Study On Key Technologies Of GaN Millimeter-Wave Power Devices

Gallium nitride（GaN）devices outperform Sidevices and GaAs devices in terms of efficiency,bandwidth,and output power due to the characteristics of GaN materials such as large bandgap width,high electron mobility,and high electron saturation speed.It has been about 20 years since the development of GaN millimeter-wave（mm W）devices.Through the improvement of material quality,device design,and process capability,the performance of GaN millimeter-wave devices has been significantly improved.In this paper,some problems of GaN millimeter-wave devices are studied,and the high power and high efficiency of millimeter-wave devices are finally realized.Firstly,the problems of low power and low efficiency commonly faced by mm W power devices are deeply analyzed.It is considered that the main factors limiting the performance of mm W devices are short channel effects,current collapse effect,low breakdown voltage and gate current leakage.The solutions to the above problems include rational design of device size,surface passivation,field plate,etc.On the basis of traditional research,a new bilayer passivation scheme,Schottky-ohmic hybrid drain and multiple T-gate field plate structure are proposed.Based on the SiN films grown by PECVD,a bilayer passivation scheme with the first layer being silicon-rich（Si-rich）SiN and the second layer being Si₃N₄ was developed.XPS analysis shows that there are more Si-H bonds in the Si-rich SiN of the first layer,which can reduce the density of Ga-O bond trap states at the SiN/GaN interface to alleviate current collapse.By comparing the reverse schottky leakage curve,we can see that the gate current leakage is dominated by the PF emission mechanism,and the Si-rich SiN/Si₃N₄ bilayer passivation reduces the original trap energy level from 0.53e V to 0.29e V.At the same time,the transient I-V test shows that the time constants of Si-rich SiN/Si₃N₄bilayer passivation are smaller than those of traditional Si₃N₄ passivation.The time constants at different temperatures are obtained by fitting the Aronneuz equation,and the activation energy of the trap is reduced from 0.49e V to 0.3e V.And then testing found on small signal characteristics degradation of small signal characteristics at high temperature was significantly suppressed.Through the small signal equivalent circuit modeling extraction,the phenomenon that the gate-source capacitance(C_gs)and gate-drain capacitance(C_gd)increase with the increase of temperature is improved.Through the effective gate length formula,the effective gate length decreases from 212nm to 180nm at 420K.Finally,the RF large signal characteristics are tested,and the output power density reaches 7W/mm and the maximum peak PAE reaches56%at 17GHz.Based on the low breakdown characteristics of thin barrier materials,we innovatively propose a Schottky-ohmic hybrid drain device structure to modulate the electric field distribution and realize the millimeter-wave device with high breakdown voltage and small current collapse.Firstly,we can see by Silvaco electric field simulation that an electric field peak is formed near the Schottky contact at the drain end,which reduces the electric field peak in the area under the gate.In the actual fabrication of a device with a source-drain spacing of 3μm,the breakdown voltage of the device with a Schottky-ohmic hybrid drain extension length of 0.7μm is increased from 41V to 91V.From the pulse output when the pulse static bias is(V_gs,q,V_ds,q=-8V,40V),it can be seen that the current collapse decreases from 12.5%to 7.5%.The small signal results of the device show that f _T and f_max are increased from the original 64GHz and 125GHz to 76GHz and 157GHz,respectively.According to the parameters extracted from the small signal equivalent circuit model,the drain resistance R_ddecreases from the original 1.3Ω·mm to 0.9Ω·mm.Then the large signal characteristics of the device are tested at 30GHz.The output power density reaches 8.5W/mm at 30V operating voltage,and the peak PAE is 45.5%.The results have reached the level of synchronization with international advanced indicators.Finally,the robustness of the device is tested,and it is found that the device has higher reliability than the traditional Ohmic-drain device.It can be seen that the Schottky-ohmic hybrid drain structure has greater advantages in performance and reliability,and provides a new way for millimeter wave power devices.Due to the low efficiency of traditional mm W devices,a multiple T-gate structure is proposed to reduce the electric field peak under the gate more effectively on the thin barrier material,which can inhibit the gate leakage and alleviate the current collapse.Firstly,through Silvaco electric field simulation,it can be seen that the multiple T-gate structure introduces multiple electric field peaks near the gate,which distributes the electric field originally concentrated in the gate foot area to form three small electric field peaks,which is helpful to improve the device characteristics.The DC test shows that the off-state current of the device decreases by about two orders of magnitude,reaching 10^-6m A/mm.The saturation output current is 1288m A/mm,and the transconductance is about 470m S/mm.The breakdown voltage in off state is increased by 42%from 67V to 95V.Through the pulse output test,the current collapse is 7.3%;Pulse transfer characteristics show that the threshold drift is less than 0.1V.By extracting the mobility of FAT-FET structure in the linear region,the mobility of electron in the channel is increased by about 9.7%.The small signal characteristics of the device are tested and the f _T and f_max are 87GHz and 158GHz,respectively.Finally,the device is tested for large signal at 30GHz,and the peak PAE is greater than 60%and the drain efficiency is greater than 70%.At the same time,the output power density can reach 6.1W/mm at 20V,which is in the leading level in the world.To sum up,this paper develops millimeter wave devices with high power density and high efficiency respectively,studies the mechanism of realizing high performance respectively,and proposes a Schottky-ohmic hybrid drain device and a multiple T-gate device,which provides a new method for the development of millimeter wave devices and has certain reference value for future research.