Performance of Lateral 4H-SiC Photoconductive Semiconductor Switches by Extrinsic Backside Trigger

Photoconductive semiconductor switch(PCSS)can be applied in pulsed high power systems and microwave techniques.However,reducing the damage and increasing the lifetime of silicon carbide(SiC)PCSS are still faced severe challenges.In this study,PCSSs with various structures were prepared on 4-inch diameter,500μm thick high-purity semi-insulating 4H-SiC substrates and their on-state resistance and damage mechanisms were investigated.It was found that the PCSS of an Au/TiW/Ni electrode system annealed at 950℃had a minimum on-state resistance of 6.0Ωat 1 kV bias voltage with a 532 nm and 170 mJ pulsed laser by backside illumination single trigger.The backside illumination single trigger could reduce on-state resistance and alleviate the damage of PCSS compared to the frontside trigger when the diameter of the laser spot was larger than the channel length of PCSS.For the 200 s trigger test by a 10 Hz laser,the black branch-like ablation on Au/TiW/Ni PCSS was mainly caused by thermal stress owing to hot carriers.Replacing metal Ni with boron gallium co-doped zinc oxide(BGZO)thin films annealed at 400℃,black branch-like ablation was alleviated while concentric arc damage was obvious at the anode.The major causes of concentric arc are both pulsed laser diffraction and thermal effect.

Peculiar Photoconduction in Semi-Insulating GaAs Photoconductive Switch Triggered by 1064nm Laser Pulse

The peculiar photoconduction in semi insulating GaAs photoconductive switch being triggered by 1064nm laser pulse is reported.The gap between two electrodes of the switch is 4mm.When it is triggered by laser pulse with energy of 0 8mJ and the pulse width of 5ns,and operated at biased electric field of 2 0 and 6 0kV/cm,both linear and nonlinear modes of the switch are observed respectively.Whereas the biased electric field adds to 9 5kV/cm,and the triggered laser is in range of 0 5～1 0mJ,the peculiar performed characteristic is observed:the switch gives a linear waveform firstly,and then after a delay time of about 20～250ns,it outputs a nonlinear waveform again.The physical mechanism of this specific phenomenon is associated with the anti site defects of semi insulating GaAs and two step single photon absorption.The delay time between linear waveform and nonlinear waveform is calculated,and the result matches the experiments.

Ultra-Wideband Electromagnetic Radiation from GaAs Photoconductive Switches

The experiment results of ultrawide band electromagnetic radiation with DC biased GaAs photoconductive semiconductor switch combining double ridge horn antenna triggered by high repeat frequency femto-second laser pulse are reported.The GaAs switches are insulated by solid multi-layer transparent dielectrics and the distance of two electrodes is 3mm.The electrode material of the switch is ohmic contact through alloy technics with definite proportion of Au/Ge/Ni.This switch and double ridge horn antenna are integrated and the receive antenna is connected with the test instrument.From receiving antenna,ultra fast electrical pulse of 200ps rise time and 500ps pulse width is obtained,the repetition rate of the pulse is about 82MHz and the frequency spectrum is in the range of 4.7MHz～14GHz.The radiation characteristic of the ultrafast electrical pulse is analyzed.

Delayed-Dipole Domain Mode of Semi-Insulating GaAs Photoconductive Semiconductor Switches

A mode for the periodicity and weakening surge in semi-insulating GaAs photoconductive semiconductor switches is proposed based on the transferred-electron effect. It is shown that the periodicity and weakening surge is caused by the interaction between the self-excitation of the resonant circuit and transferred electron oscillation of the switch. The bias electric field （larger than Gunn threshold） across the switch is modulated by the AC elec-tric field,when the instantaneous bias electric field E is swinging below Gunn electric field threshold ET but grea-ter than the sustaining field Es （the minimum electric field required to support the domain） at the time of the do-main reaching the anode, and then the delayed-dipole domain mode of switch is obtained. It is the photon-activated carriers that satisfy the requirement of charge domain formation on carrier concentration and device length prod-uct of 10^12 cm^-2,and the semi-insulating GaAs photoconductive semiconductor switch is essentially a type of pho-ton-activated charge domain device.

Optically Activated Charge Domain Model for High-Gain GaAs Photoconductive Switches

A model for theoretical analysis of nonlinear (or high gain) mode of photoconductive semiconductor switches (PCSS's) is proposed.The switching transition of high gain PCSS's can be described with an optically activated charge domain. The switching characteristics including rise time,delay and their relationship to electric field strength,optical trigger energies are discussed.The formation and radiation transit,accumulation of the charge domain are related with the triggering and sustaining phases of PCSS's,respectively.The results of the mathematical model on this mechanism agree with experimental results.

Transit Properties of High Power Ultra\|Fast Photoconductive Semiconductor Switch

Experiments of a GaAs ultra\|fast Photo\|Conductive Semiconductor Switch (PCSS) are reported. Both the linear and nonlinear modes were observed when triggered by the μJ nano\|second laser. The peak current could be as high as 560A. The rise time of the current pulse responses is less than 200ps when triggered with 76MHz femto\|second laser.

Transient Characteristics of a Nonlinear GaAs Photoconductive Semiconductor Switch

The transient resistance,voltage,and power of a nonlinear GaAs photoconductive semiconductor switch （PCSS） are presented by the finite difference formula to deal with the experiment data, based on the conversation of energy in the switch circuit. This method resolves the problem of directly measuring the transient characteristics of PCSS in nonlinear mode. The curve of transient voltage shows that the average electric field of PCSS in the lock-on period is always higher than the Gunn threshold,and increases monotonically. By comparing the transient power curves of the PCSS and the electrical source,it is demonstrated directly that the power shortage leads to the PCSS from the lock-on state into the selfturnoff state,so a controllable turnoff of the PCSS in lock-on by changing the distribution of the circuit power is predicted.

Investigation of GaAs Photoconductive Switch Irradiated by 1553nm Laser Pulse

Gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS's) with a 1.55mm gap spacing trigged by 1553nm femtosecond fiber laser pulse is presented.The switches are biased with 3.33～10.3kV/cm and irradiated by femtosecond fiber laser operated at a wavelength of 1553nm with pulse width of 200fs and pulse energy of 0.2nJ.The experiments show that,even if the semi-insulating GaAs photoconductive switch operates under the electrical field of 10.3kV/cm,it will be still linear response,and a clear corresponding output electric pulse with the peak voltage of 0.8mV is captured.From the weak photoconductivity on laser intensity,photoabsorption mediated by EL2 deep level defects is suggested,as the primary process for the photoconductivity.

Transition of photoconductive and photovoltaic operation modes in amorphous Ga_2O_3-based solar-blind detectors tuned by oxygen vacancies

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga_2O_3-based solar-blind photodetectors in metal–semiconductor–metal(MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga_2O_3 interface is realized by tuning the conductivity of amorphous Ga_2O_3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga_2O_3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity(PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA,a high detectivity of 9.82×10^(12) cm·Hz^(1/2)·W^(-1), a fast response time of 243.9 μs, and a high ultraviolet C(UVC)-toultraviolet A(UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the subgap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.

Photoconductive semiconductor switch-based triggering with 1 ns jitter for trigatron

Synchronization for multiple-pulse at nanosecond range shows a great value on the power multiplication and synchronous electric fieldsapplications. Nanosecond or sub-ns jitter synchronization is essential for the improved working efficiency of the large amounts of pulse modulesand accurate requirements for the power coherent combining applications. This paper presents a trigger generator based on a laser diodetriggered GaAs photoconductive semiconductor switch (PCSS) with low jitter and compact size characteristics. It avoids the high currentsthat are harmful to high-gain mode PCSSs. In the trigger circuit, a 200 pF capacitor is charged by a microsecond-scale 18 kV pulse and thendischarged via the high-gain mode GaAs PCSS to trigger the high-power trigatron switch. When triggered by the ~10 ns pulse generated by thePCSS, the DC-charged trigatron can operate in the 20e35 kV range with 10 ns rise time and 1 ns delay-time jitter.

Terahertz radiation of a butterfly-shaped photoconductive antenna(invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna(PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup.To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation,polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material,radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.

CARRIER PHOTOGENERATION IN POLY(N-VINYL-CARBAZOLE)-BASED PHOTOCONDUCTIVE THIN-FILM DEVICES

In this paper, photoexcitation processes in the bilayer devices based on inorganic materials and poly（N-vinylcarbazole） （PVK） were investigated. In order to clarify the roles of inorganic materials in photoconductive properties of bilayer devices, TiO2 and ZnS were chosen to combine with PVK. A model for generation of photocurrent （Iph） in single layer device of PVK was obtained. It is deduced that the recombination rate constant （Pcomb） and the ionization rate constant （y） ofexcitons should be considered as the most important factors for Iph. For inorganic materials （TiO2 or ZnS）/PVK bilayer devices, in reverse bias of-4 V, the photocurrent of 115 mA/cm^2 in the TiO2/PVK device was observed, but the photocurrent in the ZnS/PVK device was only 10 mA/cma under the illumination light of 340 nm and the light intensity of 14.2 mW/cm^2. The weaker photocurrent is attributed to the absorption of ZnS within UV region and the energy offset at the interface between PVK and ZnS, which impedes the transport of charge carriers.

Roles of voltage in semi-insulating GaAs photoconductive semiconductor switch

An experimental study of leakage current is presented in a semi-insulating（SI） Ga As photoconductive semiconductor switch（PCSS） with voltages up to 5.8 kV（average field is 19.3 kV/cm）. The leakage current increases nonlinearly with the bias voltage increasing from 1.2×10^-9 A to 3.6×10^-5A. Furthermore, the dark resistance, which is characterized as a function of electric field, does not monotonically decrease with the field but displays several distinct regimes. By eliminating the field-dependent drift velocity, the free-electron density n is extracted from the current, and then the critical field for each region of n（E） characteristic of PCSS is obtained. It must be the electric field that provides the free electron with sufficient energy to activate the carrier in the trapped state via multiple physical mechanisms, such as impurity ionization, fielddependent EL2 capture, and impact ionization of donor centers EL10 and EL2. The critical fields calculated from the activation energy of these physical processes accord well with the experimental results. Moreover, agreement between the fitting curve and experimental data of J（E）, further confirms that the dark-state characteristics are related to these field-dependent processes. The effects of voltage on SI-Ga As PCSS may give us an insight into its physical mechanism.

Application of an Al-doped zinc oxide subcontact layer on vanadium-compensated 6H–SiC photoconductive switches

Al-doped ZnO thin film （AZO） is used as a subcontact layer in 6H-SiC photoconductive semiconductor switches （PCSSs） to reduce the on-state resistance and optimize the device structure. Our photoconductive test shows that the onstate resistance of lateral PCSS with an n＋-AZO subcontact layer is 14.7% lower than that of PCSS without an n＋-AZO subcontact layer. This occurs because a heavy-doped AZO thin film can improve Ohmic contact properties, reduce contact resistance, and alleviate Joule heating. Combined with the high transparance characteristic at 532 nm of AZO film, vertical structural PCSS devices are designed and their structural superiority is discussed. This paper provides a feasible route for fabricating high performance SiC PCSS by using conductive and transparent ZnO-based materials.

Optimization of wide band mesa-type enhanced terahertz photoconductive antenna at 1550 nm

A mesa-type enhanced InGaAs/InAIAs multilayer heterostructure （MLHS） terahertz photoconductive antenna （PCA） at 1550 nm is demonstrated on an InP substrate. The InGaAs/InAIAs superlattice multilayer heterostructures are grown and studied with different temperatures and thickness ratios of InGaAs/InAIAs. The PCAs with different gap sizes and pad sizes are fabricated and characterized. The PCAs are evaluated as THz emitters in a THz time domain spectrometer and we measure the optimized THz bandwidth in excess of 2 THz.

Photovoltaic Response Characteristics of GaAs Photoconductive Switches Under High Gain Mode

Given is the experiment results in which the laser pulses of 1 046 nm and 532 nm are used to trigger the semi-insulation GaAs photoconductive semiconductor switch(PCSS) with an electrode distance of 4 mm. And made is an analysis of the switchs photovoltaic response characteristics under the high gain mode when the biased field is bigger than the Geng effect field. Also a theory is presented that the main reason for the photovoltaic pulse response delay is the transmission of charge domain, caused by the presence of EL2 energy level in the chip material. Finally, the transmission time of charge domain is calculated and a result that inosculates with the experiment is attained.

Fabrication and Characteristics of Fast Photo Response ZnO/Porous Silicon UV Photoconductive Detector

Fast response time UV photoconductive detector was fabricated based on ZnO film prepared by thermal chemical spray pyrolysis technique. The ZnO nanofilms are grown on the porous silicon (PS) nanosurface which has drastically reduced the response time of the ZnO UV detector from few seconds to few hundreds of microseconds. The surface functionalization of the ZnO film deposited on porous silicon (PS) layer by polyamide nylon has highly improved the photoresponsivity of the detector to 0.8 A/W. The normalized de-tectivity (D*) of the fabricated ZnO UV detector at wavelength of 385 nm is found to be about 2.12 × 1011 cm Hz1/2 W–1. The ZnO film grown on the porous silicon layer was oriented in the c-axis and it is found to be a p-type semiconductor, which is referred to the compensation of the excess charge carriers in the ZnO film by the nanospikes silicon layer.

Photoconductive Effect of Liquid and Utilization of Solar Energy

Experiments described in this paper show that there is the photoconductive effect of liquid, i.e. when light shines into a sort of alkali, acid or salt solution, the conductivity of the solution will increase. The mechanism of the effect is explained as follows. When hydrated ions in the solution absorb photons with their high enough energies, they will decompose to naked ions and water-molecules. The naked ions can reach an anode or a cathode more easily and faster than the hydrated ions;It is possible that when a molecule in the solution absorbs a photon with its high enough energy, it will decompose to negative and positive ions. Based on the effect, a device producing hydrogen by the solar-energy had been devised.

STUDY ON STABILITY OF STRUCTURE AND PROPERTIES OF TIOPC/SAN PHOTOCONDUCTIVE COMPOSITES

The influence of the condensed structure of poly(styrene-co-acrylonitrile) (SAN) and traces of tetrahydrofuran(THF) that remained in titanyl phthalocyanine (TiOPc)/SAN films after fabrication on the photoconductive stability ofTiOPc/SAN composites is studied. The results reveal that the existence of traces of THF results in the increase of thephotoconductivity and the charging voltage. The main facors responsible for the unstable photoconductivity of thephotoconductors are believed to be the relaxation of SAN and the slow volatilization of THF.

Modeling of terahertz pulse generation from LT-GaAs ultrafast photoconductive switches

The technique of terahertz pulses generated from the photoconductive switches has been applied in the ultrafast electrical pulse metrology recently.A lumped-element theoretical model is established to describe the performance of the LT-GaAs ultrafast photoconductive switch used in the ultrafast pulse standard.The carrier transport processes of the photoexcited semiconductor,the attenuation and dispersion during terahertz pulse propagating are considered in the theoretical model.According to the experimental parameters,the waveforms of the generated terahertz pulses are calculated under optical excitations with different wavelengths of 840 nm and 450 nm,respectively.And comparisons between the theoretical results and the experimental results are carried out.