Design And Optimization Of P-gate AlGaN/GaN Photoconductive Power Switch

GaN is regarded as the best material to fabricate high-voltage high-repetition-frequency photoconductive semiconductor switches(PCSSs)due to its advantages such as wide band gap,high saturated electron velocity and great thermal conductivity.However,it is currently impossible to grow an intrinsic GaN free-standing substrate,and only semi-insulating GaN:Fe substrates can be used to fabricate GaN photoconductive switches.Therefore,the semi-insulating GaN:Fe PCSS with any traditional structure of PCSS is unable to withstand high DC bias voltage,since the dark-state leakage current in the traditional PCSS always increases approximately linearly with the bias voltage.In this paper,a novel vertical photoconductive switch(named P-gate AlGaN/GaNPCSS)is designed,which comprehensively utilizes the structural advantages of enhanced high electron mobility transistor(HEMT)with a p-GaN gate cap layer,a junction field-effect transistor(JFET)and a Zener diode.Through device modeling and optimizing,the design goals are realized,including the withstand voltage greater than DC 10 kV,the high repetitive operating frequency greater than 10 MHz triggering with a 0.5-ns-pulsewidth laser,and the large current capacity based on parallel multiple cells and current automatic homogenization.First,a device simulation model of the P-gate AlGaN/GaN PCSS structure is established.In order to improve the reliability of the device simulation model,the suitable physical models fitting for the relevant GaN and AlGaN material are used,such as the temperature-dependent band-gap model,the high-field mobility model with negative differential mobility regions and saturation drifting regions,the impact ionization model,the polarization model of the two-dimensional electron gas,the generation rate model of the photo-generated carriers and so on.In order to ensure the credibility of the simulation results,the important parameter values in the physical models are taken from actual experiments as much as possible.Secondly,the dark-state leakage current,the static electric field distribution,and the JFET space charge area distribution at 10kV DC bias are analyzed;the dynamic characteristics of the device under the asynchronous triggering of the gate voltage pulse and the sub-bandwidth laser pulse are analyzed;the phenomenon of photocurrent tailing and its generation mechanism are discussed.Finally,the influence factors of the design parameters are analyzed based on the control variate method under the different structural modifications,for optimizing the through-current capacity,the withstand voltage and the turn-off speed.Hence,the cell area is reasonably decreased to increase the cell count per unit area inside the PCSS device.Furthermore,for decreasing the local current crowding in the cell,the specific phenomenon of GaN growth is investigated on the enrichment of unintentional-doping silicon at the interface between the substrate and the first epitaxial layer,and the silicon enrichment layer is utilized as a current-homogenization layer.In order to prove the benefits of improving the device structure,the device model of the traditional vertical PCSS with the same substrate is built as a control group.The differences between the two kinds of device are analyzed in the withstand voltage,the turn-off time,and the device operation temperature.The sensitive parameters to the key layer structure are analyzed and optimized.The switching power dissipation of the device are theoretically analyzed,the turn-on and turn-off losses of the device before and after being modified are compared,and then the Joule heat problem related to the switching loss and the factors affecting the switching loss are analyzed and optimized.The final simulation results show that the design goal of 10 kV/10 MHz is achieved.