Semi-insulating GaAs Photoconductive Switch Research Articles

Study of photoexcited-carrier dynamics in GaAs photoconductive switches using dynamic terahertz emission microscopy

We propose dynamic terahertz (THz) emission microscopy (DTEM) to visualize temporal–spatial dynamics of photoexcited carriers in electronic materials. DTEM utilizes THz pulses emitted from a sample by probe pulses irradiated after pump pulse irradiation to perform time-resolved two-dimensional mapping of the THz pulse emission, reflecting various carrier dynamics. Using this microscopy, we investigated carrier dynamics in the gap region of low-temperature-grown GaAs and semi-insulating GaAs photoconductive switches of the identical-dipole type. The observed DTEM images are well explained by the change in the electric potential distribution between the electrodes caused by the screening effect of the photoexcited electron-hole pairs.

Infrared Inhibition of Flashover in Photoconductive Semiconductor Switch in High Electric Field

Semi-insulating GaAs photoconductive switches hold great potential for use in high-voltage switching applications. However, the utility is restricted by surface flashover, which resulted in breakdown in previous experiments. In this letter, we designed a configuration based on infrared inhibition to suppress the surface flashover in high-voltage operation. Bias voltage can be up to 32 kV with a 0.9-kA switching current. Results show that transport of photo-activated charge domain is interrupted by the formation of new domains due to second laser trigger.

Peculiar Transmission Characteristics of the Large Gap Semi-Insulating GaAs Photoconductive Switch

Unique experimental phenomena are discovered in a large gap semiinsulating (SI) GaAs photoconductive semiconductor switch (PCSS) and the peculiar transmission characteristics are exhibited in the experiment. The transmission characteristics for the large gap SI-GaAs PCSS are entirely different from the commonly designed PCSS. By analyzing the differences of the transmission characteristics between the common and the large gap SI-GaAs PCSS, a detailed statistical analysis and theoretical explanations are expounded. The large gap SI-GaAs PCSS works in the overvoltage relaxation limit space charge accumulation (LSA) mode when the conditions of 5 × 104 s · cm−3 ≤ n0/f ≤ 3 × 105 s·cm−3 and n0L ≥ 1013 cm−2 must be met in the switch, with n0 being carrier concentration and f the frequency. The large gap SI-GaAs PCSS we developed has not shown the nonlinear (lock-in) behavior at high bias voltage, so the withstand voltage and service life for PCSS are improved.

Light controlled prebreakdown characteristics of a semi-insulating GaAs photoconductive switch

A 4 mm gap semi-insulating (SI) GaAs photoconductive switch (PCSS) was triggered by a pulse laser with a wavelength of 1064 nm and a pulse energy of 0.5 mJ. In the experiment, when the bias field was 4 kV, the switch did not induce self-maintained discharge but worked in nonlinear (lock-on) mode. The phenomenon is analyzed as follows: an exciton effect contributes to photoconduction in the generation and dissociation of excitons. Collision ionization, avalanche multiplication and the exciton effect can supply carrier concentration and energy when an outside light source was removed. Under the combined influence of these factors, the SI-GaAs PCSS develops into self-maintained discharge rather than just in the light-controlled prebreakdown status. The characteristics of the filament affect the degree of damage to the switch.

Transient thermal effect of semi-insulating GaAs photoconductive switch

Results of experiments of the 4 mm gap semi-insulating(SI) GaAs photoconductive switch triggered by 1064 nm, 1.0 mJ pulse laser showed the nonlinear mode when the bias field was 3800 V. Under the same bias electric field and trigger light energy conditions, the switch outputs stably nonlinear electrical pulses, and the switch surface injury mark is caused by filamentation after 1500 times triggering. Analysis shows that under given conditions of trigger energy and electric field, two transient thermal effects occur in the switch chip, namely the thermal relaxation and photoactivated charge domain-phonon drag, respectively. Thermal relaxation time is shortened to the order of picoseconds or subpicoseconds, thermal relaxation process leads to the thermal conduction relaxation. When photoactivated charge domain moves at 107cm/s speed from cathode to anode, switch chip transient temperature makes relaxation oscillations owing to these effects, and the rapid increase of temperature in the chip is constrained. Photoactivated charge domain-phonon drag effect transmits in the direction of the dislocation movement, the temperature in mobile region increases when the flow of thermal energy carried by the phonons was concentrated in the movement plane, the injury of filamentation is produced by superposition and cumulation of mobile tracks.

Photoresistances of semi-insulating GaAs photoconductive switch illuminated by 1.064 μm laser pulse

The Shockley–Read–Hall model (SRHM) and its simplified model (SSRHM) were used to describe the characteristics of a photoconductive semiconductor switch (PCSS) made from a semi-insulating (SI) gallium arsenide (GaAs) chip, biased at low voltage, and illuminated by a 1.064 μm laser pulse. These characteristics include the free carrier densities, dynamic photoresistance, and time evolution of output pulses of the PCSS. The deep donor EL2 centers in SI GaAs play a dominant role in both the SRHM and SSRHM as electrons at EL2 unionized centers are strongly excited by the subband-gap photons at the wavelength of 1.064 μm. Theoretical modeling on the evolution of the experimental measured output pulses led to a two-step micromechanism of electron excitation process within the GaAs chip. The minimum photoresistances predicted by the SSRHM are in good agreement with experimental measurements, which confirms the dominant role of EL2 in the generation of electric pulses from a SI GaAs photoconductivity switch on which the 1064 nm laser pulse is illuminated.

Current limiting effects of photoactivated charge domain in semi-insulating GaAs photoconductive switch

The photoactivated charge domain (PACD) plays an important role in the nonlinear modes of semi-insulating GaAs and LnP photoconductive switches. The formation and transporting process of photoactivated charge domain are discussed in this paper, which indicate that it is the shielded electric field that induced the unique distribution and evolution law of the PACD. The PACD restricts space-charge current in the photoconductor and the output current of the photoconductive switch by its shielded effect.

Characteristics of photoconductivity oscillation in semi-insulating GaAs photoconductive semiconductor switches

The 4 mm gap and 5 ns pulse width semi-insulating GaAs photoconductive switches were triggered by 532 nm laser pulse with gradual increase of bias voltage from 500 V in steps of 50 V until the emergence of nonlinear electrical pulse. The expermental results showed that the linear and nonlinear electrical pulse waveforms had smaller amplitude and varying degrees of oscillation reduction after going through a main pulse. Then the microscopic state and transport process of carriers （hot-electron） in the switch material were studied in detail using the quantum theory. It was found that in the DC bias electric field, when the relaxation time of the hot-electrons in the electron-electron and electron-phonon interaction process is longer than the carrier life, the photoconductivity oscillation can be caused by the change of mobility in the process of optoelectronic transport, which is the main cause for the output electrical pulse to show oscillations.

Ultrafast rising of output electric impulse of lock-on model of semi-insulated GaAs photoconductive switches

In the lock-on model of semi-insulating GaAs photoconductive switches，the output current pulse has ultra-fast rise character，the rise time can be even less than that of the laser pulse. Fluid model is used to simulate the transport of photo-induced carrier. The results show that photo-activated charge domain formed in the transport process of carriers is the main cause of ultra-fast rise time. Further studies on the establishment of photo-activated charge domain and the absorption process show：1. The photo-activated charge domains during the establishment enhance the electron density gradient. 2. When the photo-activated charge domains are absorbed by the anode，rapid decline of bias electric voltage will accelerate the rise rate of output current pulse. Due to the two processes, the output current pulse in the lock-on model shows ultra-fast rise character.

Transient and steady state simulations of internal temperature profiles in high-power semi-insulating GaAs photoconductive switches

Simulations have been performed to determine the internal temperature profiles of high-power GaAs photoconductive switches in the presence of a current filament. No thermal instability is predicted below a power generation density level of about 1.3×1014 W/m3. This prediction is in keeping with recent experimental data on photoconductive semiconductor switch devices. It is shown that this power dissipation density threshold for stability exists under both dc and transient conditions. A simple model provides qualitative support for the power density threshold, and an explanation of the filamentary current radii that have been observed experimentally.

Temperature-dependent terahertz output from semi-insulating GaAs photoconductive switches

The temperature dependence of the terahertz (THz) output power and spectra from biased photoconductive switches was measured for several antenna gap widths and applied biases. The spectrally integrated THz output had a nonmonotonic temperature dependence in all cases with the value increasing by a factor of 3 from room temperature to 150 K for low biases and 100 K at high biases. An abrupt decrease in output power occurs below 90 K, and the spectrum shifts to lower frequencies as the temperature is lowered.

Temporal development of electric field structures in photoconductive GaAs switches

The temporal development of the electric field distribution in semi-insulating GaAs photoconductive switches operated in the linear and lock-on mode has been studied. The field structure was obtained by recording a change in the absorption pattern of the switch due to the Franz–Keldysh effect at a wavelength near the band edge of GaAs. In the linear mode, a high field layer develops at the cathode contact after laser activation. With increasing applied voltage, domainlike structures become visible in the anode region and the switch transits into the lock-on state, a permanent filamentary electrical discharge. Calibration measurements show the field intensity in these domains to exceed 40 kV/cm, which is greater than three times the value of the average applied field.

A comparative study of Si- and GaAs-based devices for repetitive, high-energy, pulsed switching applications

A study is performed to assess Si and GaAs as materials for realization of repetitive, high-energy, pulsed switches in applications where the switching parameters are blocking voltages (VB) exceeding 1 kV in the off state and conduction currents (IF) in excess of 500 A in the on state, the current risetime being less than 1 μs and the pulse length being longer than 50 ns. Theoretical and technological limitations associated with the switching characteristics of Si- and GaAs-based majority carrier (unipolar) and minority carrier (bipolar) devices in the low-field, high-mobility, and high-field velocity saturation regimes are analyzed and discussed. It is concluded that for medium power applications (VBIF<300 kW,VB≳1 kV), majority carrier devices are best suited for fast switching processes in the low-field, low current density (J<100 A/cm2) regime. Under such conditions, the high drift mobility of GaAs allows for realization of field-effect devices exhibiting fast switching speeds and low on-state conduction losses. For pulsed switching in the high-power regime (300 kW<VBIF<30 MW, VB≳1 kV), bipolar structures exhibit the most desirable characteristics, while compared to their Si counterparts, the on-state conduction losses of GaAs-based devices are extremely high. These considerations are extended to stacked Si bipolar devices and semi-insulating GaAs photoconductive switches for ultrahigh-power (≳30 MW) switching applications.

Improved contacts to semi-insulating GaAs photoconductive switches using a graded layer of InGaAs

High performance photoconductive switches are demonstrated using a molecular beam epitaxy grown graded layer of InxGa1−xAs to make an ohmic contact to semi-insultating GaAs. It is found that the graded layer results in a peak signal amplitude that is more than a factor of 2 larger than what is obtained using conventional alloyed AuGeNi contacts.