Investigation Of Thermal Design Technology For High-power GaAs Photoconductive Semiconductor Switches

Miniaturization and repeat-operation is one of the main directions of current researches in pulsed power devices. PCSS is a new type of electronic devices fabricated by ultra-fast laser technology and semiconductor technology. Switch is on or off is controlled by changing the conductivity of semiconductor materials. PCSS has several merits, such as high power capacity, high repetition rate, low jitter, etc. PCSS attracts increasing attention in miniaturization, repeat-operation pulsed power device design. The Joule Heating of PCSS operation seriously affects the electrical properties and reduces lifetime of devices. The different photoconductive and work modes are discussed. Thermal characteristics vary with different photoconductive and work modes, such as the dark state heat conductor process can be depicted by Fourier Heat Conduction Law, but transient state has strong effect of Non-Fourier Effect. The heat conductor process of PCSS can be simplified by a properly assumption.The heat dissipation process of GaAs PCSS in nonlinear mode has been simulated base on Finite Difference Time Domain method (FDTD). The critical frequency was defined. Factors affecting the critical frequency, such as location, radius, number of current filament, dimensions of PCSS chip and environment temperature are discussed. The results are following:The critical frequency exponentially increases as radius, the number of current filament increases and exponentially decreases as distance between location of current filament and surface of chip, thickness of chip increases. The critical frequency linear decreases as environment temperature raising in a special range. When the temperature of PCSS is over the critical temperature, the PCSS is destroyed in first shot.PCSS chip temperature rising and hot spot forming caused by Joule Heating restrain the PCSS's power, repetition rate and lifetime, so initiative cooling technology is necessarily. In the paper, a high efficiency silicon rectangular micro-channels cooler is designed and experimental tested. The cooler has two sections:principal frame and cover, the front has an array of micro-channel between distributary groove and conflux groove, the latter is punched holes by semiconductor technology.Both are joined by Si-Si bonding. As water and FC-40 are coolants respectively, heat flux, temperature uniformity and fluid pressure drop in diverse coolant volumetric flow rate, inlet temperature are tested. The experiment results indicate that the cooler has a high heat transfer efficiency, low fluid pressure drop and nicer temperature uniform in a moderate coolant volumetric flow rate. The silicon rectangular micro-channels cooler satisfy repetition-rate high power PCSS chip cooling.In the meantime, a packaging structure for PCSS and semiconductor diode laser is discussed. It has two cabins separated by K9 optic glass window respectively, one equipped with semiconductor diode laser and the other equipped with PCSS. The FC-40 cooling by exterior heat radiator is pumped into Silicon Micro-Channel Cooler and semiconductor diode laser cooler. The PCSS cabin is filled by FC-40 and FC-40 is a kind of befitting insulating liquid for PCSS.Lastly, the principle, technics process and degradation mechanism of ohmic contact are discussed. Three hypotheses are proposed. They are:high temperature increase the mutli-ion-doping Interdiffusion; The thermal fatigue caused by the circular thermal stress of ohmic contact electrode and semiconductor material thermal mismatch leads to strip and off of ohmic contact electrodes; Thermal mismatch is a main factor of strip and off of Si3N4 protective layer package in some PCSS chips.