The Fast Turn-off Control Study On Photoconductive Semiconductor Power Switches

A semi-insulation photoconductive semiconductor switch(PCSS)has many advantages,such as low-jitter when triggering,high-speed response,high repetition frequency,high power,and anti-electromagnetic interference,etc.Therefore,it has broad application prospects in ultra-fast electronics,pulsed power technology and THz technology.Especially,GaAs PCSSs can operate in the high-gain mode triggered with a significantly lower laser eneryg than that needed in the linear mode,whereby the high-gain operating mode can greatly improve the system portability and reduce the cost.The past studied on high-voltage PCSSs mainly focused on the turn-on velocity and the low-jitter characteristics.However,this thesis focuses on the natural turn-off characteristic and the forced turn-off method to obtain a higher repetition frequency.The optimization premise of the structure and the parameter is without obvious power-peak loss.Based on the Atlas software of Silvaco TCAD,a semiconductor simulation platform,in this thesis some quantitative analysis on the turn-off characteristic of PCSSs under linear and nonlinear models respectively and offers relative control plans.In the first part,the turn-off characteristic of PCSSs and its sensitive factors are analyzed under the linear mode based on the two basic structures of PCSSs to optimize the design for accelerating the device turn-off velocity.First,the relevant physical simulation models of the structures are created based on the Atlas software,one module of the Silvaco TCAD semiconductor simulation platform,and the advantages and disadvantages of the structures are analyzed and compared.Second,the effects are quantitatively analyzed varying the carrier lifetime,the laser wavelength,the laser triggering position and the triggering methods of the PCSS simulation models.Finally,some of the simulation conclusions are compared with the previous experiments and they are in agreement with each other,which validates the simulation models.There is an internationally accepted hypothesis of Photon-Activated Charge Domain(PACD)by Wei Shi,to explain that the phenomena experimental phenomena of the nonlinear mode when such a GaAs PCSS is biased under a high electric field.In the second part of this thesis,based on the previous hypothesis and the simulation model made with Atlas,the detailed space-time-evolutionary cyclic processes of the PACDs are simulated and then analyzed optical excitation of a charge domain → to grow enough and then begin to impact ionization → to become an avalanche luminous domain → to compete voltage with a new charge domain →to be substituted by the new domains → the new domain growing until avalanche ionization.Next,the experimental comparative analysis proves that only those PCSSs made on a direct-bandgap semiconductor materials with the transferred-electron effect can operate into the nonlinear mode.Finally,Thermodynamics reason for PCSS to quit from the nonlinear mode is analyzed through quantitative calculation.The comparison between the above simulation results and the previous experiments proves that the effectiveness of simulation models are valid.Furthermore,The relevant lumped-parameter models of the experimental circuit is built.Based on the transient photocurrent waveforms from the previous PCSS experiment and the Matlab tools,the periodic changes of volt-ampere characteristics during the "lock-on" state is calculated to a GaAs PCSS operating in the nonlinear mode.The periodic changes validate the PACD theory since the oscillation period predicted with the PACD theory is close to the above calculated period.All of the above research results provide clear evidences for the PACD theory and the Atlas simulation PCSS models is helpful to the following studies on the control methods of the PCSS high-gain process.The lock-on time under the high-gain mode reaches up to microsecond range if without any artificial control,which greatly limits the switching repetition frequency and the device lifetime.Therefore,in the third part of this thesis,the control methods of the high-gain quenching are studied in order to improve the maximum peak power and the repetition frequency,First,based on the above physical mechanism analysis of the nonlinear mode and the traditional vertical PCSS structure,a novel PCSS structure named Insulated-Gate PCSS(IGPCSS)is presented,in which the multi-cell-MISFET structure is introduced.Next,the IGPCSS structure models are simulated with the Atlas and then the design parameters are optimized to ensure the voltage ratio of the internal electrically-controlled area and the optically-controlled area.Finally,the IGPCSS expected performances are demonstrated with the Atlas simulation results.