Photoconductive Switches Research Articles

Investigation on the Switching Transient of GaAs PCSS Operating in the Transition From Linear to Nonlinear Mode

The switching transient of gallium arsenide photoconductive semiconductor switch (GaAs PCSS) undergoing the transition from linear mode to nonlinear mode is investigated based on a “two-channel” model. Excellent agreement between the simulated and the experimental output waveforms is achieved first in the study of this transitional mode. The numerical results have revealed that a competitive relationship exists between the switching and nonswitched channels of the PCSS, and the formation and evolution of avalanche domains boost the nonlinear mode developing in the switching channel to win out from the linear mode dominated by the nonswitched channel. Based on the numerical model, the effects of the cross section of current channel on the output characteristics are well analyzed and discussed, which have also been demonstrated experimentally.

Investigation of Delay Jitter of Avalanche GaAs PCSS for Triggering High-Voltage Gas Switch

The delay jitter characteristics of gallium arsenide (GaAs) photoconductive semiconductor switches (PCSSs) triggered in avalanche mode were experimentally investigated under dc bias. In the dark-state measurement, the PCSS was tested under a 40-kV bias voltage to verify its withstanding voltage and reliability. In the ON-state measurement, the delay jitter time of PCSS at different triggering positions was measured. The experimental results showed that the shortest delay jitter time was obtained when the laser was applied to the middle position of the cathode electrode. To evaluate the long-time stability, the delay-jitter time was recorded after continuous shots. It shows an ascending trend in delay jitter time with the increase in operating shots. The degeneration of the delay jitter time of the PCSS was mainly attributed to the device damage, including electrode ablation and filament current ablation, which was verified by the analysis of scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). Furthermore, a bipolar high-voltage gas switch integrated with PCSSs was successfully triggered by a low-energy fiber laser.

Breakdown and Photoconductivity Enhancement by Mixed Reflective Al-Doped ZnO/Ag Electrode in Vertical SiC Photoconductive Switch

an axial-triggering vertical SiC photoconductive switch with Al-doped ZnO placed under the Ag electrode is developed to enhance breakdown strength and photoconductivity. Traditional and new devices with the AZO/SiC/Ag and AZO/SiC/AZO/Ag structures, respectively, were prepared. Test results show that the conductivity of the new device is approximately 1.2 times higher than that of the old one, attributable to the diffuse reflection effects of the AZO/Ag mirror. Under high bias voltages of up to 16.5 kV/mm and a 30-ns pulse width laser input, the new device shows significantly enhanced breakdown strength. The new device has a lifetime of >1E4 shots, whereas the old device has a maximum lifetime of approximately 1E2 shots. The new device can continuously output electrical pulses for 10 minutes at a 100 Hz repetition rate, 30-ns pulse width, and 100 A peak current without breakdown.

On-Chip Time-Domain Terahertz Spectroscopy of Superconducting Films below the Diffraction Limit.

Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ∼0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 to 750 GHz. We measure the optical conductivity of a 7.5-μm wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.

Si3 N4 Passivation and Side Illumination of High-Power Photoconductive Semiconductor Switch Based on Free-Standing SI-GaN

To meet the needs of wide band gap (WBG) gallium nitride (GaN)-based photoconductive semiconductor switch (PCSS) in high-voltage and high-power microwave systems. Herein, silicon nitride (Si3N4) dielectric films were deposited on GaN PCSS using plasma-enhanced chemical vapor deposition (PECVD) and illuminated from the side of the device. A high refractive index of 2.03 and a high permittivity of 7.97 were obtained from Si3N4 films, which are characterized by high optical and dielectric properties. According to the electrostatic field simulation, Si3N4’s high permittivity minimizes the peak electric field at the electrode boundary of GaN PCSSs. When the 532-nm incident light is side illuminated for the device, the output peak power and rise time are 59.8 kW and 0.52 ns, respectively. At a future date, the proposed GaN-based PCSS will have a practical application for radio frequency microwave systems.

Modeling of Changes in the Resistivity of Semi Insulating Gallium Phosphide under the Influence of Lighting

The article presents the results of a simulation of changes in gallium phosphide (GaP) resistivity under the influence of lighting. The adopted model of the defect structure is presented along with the defect parameters. Initial conditions created on the basis of a tested material sample, labeled GaP-1, made of monocrystals of semi-insulating gallium phosphide (SI GaP), are presented. The simulation methodology and the created model of the kinetics equations are described. As a result of the simulation, the values of the photocurrent and the electron-hole pair generation coefficient G were assigned to data obtained experimentally depending on the carrier lifetime coefficient τ. Changes in resistivity and concentration of electrons and holes in the bands for gallium phosphide with a structure consisting of five defects are presented. The proposed simulation method can be used to calculate switch-on and -off times and photocurrent values for the semiconductor materials used to construct PCSS (photoconductive semiconductor switches) and other electronic devices.

Pulse Width Control Based on Blumlein Pulse Forming Line and SI-GaAs PCSS

In this paper, the output electrical pulse width of semi-insulating gallium arsenide photoconductive semiconductor switch (SI-GaAs PCSS) is controlled by means of Blumlein pulse formation line. Under the condition that the bias voltage is 28 kV and the laser pulse width is 9.5 ns, the electric pulse width obtained by using high-power pulse system transmission is 10 ns and the output voltage is 23 kV. Based on the Blumlein pulse formation line theory, the output pulse width and transient impedance are analyzed. The results show that the holding time of carriers avalanche multiplication can be controlled.

Performance of a Novel Rear-Triggered 4H-SiC Photoconductive Semiconductor Switch

Radial lateral structure photoconductive switches are fabricated in this work via vanadium-doped 4H-SiC and high-purity 4H-SiC materials. The switches are triggered by 355- and 532-nm laser, and the performance of the switch is compared in the applied voltage range of 1–10 kV and laser energy range of 0.3–14 mJ. The experimental results show that the conduction current of the laser incident from the rear is larger. Simulations of the current density distribution of a 2-D cross section of the photoconductive switch provide theoretical support for this phenomenon. Additionally, the conduction current of the high-purity material is found to be higher than that of the vanadium-doped material under the same conditions.

Improved Current and Jitter Performances of Photoconductive Semiconductor Switch Based on Reduced Graphene Oxide/Metal Electrode

This letter investigated the characteristics of avalanching gallium arsenide (GaAs) photoconductive semiconductor switches (PCSSs) based on composite electrode. The composite electrode was constructed by a layer of reticular reduced graphene oxide (RGO) and a multi-layer of metal (Ni/Ge/Au/Ni/Au). The structure promotes heat dissipation and improves contact performance between GaAs and metal. The reduction of conducting current and delay jitter degeneration was investigated experimentally with increased shots, and the results demonstrated improved reliability of the PCSS with a reticular-RGO /metal electrode. The maximum of conducting current reduces from 127 A to 120 A after continuously 5000 shots under 38 kV DC bias, and the delay jitter stabilizes at 0.3 ns. The significant improvement of the PCSS with a reticular-RGO /metal electrode was attributed to reduction of interface damage between the substrate and electrode. The stability of PCSS with a reticular-RGO/metal electrode was improved comparing with the conventional PCSS without RGO layer, and the physical reason was also discussed.

The Initial Test of a Micro-Joules Trigger, Picosecond Response, Vertical GaN PCSS

To meet the miniaturization of all-solid-state high-power microwave drive technology based on photoconductive semiconductor switches (PCSSs), a vertical gallium nitride (GaN) PCSS that is triggered by a micro-joules energy and picoseconds pulse width laser is presented. The device has showed low triggering optical energy and fast response. When the bias-voltage is 3 kV and optical energy is 33 <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 photocurrent, on-state resistance, rise-fall time and quantum efficiency of the PCSS are 4.28 A, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$650.5~\Omega $ </tex-math></inline-formula> , 412/370 ps and 0.28%, respectively. It is anticipated that GaN-based PCSS will have a practical application in future radio frequency (RF) microwave systems for military purposes.

Mechanism of avalanche charge domain transport for nonlinear mode of GaAs photoconductive semiconductor switches

Photoconductive semiconductor switch is of significance in the fields of ultafast electronics, high-repetition rate and high-power pulse power system, and THz radiation. The mechanism of the nonlinear mode of the switch is an important area of study. In this work, stable nonlinear wave forms are obtained by a semi-insulating GaAs photoconductive semiconductor switch triggered by a 5-ns laser pulse with pulsed energy of 1 mJ at a wavelength of 1064 nm under a bias of 2750 V. Based on two-photon absorption model, the photogenerated carrier concentration is calculated. The theory analysis and calculation result show that the photogenerated carrier can compensate for the lack of intrinsic carrier, and lead to the nucleation of photo-activated charge domain. According to transferred-electron effect principium, the electric field inside and outside the domain are calculated, indicating that the electric field within the domain can reach the electric field which is much larger than intrinsic breakdown electric field of GaAs material, and results in strong impact avalanche ionization in the bulk of the GaAs switch. According to the avalanche space charge domain, the typical experimental phenomena of nonlinear mode for GaAs switch are analyzed and calculated, the analysis and calculations are in excellent agreement with the experimental results. Based on drift-diffusion model and negative differential conductivity effect, the transient electric field in the bulk of the switch is simulated numerically under the optical triggering condition. The simulation results show that there are moving multiple charge domains with a peak electric filed as high as the intrinsic breakdown electric field of GaAs within the switch. This work provides the experimental evidence and theoretical support for studying the generation mechanism of the nonlinear photoconductive semiconductor switch and the improvement of the photo-activated charge domain theory.

Combining negative photoconductivity and resistive switching towards in-memory logic operations.

A family of rudorffites based on silver-bismuth-iodide shows a transition from a conventional positive photoconductivity (PPC) to an unusual negative photoconductivity (NPC) upon variation in the precursor stoichiometry while forming the rudorffites. The NPC has arisen in silver-rich rudorffites due to the generation of illumination-induced trap-states which prompted the recombination of charge carriers and thereby a decrease in the conductivity of the compounds. In addition to photoconductivity, sandwiched devices based on all the rudorffites exhibited resistive switching between a pristine high resistive state (HRS) and a low resistive state (LRS) under a suitable voltage pulse; the switching process, which is reversible, is associated with a memory phenomenon. The devices based on NPC-exhibiting rudorffites switched to the HRS under illumination as well. That is, the resistive state of the devices could be controlled through both electrical and optical inputs. We employed such interesting optoelectronic properties of NPC-exhibiting rudorffites to exhibit OR logic gate operation. Because the devices could function as a logic gate and store the resistive state as well, we concluded that the materials could be an ideal candidate for in-memory logic operations.

Study on Output Characteristics of 20 kV/100 ns Nonlinear GaAs Photoconductive Semiconductor Switch

In this paper, the output electric pulse characteristics of gallium arsenide photoconductive semiconductor switch (GaAs PCSS) with bias voltage of 20 kV and pulse width of about 100 ns are studied. By designing the reflection and transmission optical paths, the absorption rate of the trigger optical of the GaAs PCSS at a wavelength of 1064 nm was measured to be 24.8%. When the bias voltage is 16 kV-20 kV, the avalanche multiplication rates of carriers are calculated respectively. The relationship between the output current waveform and the residual charge in the energy storage capacitor is analyzed when the bias voltage is 20 kV. And the trend of GaAs PCSS electric field intensity and current waveform change with time. This research will be helpful for the application design of GaAs PCSS under the nonlinear mode with special requirements, and has important significance for the application of frontier science.

Spin-Orbit-Torque Switching of Ferrimagnets by Terahertz Electrical Pulses

In conventional spintronic devices, ferromagnetic materials are used, which have a magnetization dynamics timescale of around nanoseconds, setting a limit for the switching speed. Increasing the magnetization switching speed has been one of the major challenges for spintronic research. In this work we take advantage of the ultrafast magnetic dynamics in ferrimagnetic materials instead of ferromagnets, and we use femtosecond laser pulses and a plasmonic photoconductive switch to create THz electrical pulses for ferrimagnetic switching by spin-orbit torque. By anomalous Hall and magneto-optic Kerr effect (MOKE) measurement, we demonstrate the robust THz-electrical-pulse-driven magnetization switching of ferrimagnetic $\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$. The time-resolved MOKE shows more than 50-GHz magnetic resonance frequency of $\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Co}$, indicating faster than 20-ps magnetic dynamics. X-ray magnetic circular dichroism demonstrates the antiferromagnetically coupled $\mathrm{Fe}$ and $\mathrm{Gd}$ sublattices. Our work provides a promising route to realize ultrafast operation speed for nonvolatile magnetic memory and logic applications.

A Type of SiC Photoconductive Switch With Both High Uniformity and High Absorption Rate

In order to make the SiC photoconductive switch have better synchronization of photoelectric response and improve the working life of the device, a mathematical model of ring total reflection structure is established based on SiC semiconductor material. The influence of structural parameters on the absorption rate and uniformity of the device is analyzed. Two sets of structural parameters with high absorption and uniformity are simulated, one of which has a high absorption of 91.41% and a uniformity of 69.35%, and the other has an absorption of 86.67% and a uniformity of 70.73%. Additionally, the compensation of the regions with less light distribution of the device can be achieved by adding reflectors.

Temporal shaping of high‐voltage picosecond electric pulses for electronic spectroscopy and bioelectric applications

AbstractIn this paper, we present the temporal and spectral shaping of kilovolt picosecond electrical pulses obtained by using the linear switching process with femtosecond laser source. The frozen wave generator integrating up to four photoconductive semiconductor switches is activated by varying the optical pulse amplitude and time delay. Electric pulses with rise time and duration as short as 60 and 100 ps, respectively are used to implement a microwave spectroscopy experiment. This optoelectronic device is synchronous with the optical pulses which can be used for optical spectroscopy, after nonlinear frequency conversion. This single‐shot microwave spectroscopy technique was used to characterize the resonances of a passive dielectric resonator. Pulse shaping integrating coherent wave combining is also demonstrated.

Strongly enhanced THz generation enabled by a graphene hot-carrier fast lane

Semiconductor photoconductive switches are useful and versatile emitters of terahertz (THz) radiation with a broad range of applications in THz imaging and time-domain spectroscopy. One fundamental challenge for achieving efficient ultrafast switching, however, is the relatively long carrier lifetime in most common semiconductors. To obtain picosecond ultrafast pulses, especially when coupled with waveguides/transmission lines, semiconductors are typically engineered with high defect density to reduce the carrier lifetimes, which in turn lowers the overall power output of the photoconductive switches. To overcome this fundamental trade-off, here we present a new hybrid photoconductive switch design by engineering a hot-carrier fast lane using graphene on silicon. While photoexcited carriers are generated in the silicon layer, similar to a conventional switch, the hot carriers are transferred to the graphene layer for efficient collection at the contacts. As a result, the graphene-silicon hybrid photoconductive switch emits THz fields with up to 80 times amplitude enhancement compared to its graphene-free counterpart. These results both further the understanding of ultrafast hot carrier transport in such hybrid systems and lay the groundwork toward intrinsically more powerful THz devices based on 2D-3D hybrid heterostructures.

Low Specific Contact Resistivity of 10−3Ω·cm2 for Ti/Al/Ni/Au Multilayer Metals on SI-GaN:Fe Substrate

For the application of third-generation compound semiconductors based on high-voltage, high-power, and high-frequency devices, particularly in photoconductive semiconductor switches (PCSSs), the ohmic contact preparation technology on semi-insulating substrates is crucial. However, iron-doped semi-insulating gallium nitride (SI-GaN:Fe) with low carrier concentration and high bulk resistivity has a very difficult time achieving good ohmic contact characteristics. Here, we have demonstrated preliminary results on an interesting class of good ohmic contact of Ti/Al/Ni/Au on Ga-face of SI-GaN:Fe. The results indicated that the Ti and Al thickness ratio is a critical parameter when determining the optimum annealing temperature and time for ohmic contact. With alloyed Ti/Al/Ni/Au stacks of 20 nm/120 nm/55 nm/45 nm on SI-GaN:Fe after annealing at 850 °C for 35 s in N2 atmosphere, a low specific contact resistivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-3}\,\,\Omega \cdot $ </tex-math></inline-formula> cm2 is achieved. At a future date, such excellent ohmic contact characteristics can be applied to the SI-GaN:Fe-based high-power PCSS.

Properties of Switching Transient in the Semi-Insulating GaAs Photoconductive Semiconductor Switch With Opposed Contacts

The properties of switching transient in the gallium arsenide photoconductive semiconductor switch (GaAs PCSS) are significant for improving the device lifetime. In this article, the properties of switching transient in the opposed-contact semi-insulating (SI) GaAs PCSS, including the physical process of the ultrafast switching, the characteristics of delayed breakdown, and the current density distribution, are studied, based on an effective 2-D, time-dependent numerical model. The numerical results demonstrate that there exists a positive feedback loop between the increase of the carrier density and the formation and evolution of the avalanche domains, which achieves the ultrafast switching of the opposed-contact SI-GaAs PCSS. The higher bias voltage and triggering optical intensity can promote the earlier formation of the avalanche domains and accelerate the evolution of the domains, leading to a shorter delay time and switching time. The triggering positions determine the distribution of the avalanche domains in the opposed structure, affecting the delay time, among which the cathode triggering leads to the shortest delay time. Current crowding in the opposed structure mainly occurs in the vicinity of the contacts, which first damages the edges of the contacts and makes the areas near the contacts present different failure characteristics. Our analysis indicates that anode triggering is effective for suppressing the current crowding to improve the lifetime of the opposed-contact PCSS.

Density-Dependent Effects on Pulse Compression in GaN Photodetectors Probed by Monte Carlo Studies

With increases in the demand for faster electronic switching, requirements for higher operating voltages and currents, and the need to perform under harsh environments while operating at even higher frequencies, the research focus in photoconductive semiconductor switch (PCSS) technology has shifted to wide bandgap semiconductors. Here, we examine the possibility of pulse compression in carbon-doped PCSS devices based on the negative differential mobility concept for faster operation. Monte Carlo simulations are used to build in and model various effects on electron transport including degeneracy, charge polarization, and scattering within a three-valley model fitted to bandstructure calculations. The focus is on exploring the density dependence of pulse compression. Thresholds for the biasing fields naturally emerge. Predictive analysis of the output full-width half-maximum (FWHM) current waveforms, as well as the dynamics of the internal charge cloud behavior, and occupancy of the various valleys within GaN are all obtained. Our results show that an increase in carrier density can increase pulse compression and create pulse-widths that are smaller than the FWHM of the input optical excitation. This bodes well for enhanced repetition rates. Variations produced by moving the laser spot along the GaN PCSS length are also examined. Though data for GaN are not yet available, the trends compare well qualitatively with previous reports for GaAs.