| Among the pulse power devices,switch technology plays a very important position.With the development of the pulse power,photoconductive semiconductor switches(PCSS)have received the attention for its excellent characteristics.PCSS is one of the most promising devices which include a photoconductive semiconductor and an ultra-fast laser.One of its advantage is compact,which is same to the other solid switchs,and the other advantages includes that picosecond order of magnitude of closing time,the scale of picosecond time jitter,excellent synchronization precision,and subnanohenry inductance and photoelectric isolation,which the traditional switch does not have.Especially,because of the high-gain mode(also known as nonlinear mode)in ?-?compound semiconductors such as Ga As,In P,et al,so PCSS can be triggered by weak light and make the switch system miniaturization,integration and cost savings.In view of the problems existing in the theory and technology research on semi-insulating(SI)Ga As PCSS,in this paper,the transport law of SI-Ga As PCSS biased at high electric field and the mechanism of surface flashover are investigated by combining theoretical and experimental study.Three aspects of research work have been completed in this paper.(1)Study of the mechanism of the nonlinear modelPhotoactivated charge domain model can well explain high gain PCSS experimental phenomenon,but the model did not consider the properties of LEC SI-Ga As material,the existence of high-density native defects EL2 energy level can affect the carrier transport and output characteristic of the switch.By the test of SI-Ga As PCSS at high field,we observed delay phenomenon and current overshoot phenomenon of Lock-on when triggered near the thresholds of optical energy and the field.Based on electric-field–enhanced trapping and electric-field–enhanced emission of EL2 level,thorough theoretical analyses are made.The result points out that carrier density attenuation and the extension of hole carrier life due to EL2 trapping restrain occurrence of avalanche collision ionization,and then phonon assisted tunneling effect of neutral EL2 level makes the carrier heat emission form the high carrier concentration needed for domain,and the avalanche domain forms once again.The Lock-on delay time depends on the drift time of hole to the cathode and the time of electric field up to the threshold of avalanche collision ionization.So the biased electric field is greater,the Lock-on delay time is smaller.Current overshoot phenomenon of Lock-on through the characteristics of EL2 level at high field is analyzed.At high field,phonon assisted tunneling effect of EL2 level causes the carriers instantaneous detrapping from neutral EL2,and high energy carrier concentration increases at the front of the domain,which results in the instantaneous increase of electric field in the front region of domain,high carrier density and high field surpass the threshold of avalanche collision ionization,which causes an avalanche ionization rate rising,and leads to more intense collision,and makes the carrier multiplication and the current increases.We point out that the delay mode and current overshoot presence under the condition of the electric field or light near their thresholds.Combination with the characteristics of EL2 energy level,the two phenomena was successfully explained.These analyses complement and perfect photoactivated charge domain theory.(2)Study of the mechanism of the surface flashover of SI-Ga As PCSSDamage of PCSS contains a very complex physical process,include surface flashover,local electric breakdown(which can be generalized to flashover),and thermal breakdown.These processes can cause various damage phenomenon of switch,limit the lifetime of the switch.Firstly,the lifetime test experiment is carried on in this paper the experimental law are summarized that the flashover is often occurred when the switch works at the nonlinear model,and flashover current lags behind the light current.Comparing the flashover and air breakdown luminescence spectroscopy,we put forward the material itself involved in the flashover and the surface electric field distortion is the key factor of the flashover.The gas desorption due to the heating effect of the current led to the decrease of flashover voltage.We proposed that the surface flashover occurred at the interface between switch insulation encapsulation and air,is essentially the breakdown of the gas discharge.And compared the flashover voltage of PCSS at SF6 environment with the pressure of 3 atm,we find that the flashover voltage doubled relative to the air.These experimental results verify the validity of the flashover mechanism which was put forward.Moreover,we analyzed the influence on flashover voltage of the passivation layer and solid insulation encapsulation and draw the conclusion that passivation reduces the switching surface state and inhibits the surface electric field distortion caused by the surface defect capture carrier,so that the flashover voltage increase.And the uses of transparent insulation encapsulation of very high dielectric strength inhibit the formation of flashover from two aspects.Firstly,the large dielectric constant of insulation encapsulation material will decrease the surface field intensity because of dielectric polarization;Secondly,the use of insulation encapsulation suppress the gas desorption,thus increase the flashover voltage and this conclusion has been testified by our experiment.The experiments showed the flashover voltage is much larger with insulation encapsulation than without the encapsulation insulation.Lastly,breakdown characteristic of the switch body is analyzed.Essentially,the body breakdown of PCSS is the thermal breakdown when the switch is triggered based at the electric field far less than breakdown field.The electrode loss and the crack of PCSS material are caused by thermal expansion.(3)Experiment study of SI-Ga As PCSS biased at high electric fieldUltra-wideband radiation experiments are carried on in this paper on the condition of SI-Ga As PCSS biased at 5k V voltage,and ultra wideband electromagnetic wave with the bandwidth of 8 GHz was obtained.Through analysis of the experimental results,the conclusion we have drawn is that the radiation bandwidth determined by switching the rise time of output electrical pulse and improving the response time of the switch can improve the bandwidth of the output pulse.But the power of the output pulse has been curbed as improving the response time of the switch.SI-Ga As photoconductive switch was triggered by a laser diode with the single pulse energy of 1.6?J,and we got linear output pulse and nonlinear pulse under different fields bias and got the large output current of 1k A at least when the PCSS biased at 4k V.Under high field bias,PCSS is prone to the surface flashover when PCSS work at high gain mode.Based on EL2 energy capture and thermal emission mechanism,the broadening characteristic of the linear mode output pulse are analyzed.Meaningfully,in the experiment,the narrow output pulses were got when PCSS worked at nonlinear mode,and its mechanism is the result of high gain quenching caused by the insufficient of capacitance voltage. |
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