Research On Gate Of GaN Millimeter Wave HEMT Devices

With the progress of science and technology as well as the age of 5g communication,the existing first and second generation semiconductor devices can no longer meet the needs of technological development in the field of higher frequency,higher power,and lower power consumption.GaN,a novel compound semiconductor material,has a wide direct band gap,high electron saturation rate,high electron mobility and other excellent properties as well as the unique two-dimensional electron gas（2DEG）structure,which makes the third-generation semiconductor materials represented by GaN have broad application potential in the high-frequency field.In order to give full play to the advantages of GaN HEMTs in the high frequency field,this thesis based on the gate structure of the device,and obtains higher frequency characteristics by changing the gate structure of the device.Finally,the optimal structure and key processes are adopted to produce high-performance GaN millimeter wave devices.The I-gate process is developed and studied.By selecting the electron beam photoresist and controlling the lithography process,the experimental scheme of making the minimum gate length is obtained.Al GaN/GaN HEMT devices and In Al N/GaN HEMT devices were fabricated by the developed I gate fabrication process respectively.In Al N/GaN HEMT devices are superior to Al GaN/GaN HEMT devices in terms of maximum peak transconductance,maximum saturation current and maximum cut-off frequency for devices with gate length of 20 nm and source drain spacing of 3μm.The maximum cutoff frequency of Al GaN/GaN HEMT devices is 101 GHz,and that of In Al N/GaN HEMT devices is167GHz.The manufacturing process of the floating T-gate was optimized.The proximity effect distance between the gate cap and the gate foot is also optimized;the technology of removing the metal of the gate with blue film was proposed to avoid the phenomenon of the gate cap falling off in the wet stripping process;a process of lithography development is optimized to improve the yield of gate greatly.The optimized floating T-gate manufacturing process is used to fabricate Al GaN/GaN HEMT devices with a gate length of 120 nm and a source-drain spacing of 3μm,with a maximum peak transconductance of 308 m S/mm.When the gate voltage is 2 V,the saturation current is about 907 m A/mm.When the current density is1 m A/mm,the breakdown voltage of the device is 37 V.the maximum cut-off frequencyf_Tis 80 GHZ,and the maximum oscillation frequencyf_max is 200 GHZ.One method of using two kinds of photoresist spin three layers,by changing the exposure dose distribution of the second and third floor,by the time exposure and one development to directly get floating Y-gate structure is put forward,which obtained the tilted gate cap structure directly through study on characteristics of the upper gum exposure and the photoresist exposure dose tiered distribution.The Al GaN/GaN HEMT device manufactured by the floating Y-gate process,the device with a gate length of 120 nm and a source-drain distance of 3μm,has a maximum peak transconductance of 329 m S/mm.When the gate voltage is 2 V,the saturation current is about 859 m A/mm.The breakdown voltage of the device is 50 V when the current density is 1 m A/mm,maximum cut-off frequencyf_Tis 103 GHz,and the maximum oscillation frequencyf_max is 230 GHz.Compared with the floating T-gate device,the breakdown voltage of the device at the current density of1m A/mm is increased by about 13V,f_T of the device can be increased by about 23 GHz andf_max can be increased by about 30 GHz.Therefore,the floating Y-gate structure can greatly improve the breakdown voltage and frequency characteristics of the device,and will be widely used in the field of millimeter wave in the future.Finally,the thin-layer passivation of the floating gate is studied.It is found that the thickness of 20nm passivation is the optimal thickness for the structure of the floating gate with thin barrier,which can effectively suppress the current collapse and increase the peak transconductance.After passivation,the device appears the deterioration of subthreshold and off state characteristics,and then some pretreatment processes are added before passivation,in which N₂O+Si N20 pretreatment is considered to be the optimal condition from the characteristics of off state,subthreshold and breakdown,and from the saturation current and peak transconductance characteristics,Si N20 is considered to be the optimal condition.