Research On Breakdown Mechanism And New Structures Of GaN-based Power Switching Devices

GaN materials's properties are high electron mobility,low dielectric constant,good thermal conductivity and radiation resistance etc which behaving excellent physical and chemical properties.At present,they are widely used in fields such as high current and high withstand voltage.However,with the in-depth research on GaN-based power devices,there is still a large gap between their performance and the theoretical limit.The main problem is:when the ordinary GaN HEMT is turned off,there is a phenomenon of electric field concentration under the edge of the gate metal,which easily causes the avalanche breakdown in the device to occur early;the device has a large delay and a slow switching speed during the turn-on and turn-off process of the device.In view of the above-mentioned problems,this article has conducted in-depth research on the new structure and switching characteristics of the device,and obtained the following results.First of all,this article innovatively proposes a GaN-based high electron mobility transistor(GaN PN-HEMT)with a junction field plate withstand voltage structure.The structure is characterized by the conventional gallium nitride HEMT device.A vertical PN junction is formed above the barrier layer as the channel electric field of the withstand voltage structure modulation device.When the gate is in the blocking state,the vertical PN junction diode will help deplete the two-dimensional electron gas in the device channel and reduce The voltage on the drain side of the gate edge reduces the peak electric field there.The simulation results show that the breakdown voltage of the proposed GaN PN-HEMT is 785V(@I_DS=1?A/mm)when the gate-to-drain spacing is 6?m.Compared with the conventional GaN HEMT and the gate field plate device(GP-HEMT),the breakdown voltage is increased by 85%and 32%.The on-resistance of the GaN PN-HEMT device is 0.55 m?·cm²,which is basically the same as the conventional device.The power figure of merit FOM of the new structure device has also been significantly improved,and its value has been increased from 0.384GW/cm² to 1.13GW/cm² of the conventional structure device,an increase of 195%.Secondly,this paper proposes a device-circuit hybrid simulation method(Mixed Mode)for the study of switching characteristics of GaN HEMT devices.Using Silvaco simulation software to build a peripheral simulation circuit,through in-depth analysis of the changes in the electrical characteristics of the gate-drain voltage,channel current,and switching speed during the turn-on and turn-off of the device,the study found that:the turn-on speed of the device with the PN junction field plate structure has increased from 304.8V/ns(t_r=1.05ns)of the conventional device to 363.6V/ns(t_r=0.88ns),and the turn-off speed has increased from the 217.7V/ns of the conventional device(t_f=1.47ns)to 268.9V/ns(t_f=1.19ns),the turn-on and turn-off speeds of GaN PN-HEMT devices are improved.Finally,in order to study the influence of the gate dielectric on the switching speed of the device,a gate dielectric is introduced between the gate metal of the p-GaN enhancement mode device and the p-GaN.The analysis of the results shows that the introduction of the gate dielectric can suppress the gate leakage current that exists when the device is turned on.The turn-off speed of the device has increased from 18.1V/ns(t_f=17.68ns)without a gate dielectric device to 174.9V/ns(t_f=1.83ns)with a gate dielectric device.Further research found that the structure of the gate field plate and the source field plate can also increase the switching speed of the device,while the drain field plate has almost no effect on the switching speed of the device.The simulation results show that after adding the gate field plate,the turn-on speed of the device increases from 264.5V/ns to 438.4V/ns,and the turn-off speed increases from 18.1V/ns to 69.4V/ns.