Research On Floating T-Gate Technology Of GaN Millimeter Wave HEMT Device

GaN-based HEMT devices have broad application prospects in the field of microwave and millimeter-wave devices because of the excellent properties of GaN materials,such as high breakdown electric fields,high electron mobility,and electron saturation rate,and unique two-dimensional electron gas(2DEG)structures.With the further demands of wireless communication technology on the frequency characteristics of devices,the frequency performance of GaN-based HEMT devices also needs to be further improved.Therefore,in this paper,starting from the optimization of the device gate structure,the key technology of the floating T-gate is studied to improve the frequency characteristics of the device,and finally a GaN millimeter-wave HEMT device is fabricated.This article first briefly introduces GaN materials and GaN HEMTs;analyzes the effect of gate parasitic parameters on the frequency performance of the device,and proposes methods to optimize the various parasitic parameters of the gate;introduces the significance of gate structure research,and the application advantages of floating T-gates in millimeter-wave devices and the development process of floating T-gates;the photolithography process and the manufacturing process of millimeter-wave devices are described in detail.Development and research of floating T-gate technology.A process method of spin-coating three layers with two types of photoresist was proposed to directly obtain the floating T-shaped gate structure after one exposure and one development,which greatly simplifies the complicated process of traditional floating T-shaped gate require multiple exposures or multiple developments.This method utilizes the effect of temperature on the sensitivity of photoresist,reduces the mutual soluble layer between photoresist by increasing the photoresist baking temperature,and improves the flatness of the gate structure;different baking temperatures are used for the same photoresist to obtain different sensitivities,so that an undercut structure that is favorable for lift-off is directly obtained during the exposure process.The floating T-shaped gate produced by this process has a self-supporting structure,is mechanically stable,and has low gate resistance.The T-shaped gate structure is flat and the air is below the gate cap,which greatly reduces the parasitic capacitance of the gate to achieve an purpose of increase frequency,very suitable for application in millimeter wave HEMT devices.An Al GaN/GaN HEMT device was fabricated using a floating T-gate process.The device has a gate length of 100 nm and a source-drain spacing of 2 ?m.The maximum peak transconductance is 214 m S / mm.When the gate voltage is 2V,the saturation current is about 879 m A/mm;saturation current cut-off frequency ?_T is 60 GHz,and the maximum gain cut-off frequency ?_max can reach 115 GHz.Compared with the conventional T-gate device,?_T of the device can be increased by about 22 GHz;?_max can be increased by about 15 GHz.It can be seen from the experimental results that the frequency characteristics of the devices manufactured by the floating T-gate process experimentally developed in this paper have been greatly improved.A study of floating T-gate devices of different sizes found that as the gate length increases,the saturation current of the device decreases,and the threshold voltage increase,but the gate current basically does not change with the increase of the gate length,and they are all in the same order of magnitude.As the source-drain spacing increases,the device's maximum peak transconductance gradually decreases,the saturation current of the device decreases significantly,and the threshold voltage increase.The floating T-gate process was used to develop In Al N/GaN millimeter-wave HEMT devices.For devices with a gate length of 100 nm and a source-drain spacing of 1.5 ?m,the maximum saturation current is greater than 1300 m A/mm;the maximum peak transconductance is 360 m S/mm.The saturation current cut-off frequency ?_T of the device reaches 90 GHz,and the maximum gain cut-off frequency ?_max reaches 200 GHz.Therefore,the floating T-gate process has great advantages in the fabrication of millimeter-wave HEMT devices.It is expected that a higher-frequency GaN-based HEMT device can be produced by reducing the gate length and the source-drain spacing to meet the application requirements for the frequency characteristics of the device.