Lock-on Effect Research Articles

Analysis of carrier dynamics and thermal effect of the GaAs photoconductive semiconductor switch in lock-on mode

Abstract The lock-on effect of the gallium arsenide photoconductive semiconductor switch (GaAs PCSS) at repetition rate aggravates the current crowding and electric field distortion, which significantly increases the risk of switch damage or even failure. Therefore, it is of great significance to investigate the carrier transport and the heat generation mechanism for improving the performance and longevity of GaAs PCSS in lock-on mode. The internal physical process of an opposed-electrode GaAs PCSS at low optical energy and strong electric field is analysed and discussed by experiment and simulation. A device-circuit hybrid simulation is employed to investigate the transient electric field, carrier concentration, and lattice temperature distribution within the GaAs PCSS in lock-on mode. The device temperature exhibits a positive correlation with the applied bias electric field, resulting in a peak temperature of 1037.25 K at an electric field of 38 kV/cm. The temperature distribution within the GaAs PCSS indicates a greater possibility for thermal breakdown and damage near the electrodes.

Deep reinforcement learning finds a new strategy for vortex-induced vibration control

As a promising machine learning method for active flow control (AFC), deep reinforcement learning (DRL) has been successfully applied in various scenarios, such as the drag reduction for stationary cylinders under both laminar and weakly turbulent conditions. However, current applications of DRL in AFC still suffer from drawbacks including excessive sensor usage, unclear search paths and insufficient robustness tests. In this study, we aim to tackle these issues by applying DRL-guided self-rotation to suppress the vortex-induced vibration (VIV) of a circular cylinder under the lock-in condition. With a state space consisting only of the acceleration, velocity and displacement of the cylinder, the DRL agent learns an effective control strategy that successfully suppresses the VIV amplitude by $99.6\,\%$ . Through systematic comparisons between different combinations of sensory-motor cues as well as sensitivity analysis, we identify three distinct stages of the search path related to the flow physics, in which the DRL agent adjusts the amplitude, frequency and phase lag of the actions. Under the deterministic control, only a little forcing is required to maintain the control performance, and the VIV frequency is only slightly affected, showing that the present control strategy is distinct from those utilizing the lock-on effect. Through dynamic mode decomposition analysis, we observe that the growth rates of the dominant modes in the controlled case all become negative, indicating that DRL remarkably enhances the system stability. Furthermore, tests involving various Reynolds numbers and upstream perturbations confirm that the learned control strategy is robust. Finally, the present study shows that DRL is capable of controlling VIV with a very small number of sensors, making it effective, efficient, interpretable and robust. We anticipate that DRL could provide a general framework for AFC and a deeper understanding of the underlying physics.

The lock-on effect and collapsing bipolar Gunn domains in high-voltage GaAs avalanche p-n junction diode

We present experimental evidence and physics-based simulations of the lock-on effect in high-voltage GaAs avalanche diodes. The avalanche triggering is initiated by steep voltage ramp applied to the diode and in-series 50 Ω load. After subnanosecond avalanche switching the reversely biased GaAs diode remains in the conducting state for the whole duration of the applied pulse (dozens of nanoseconds). There is no indication of the p-n junction recovery that is commonly expected to develop on the nanosecond scale due to the drift extraction of non-equilibrium carriers. The diode voltage keeps a constant value of ∼70 V much lower than the stationary breakdown voltage of 400 V. Numerical simulations reveal that the conducting state is supported by impact ionization in narrow high-field collapsing Gunn domains as well as in quasi-stationary cathode and anode ionizing domains. Collapsing Gunn domains spontaneously appear in the dense electron-hole plasma due to the negative differential mobility of electrons in GaAs. The effect resembles the lock-on effect of GaAs bulk photoconductive switches but is observed in reversely biased p-n junction diode switched by a non-optical method.

Pulsewidth Control of Nonlinear GaAs Photoconductive Semiconductor Switch

The nonlinear gallium arsenide photoconductivity semiconductor switch (GaAs PCSS) can generate high-voltage and strong current electrical pulses with low light triggering at the order of microjoule. Due to the lock-on effect of the GaAs PCSS, the output pulsewidth is usually on the order of hundreds of nanoseconds or even tens of microseconds. This severely limits the application of nonlinear GaAs PCSS. This article presents the output pulsewidth modulation of nonlinear GaAs PCSS through the dual regulation of energy-storage capacitance and current-limiting resistance. When triggered by a pulse laser with a width of 10 ns and a single pulse energy of 200 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{J}$ </tex-math></inline-formula> , the switch can output the minimum pulsewidth of 6.3 ns and the maximum pulsewidth of 196 ns. In the meantime, combined with the theoretical analysis of photon-activated charge domain (PACD), the electric field intensity of nonlinear GaAs PCSS is less than the threshold electric field of PACD, and the switch will be turned off quickly so that different pulsewidths can be output. The experimental results indicate that adding different current-limiting resistances and energy-storage capacitance in the circuit can control the electric field intensity so that different pulsewidths can be output.

Collapsing Gunn Domains as a Mechanism of Self-Supporting Conducting State in Reversely Biased High-Voltage GaAs Diodes

Switching of a high-voltage GaAs diode to the conducting state in the delayed impact-ionization mode is simulated and the results are compared with experimental data. It is shown that the effect of long-term (up to 100 ns) sustaining of the conducting state of the diode after switching is due to the appearance of narrow (of the order of a micrometer) ionizing Gunn domains, the so-called collapsing domains, in the electron-hole plasma. Impact ionization in collapsing domains and in the edge (cathode and anode) domains of a strong electric field (~ 300 kV/cm) maintains a high concentration of nonequilibrium carriers (≥ 1017 cm-3) during the entire duration of the applied reverse polarity voltage pulse. Keywords: high-voltage GaAs diodes, impact ionization, subnanosecond switches, Gunn effect, lock-on effect.

Эффект самоподдержания проводящего состояния в обратносмещенных GaAs-диодах, переключаемых в режиме задержанного лавинного пробоя

It is experimentally established that high-voltage GaAs diodes maintain conducting state with low residual voltage after switching to the conducting state in the delayed impact-ionization mode. The duration of the constant current in the reversely-biased structure is determined by the duration of the applied rectangular voltage pulse (up to 100 ns) and significantly exceeds drift extraction and recombination times. The discovered effect of self-supporting conducting state resembles the lock-on effect in optically activated GaAs semiconductor switches and S-diodes with deep centers. The effect can be explained by shock ionization in narrow collapsing Gann domains.

A hierarchical model of visual processing simulates neural mechanisms underlying reflexive attention.

The visual system deploys attention reflexively in response to important stimuli to facilitate rapid selection. To better understand how a system that is distributed across a hierarchy of brain areas coordinates this rapid response, we have developed a computational model that simulates reflexive attention as a recurrent lock-on state between topographically aligned cortical areas. This model provokes key questions about reflexive attention that we study through the N2pc, an EEG component that indexes the timing and lateralization of stimulus-driven attention. A key finding from such research is that for two sequential targets sharing a location, the second elicits no N2pc, despite being easily perceivable. This suggests that attention locks-on to a target's location and carries forward in time to a second target without being redeployed. Here, we assess key properties of reflexive attention in four EEG experiments. First, the lock-on N2pc effect generalizes to feature target types. Second, the lock-on state is specific to a location, not just a visual hemifield. Third, reflexive attention decays as the interval between two targets increases, such that a second target will once-again elicit an N2pc at longer intervals. Finally, the lock-on state is not specific to a particular target type, which implies that reflexive attention is mediated using representations that are not stimulus-specific. These results provide important constraints on our understanding of visual attention. We incorporate those constraints in a formalized model of attention that elucidates the link between neural mechanisms and a key neural correlate of attention. (PsycINFO Database Record

Picosecond-Range Avalanche Switching of High-Voltage Diodes: Si Versus GaAs Structures

We present a comparative study of Si and GaAs high-voltage diodes operated in the delayed impact ionization breakdown mode. We use an experimental setup that allows measuring current and voltage on the diode simultaneously and independently during a 100-ps high-voltage switching transient. Si and GaAs structures with identical geometries and a stationary breakdown voltage of $\sim 1$ kV were investigated. All devices trigger at close to 2 kV and are capable of forming a voltage ramp with a kilovolt amplitude and a 100-ps rise time. We found that Si p+nn + and p+pnn + structures differ in residual voltage: a relatively low residual voltage $V_{\mathrm {res}}$ of 100–200 V was observed only for p+nn + structures, whereas for p+pnn + structures $V_{\mathrm {res}}$ is about 1 kV. We report observing the lock-on effect in GaAs structures: after 100-ps avalanche switching GaAs diodes remain in a high conducting state as long as the applied voltage pulse lasts, whereas within the same time of 2 ns reference Si diodes fully recover the blocking capability.

Anomalous dynamics of the residual voltage across a gallium-arsenide diode upon subnanosecond avalanche switching

A qualitative difference between high-voltage gallium-arsenide diodes and similar silicon devices is found experimentally upon ultrafast switching in the delayed avalanche breakdown regime. It is shown that, following switching, a gallium-arsenide diode remains in a highly conductive state throughout the entire duration of the applied voltage pulse and the recovery of the reverse voltage across the p-n junction due to the dispersal of nonequilibrium electron-hole plasma is not observed. In the same interval of time (2 ns in our experiment), a silicon diode passes completely into a blocking state. The residual voltage amplitude for a gallium-arsenide diode is an order of magnitude lower than that for a silicon device. The discovered effect is similar to a known effect of “sticking” of gallium-arsenide diode switches (the lock-on effect), which are triggered by a laser pulse, in a conductive state.

Lower Bound of Electrical Field for Maintaining a GaAs Photoconductive Semiconductor Switch in High-Gain Operating Mode

The high-gain operating mode of gallium arsenide (GaAs) photoconductive semiconductor switch (PCSS) brings the hope that a higher power PCSS will be triggered by a lower energy laser. However, the lock-on effect of high-gain PCSS restricts its lifetime and some other applications. In this paper, it is experimentally demonstrated that dynamically controlling the electrical field across a GaAs:EL2 PCSS can terminate the carrier avalanche and that there exists a lower bound of electrical field (about 3.64 kV/cm) for maintaining the high-gain operating mode of the PCSS under different conditions. This lower bound of electrical field is approximately equal to that of the transferred-electron effect; hence, it supports the hypothesis of a photon-activated charge domain to explain the physical phenomena of the high-gain PCSS.

Research on electrical pulse of 20-kV/30-Hz GaAs photoconductive switches

Photoconductive semiconductor switches (PCSSs) are widely used in high power ultra-wideband source applications and precise synchronization control due to their high power low-jitter high-repetition-frequency. In this letter, a 14-mm gap semi-insulating GaAs PCSS biased under 20 kV is triggered by a 1064-nm laser with a repetition frequency of 30 Hz. Although the trigger condition is greater than the threshold of the lock-on effect, the high gain mode is not observed. The results indicate that the high gain mode of the PCSS is quenched by decreasing the remnant voltage of pulsed energy storage capacitor.

A Novel Application of GaAs Photoconductive Semiconductor Switch in Triggering Spark Gap

Based on the characteristics of the GaAs photoconductive semiconductor switch (PCSS) and the spark gap, a new combined switch is developed. The combined switch integrates a traditional GaAs PCSS and a normal spark gap, which could offer two to three times of self-breakdown voltage for the spark gap in nanoseconds. Compared with the traditional single GaAs PCSS, a higher peak current at the output terminal was observed in the experiments when the combined switch was used. Based on the transferred electron effect and the runaway fast electron model, the turn-on process of the GaAs PCSS and the mechanism for the formation of plasma channel in the spark gap are discussed. In addition, due to the interaction between the GaAs PCSS and the spark gap, lock-on effect is avoided in a bulk GaAs PCSS when the bias electric field is larger than the lock-on threshold, which indicates that this combined switch could improve the longevity of the GaAs PCSS under high applied voltage.

Analysis of operation mechanism of semi-insulating GaAs photoconductive semiconductor switches

This paper reports that the quenched-domain mode of luminous charge domain has been observed in semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) and the typical characteristics of lock-on effect have been explained based on the model of luminous charge domain. It is shown that the domain is formed by photogenerated carriers and the quenched domain is due to the interaction of the circuit self-excitation and transferred-electron oscillation in the bulk of switch. During the transit of the domain, the bias electric field (larger than Gunn threshold) across the switch is modulated by the ac electric field, when the instantaneous bias electric field is swinging below the sustaining field (the minimum electric field required to support the domain), and then the quenched-domain mode is obtained. When PCSSs operate in nonlinear mode, the ultrahigh electric field of domain induced by photogenerated carriers leads to strong impact ionization accompanied by electron-hole recombination radiation in the switch. Therefore new avalanche domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation which causes the effective carrier velocities to be larger than the saturation velocity. Lock-on field resulted from the length proportional number of domains and steadfast electric fields inside and outside the domains, and the recovery of lock-on effect is caused by the domain quenching. The calculations agree with the experimental results, and the analysis also indicates that SI-GaAs PCSS is a photoactivated charge domain device.

Multiple charge domains model for the lock-on effect in GaAs power photoconductive switches

This paper reports that the lock-on field of semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) was measured under different bias voltages. Based on the experimental results and the transferred-electron effect, a model for the lock-on effect in GaAs PCSSs is proposed. It is shown that the charge domain with an ultrahigh electric field is due to a high photogenerated carrier density, which gives rise to intensive impact ionization accompanied by electron–hole recombination radiation within the domain. Since new domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation, the forefront domain crosses the switch at a speed alternating between the photon velocity and the carrier saturated drift velocity, which makes the observed velocity of carriers larger than the saturated drift velocity. The lock-on field results from the fixed number of a moving train of avalanching charge domains, the steady-state domains electric fields and the steadfast external electric field of the domains. The recovery of the lock-on effect is caused by domain quenching. The calculations agree with the experimental results. Moreover, the analytical results indicate that SI-GaAs PCSS is essentially a type of photo-activated charge domain device.

Effects of Field-Dependent Trapping and Detrapping on the Responses of Compensated GaAs Photoconductive Switches

In this paper, the response of a semi-insulating gallium arsenide photoconductive switch used in high-power microwave generation has been analyzed through experiment and simulation. The switch characteristics as a result of trapping and detrapping phenomena during a transient are studied. Analysis show that field-enhanced capture at high field locations during the onstate creates a temporary reservoir of trapped charge at different pockets in the device. Release of trapped charge and the subsequent injection of carriers from the contacts to maintain charge neutrality helps maintain conduction, which persists even after the laser pulse until all charges are transferred across the device. A model has been presented which can explain such phenomena as lock-on effect, persistent conductivity, and low-power requirement during transients. This phenomenon is unique to semi-insulating gallium arsenide and is primarily due to field-enhanced trapping and could be applicable to other materials that have similar characteristics

Monopole Charge Domain in High-Gain Gallium ArsenidePhotoconductive Switches

Considering that semi-insulating galliumarsenide photoconductive switches can be triggered into the high gainmode and no reliable theories can account for the observed transientcharacteristics, we propose the monopole charge domain model to explainthe peculiar switching phenomena occurring in the high gain mode and wediscuss the requirements for the lock-on switching. During operation onthis mode, the applied field across the switch and the lock-on fieldare all larger than the Gunn threshold field. Our developed monopolecharge domain is based on the transferred-electron effect, but thedomain is only composed of large numbers of electrons piled up due tothe negative differential mobility. Using the model and taking thephysical mechanism of the avalanche impact ionization and recombinationradiation into consideration, we interpret the typical phenomena ofthe lock-on effect, such as the time delay between the beginning of opticalillumination and turning-on of the switch, and the conduction mechanism of thesustaining phase. Under different conditions of bias field intensityand incident light energy, the time delay of the switching iscalculated. The results show that the physical mechanisms of impactionization and recombination radiation occurring in the monopole chargedomain are responsible for the lock-on switching.

Experimental Study of the Effect of Transverse Oscillation on Convection Heat Transfer From a Circular Cylinder

The present study is concerned with heat transfer characteristics of forced convective flows over a transversely oscillating cylinder. The effect of oscillation of the cylinder on heat transfer coefficient and flow pattern is evaluated by experimental measurement. A modified transient test method and a flow visualization technique are employed to determine the heat transfer coefficient and to observe the flow pattern, respectively, for various dimensionless oscillation frequencies (Sc), dimensionless oscillation amplitudes (A/D), and Reynolds numbers (Re). The ranges of these parameters considered in this study are 0 ≤ Sc ≤ 0.65, 0 ≤ A/D ≤ 0.628, and 0 ≤ Re ≤ 4000. Results show that the heat transfer coefficient can be significantly increased by the oscillation of the cylinder. Two effects that enhance the heat transfer performance are found, namely, the lock-on effect and the turbulence effect. A maximum of 34 percent increase in heat transfer is found within the parameter ranges considered in this study. Agreement of the present data is found with previous numerical studies; however, an existing approximate method, which ignores the lock-on phenomenon but which is commonly used to estimate the effect of oscillation on heat transfer, is found to be inapplicable in this study.

Numerical prediction of lock-on effect on convective heat transfer from a transversely oscillating circular cylinder

Abstract Heat transfer characteristics and the flow behavior of cross flow over a transversely oscillating cylinder are investigated. The lock-on phenomenon has been predicted numerically and its influence on the heat transfer performance of the cylinder is evaluated. The SOLA method is employed to solve the unsteady velocity field in a non-inertial reference frame, and the energy equation is solved by a finite-volume method. Transient variations of the Nusselt number and the drag and lift coefficients are calculated for various oscillation conditions. The ranges of the dominant parameters considered in this study are 0 ⩽Re ⩽ 300, 0⩽ Sc ⩽ 0.3 and 0 ⩽A/D ⩽ 0.7.. The Prandtl number is considered to be 0.71 or 7.0. In the lock-on regime, an appreciable heat transfer increase caused by the oscillation is observed; however, outside this regime, the heat transfer is almost unaffected by the oscillation. A correlation formula expressing the dependence of heat transfer on these dominant parameters in this lock-on regime is presented. The numerical predictions have been compared with the existing information, and good agreement has been found.

Lock-on effect in pulsed-power semiconductor switches

Certain high-voltage pulsed-power switches based on semi-insulating GaAs or InP exhibit a ‘‘lock-on’’ effect. In this paper, this effect is argued to be fundamentally a transferred-electron effect, and its experimentally observed characteristics are explained. The lock-on effect causes high forward drop and high power dissipation for certain pulsed-power switches based on GaAs and various other direct-gap materials.

The lock-on effect in electron-beam-controlled gallium arsenide switches

The authors analyze the lock-on effect, which is the inability of photoconductive or electron-beam-controlled semiconductor switches to recover to their initial hold-off voltages following the application of the laser or electron-beam pulse, if the applied voltage exceeds a certain value. For GaAs this threshold voltage corresponds to average electric fields in the range from 4 to 12 kV/cm. Experimental results on semi-insulating GaAs switches indicate that the corresponding lock-on current after e-beam irradiation is identical with the steady-state dark current. The highly resistive state of the switch before e-beam irradiation is shown to be a transient phase towards the much lower steady-state dark resistance, with a duration which depends on the impurity content of the switch material and the applied voltage. The irradiation of the GaAs samples with electrons or photons causes an acceleration of this temporal evolution; at sufficiently high laser or e-beam intensities, lock-on of the dark current after termination of the driving ionization source is observed. Based on the experimental results, a model is developed which describes the lock-on effect in terms of double injection and carrier trapping in deep intraband levels. The model explains the major characteristics of the lock-up effects and is supported by the qualitative agreement of the calculated current-voltage curves with the experimental data. >