Research On Current Filament Mechanism Of Non-linear Semi-insulation GaAs Photoconductive Semiconductor Switches

With the rapid development of pulsed power technology,the Ga As photoconductive semiconductor switch(PCSS)with power and band width arouses broad attention.On the nonlinear mode,the current filament phenomena will occur in the Ga As PCSS.Because the phenomena are so complicated and uncertain,up to now the latent m echanism is not very clear.Based on some experiments,the paper discusses forming mechanism of current filament on the nonlinear mode.Firstly,a new streamer model for the Ga As PCSS is established.Secondly,this paper studies on changes of the dark resistance which is caused by current filament.Thirdly,the mechanism of the flashover phenomenon is analyzed.Lastly,the failure reasons which are caused by current filament are analyzed.In this paper,the specific do the following:(1)The paper proposes a new streamer model to explain current filament phenomenon.The model includes three aspects:the streamer generation,the streamer development and the streamer flowing through PCSS.There are two conditions for streamer formation:one is the carrier density which should be not less than 10¹⁷cm^-3,and another is the electric field which should be much higher than 250 k V/cm.Avalanche ionization and compound radiation are the physical factors of the streamer development.When the current filament flows through PCSS,Lock-on effect is caused by dynamic distribution of transient voltages of PCSS and the load.On a micro level,Lock-on effect is caused by dynamic change of production rate and recombination rate of carries.Based on this streamer mode,the current filament velocity is calculated as about 2×10⁸cm/s.Then the current filament phenomenon is explained by Franz-Keldysh effect.(2)The physical mechanism of dark resistance change of the Ga As PCSS is explained.After the Ga As PCSS was damaged,there are two different influences of current filament on Ga As dark resistivity.One is reduction of dark state resistance,but the order of magnitude is not change,another is dark state resistance decreased by two orders of magnitude.The paper deduces the concentration of EL2 varies proportionally to the resistivity of the material.The heating effect of current filament causes dissociation of Ga As crystals and the evaporation of arsenide;moreover,it also leads to reduction of EL2 concentration.The reduction of EL2 concentration ultimately results in the reduction of dark res istivity of the Ga As material.Under a large current,the body of current filament excited a large number of hot electrons.These energetic electrons caused defects in the bulk of PCSS.The catastrophic ablation occurs when these defects reach a critical density level.(3)Based on the experiment,the physical mechanism of the flashover phenomena is studied.The paper proposes two conditions for flashover formation:one is the Ga As PCSS working on the nonlinear,and another is the electric field which should be much higher than electric field of air-ionization.The mechanism of flashover formation is quantitatively described.The main causes of flashover formation are hot electrion and electric field distortion caused by current filament.The flashover is not air-breakdown,contrast with spectrum of flashover and spectrum of air-breakdown.The pinch effect is the main reason for the periodic oscillation of time waveform of the surface flash over.It was pointed out that the flashover channel was pinched with the self-excited magnetic field.During the pinch process,the interaction of the magnetic field and dynamic pressure caused periodic oscillation of radius of flashover channel,which led to time waveform periodic oscillation.Under the low current,the local flashover near the electrodes caused damage of Ga As PCSS.Under the high current,the thermal stress is the main causes of cracks of Ga As PCSS.(4)The damage mechanism of the Ga As PCSS in different current intensity was studied in this paper.Operating under low current,a percolation theory is used to explain the failure.On the passage of energetic carriers,local thermal breakdowns are generated randomly.These local thermal breakdowns cause dot damage on the surface of PCSS.A conduction path is created and catastrophic breakdown occurs when the dot damage is dense enough to form a continuous chain connecting the PCSS electrodes.Operating under the high current,the thermal stress is the main cause of the failure of high-power Ga As PCSS.It shows that the high stress is mainly concentrated around the current filament and electrode.The stress causes local fracture on the surface of PCSS,and it further leads to disintegration of high-power PCSS.