown to alter microbial growth in the human colon and result in altered stool output, on a constant diet. Factors which affect transit may be as important as diet in determining large bowel function and hence susceptibility to disease.

A new transit-time device using quantum-well injection

The use of such techniques as molecular beam epitaxy has allowed the fabrication of devices in which tunneling is the dominant transport mechanism. In this paper a new transit-time device which uses resonant tunneling through a quantum well is proposed and analyzed. Depending on the bias level, this device may permit injection of carriers into the drift region at more favorable phase angles (hence higher efficiencies) than other transit-time devices. The device promises low noise performance and should be capable of operating at high millimeter-wave frequencies with higher output power than other transit-time devices or pure quantum-well oscillators. Since the device uses quantum-well injection and transit-time effects, it is called a QWITT diode.

AC Corona Charging of Particles

Measurements of the mobility statistics of dioctyl-phthalate (DOP) particles leaving an ac corona charger with various electrode configurations are reported. Mobilities are determined using a laser-Doppler velocimeter (LDV) to measure particle migration velocities in response to a transverse electric field imposed downstream of the charger. Charging models, which do not include effects of finite ion transit times, nonuniform fields, or differences between the distributions of positive and negative wire coronas, predict some of the mobility statistics, especially below 100 Hz, but cannot explain the high-frequency charging. In the turbulent flow with significant corona wind effects the disparate corona current distributions are shown to have a significant effect on the ac corona charging processes.

High-speed electrooptic modulation in GaAs/GaAlAs waveguide devices

A figure of merit is constructed for broad-band electrooptic modulators which compares bandwidth with launched generator voltage standardized to a common wavelength. Comparison of various published results in terms of this figure shows that a lumped-element III-V semiconductor device performance may be no more than a factor of two below that of typical (i.e., LiNbO 3 ) traveling wave devices and are probably easier to implement-especially in an integrated format. Accurate modeling, which incorporates all transit time and velocity match effects, is described and found to agree well with experimental results. Experimental GaAs/GaAlAs modulators have been made, using a Mach-Zehnder interferometer configuration. At a wavelength of 1.15 μm and with unterminated drive a bandwidth of 6.5 GHz was obtained with V pi of 17.3 V. A shorter (34.6-v) device was ∼ 1.25-dB down at 8.4 GHz. The corresponding figures of merit are close to the maximum expected for the configurations used.

A new ultra-high-speed heterojunction transistor

A new type of heterojunction bipolar transistor, designated the BICFET, (for Bipolar Inversion Channel Field Effect Transistor), is proposed. Unlike the conventional heterojunction bipolar transistor (HBT), the BICFET does not have a base. Instead, it relies on a field effect action to induce and modulate an inversion channel (which replaces the base) at the interface between the wide- and narrow- bandgap materials, to control current flow in the system.The BICFET has the high current gain associated with HBTs. However, since the BICFET does not have a base layer, it does not experience the deleterious base layer effects, such as charge storage, recombination, base transit time, etc. In particular, it has a much lower input capacitance and the capability of much higher current drive than a HBT, the combination of which augurs well for very high speed operation.In addition to the basic theory, common-emitter (CE) output and input characteristics are generated for two practical systems, and a possible integrated structure is indicated. A small-signal CE model for the device is also presented. The model predicts h cut-off frequencies as high as 10,000 GHz; however, in practice, collector transit time effects will reduce the cut-off frequency to about 600 GHz.

Energy and time-resolved photoemission in a promising new approach for contactless integrated-circuit testing

A novel approach to contactless measurement of voltages on internal nodes of integrated circuits is presented. The method is based on time-resolved photoemission exploiting the single-photon process with short laser pulses in the ultra-violet. The method allows spatial resolution in the submicron range given by the diffraction limit for UV photons, a time resolution of a few picoseconds given by the width of the laser pulses and the electron transit-time effect, and a voltage resolution of a few microvolts within a signal integration time of one second.

6247. Analysis of the transit time effect on the stroboscopic voltage contrast in the scanning electron microscope

6247. Analysis of the transit time effect on the stroboscopic voltage contrast in the scanning electron microscope

Ghost’s shadows and superluminal occultations

The field intensity behind a moving (nonrelativistic) occulting object has a rather counterintuitive behavior at sufficiently large distance: finite transit time effects yield two simultaneous shadow paths on a plane, which separate from each other with superluminal velocity.

Space-charge effect on hole transport in resistive hydrogenated amorphous silicon

Hole transport properties of resistive hydrogenated amorphous silicon (1012 Ω cm) are studied by a pulsed photoconductivity technique. Transit time is analyzed by the Scher–Montroll plotting method. Estimation of mobility is done with photocurrent measurement under a space-charge-limited condition, and it is confirmed that the mobility value measured under this condition agrees with the calculated value found from space-charge-limited current. With no space-charge effect, temperature dependence and electric field dependence of transit time is represented by τ=τ0 E−1/α′ exp(Δ/kT).

Radiation‐induced conductivity in polystyrene as a hopping phenomenon

AbstractTransient radiation‐induced conductivity of polystyrene resulting from single pulses (0.3 ms) of 65 keV electrons in vacuum has been investigated. The activation energy of the delayed component was found to be 0.17 eV. Doping of polystyrene with additives (rhodamine‐6G, tetracyanoethylene) is consistent with the fact that this polymer is hole conducting. Transit time effects in polymer films of 20 μm thickness are practically absent at fields less then 108 V/m. The application of the proposed theory of the transient hopping conduction and its relationship to the conventional band model of photo‐ and radiation‐induced conductivity in amorphous semiconductors is also discussed.

Transit-time effects in saturated absorption resonances

The main characteristics of the saturated absorption resonance (its line shape, its amplitude, power and collisional broadening) are studied in detail, experimentally and theoretically, covering both the domain of very small and relatively large saturation levels. In contrast with numerous previous papers, it is shown that the width of the resonance in the free-flight regime is governed only by the typical transit time of molecules across the beam, while the width of the first derivative is equal to the square root of the product of the collisional and transit-time linewidths. The effect of slow molecules on the shape of the resonance and the second-order Doppler shift has been detected and studied experimentally for the first time. The unprecedented accuracy of about 3 parts in 10 13 is reported for the laser frequency standard Etude theorique et experimentale des caracteristiques principales en resonances d'absorption saturee (forme, amplitude, elargissement de puissance et collisionnel) pour des niveaux de saturation tres petits et relativement grands: la largeur de la resonance en region de vol libre n'est determinee que par le temps typique de transit des molecules a travers le faisceau lumineux et la largeur de la derivee premiere est proportionnelle a la racine carree du produit des largeurs de collision et de transit. Detection et etude de l'influence des molecules lentes sur la forme de resonance et le deplacement Doppler du second ordre

Carrier mobilities in poly(vinylidene fluoride)

Irradiation of poly(vinylidene fluoride) samples with a partially penetrating electron beam generates an ionised region adjacent to the electrode of incidence. An external electric field extracts carrier from the irradiated into the non-irradiated region. In the steady state the field-induced drift of carriers through the non-irradiated region can be described in terms of the conventional theory of space-charge-limited currents. Emission of the carriers occurs from a virtual electrode located at the interface between the irradiated and the non-irradiated region. The mobility of both carrier species is found to be the same and has a value mu =2*10-8 cm2 V-1 s-1. Attainment of the steady state is preceded by long-lasting and complex transient effects which depend on the density of the incident electron beam current and on the magnitude of the applied electric field. Carrier transit time effects appear to have a small influence.

Noise properties and time response of the staircase avalanche photodiode

The staircase avalanche photodiode is a novel graded-gap superlattice device that is expected to detect photons quite noiselessly. It is designed in such a way that only electrons impact-ionize, thereby eliminating the feedback noise associated with conventional two-carrier avalanche devices. Because the electron multiplication can occur only at a small number of discrete locations in the device, the variability of the number of electrons generated per detected photon is minimized. The excess noise and the electron counting distribution are obtained as a function of the number of stages in the device and the impact-ionization probability per stage, for instantaneous multiplication. The (single-photon) impulse response function is calculated when the effects of (random) transit time are incorporated into the carrier multiplication process. Inclusion of the time dynamics is essential for determining the time course of the current generated by the device in response to pulses of light. This, in turn, permits bit error rates to be calculated for systems incorporating the device. For a five-stage quaternary device the gain-bandwidth product is calculated to be in the vicinity of 600 GHz.

Small-signal model and high-frequency performance of the BICFET

The small-signal model for the bipolar inversion channel field-effect transistor (BICFET) is presented. Expressions are obtained for all the common-emitter small-signal parameters in terms of the physical parameters of the device. The unity current gain bandwidth product frequency f T is derived and calculated for the device and found to be 10 000 GHz. However, frequency limitations imposed by transit time effects are shown to be much lower than this, of the order 530 GHz. The unity power transfer ratio frequency ω PT , which is considered to be a much more practical figure of merit than f T , is derived for the BICFET. Unlike f T , which is simply the ratio of the transconductance and base diffusion capacitance this frequency incorporates all the small-signal parameters including internodal capacitance and source resistance. Assuming internodal capacitance of the same order as the input capacitance and using the source resistance of 1000 Ω, f PT is found to be approximately 400 GHz. It is also shown that due to the high current gain, current crowding and, hence, the channel resistance in the BICFET is insignificant for any practical operating conditions, and for current densities up to 106A/ cm2.

A study of system characteristics in a manufacturing cell

Using a comprehensive simulation model, a number of important parameters relating to manufacturing cells are investigated. These include: (i) the proposal by Burbidge that ‘period batch control’ is more appropriate for group technology cells, (ii) the effects of different numbers of tool set-up types or part families, (iii) the comparative merits of a larger 18-machine cell and 3 parallel 6-machine cells and (iv) the effects of inter-operation transit time within cells. System performance obtained showed that, with an efficient scheduling rule, the smaller batches product better facility utilization and job throughput. The results also suggest that, expect for extremely small, restrictive cells (with one or two tool setup types), the addition of further part families does not cause a serious deterioration in the productivity of the cell. The larger cells were found to be more effective than the small cells placed in parallel in all the system performance characteristics recorded.

Energy broadening in a pulsed field emission gun by the transit time effect

The broadening of electron energy by the transit time effect is analysed for two types of electron guns, a planar triode gun and a pulsed field emission gun whose grid or first-anode potential is modulated by a pulse. In the calculation, the potential is assumed to be quasi-static and the equation of motion is normalised to the transit angle. When the switching time constant of the pulse voltage is comparable with the transit time, the energy broadening by the transit time effect is large. When the variation of the potential during the electron transit is slow enough, the energy deviation is of the same order as that variation. The current bunching and the energy spectrum are also calculated for the pulsed field emission gun used in stroboscopic TEM, and it is shown that the transit time effect might become a limiting factor as regards the pulse width.