Research On The Scaling-down Laws Of Ga-face GaN HEMTs Related New Device Structures

GaN materials show outstanding performance in the field of high power devices and high frequency devices,and have been considered one of the ideal semiconductor choices for the new generation of microwave or millimeter-wave power devices.The target characteristics of GaN HEMT devices include high output impedance,high breakdown voltage,high transconductance,good ohmic contact,good pinch characteristics,high and stable cutoff frequency and maximum oscillation frequency in large ranges of bias and output current,and high reliability.In recent years,high-speed and high-frequency power applications have raised challenging demand on the operation frequency of the devices,so the size of GaN HEMT device is aggressively reduced to promote the frequency characteristics.At the same time,some negative phenomena emerged such as the large leakage current and the drain induced barrier lowering effect.Therefore,it is necessary to study the internal physical relationship between the parameters such as gate length,source drain interspace,characteristic frequency and so on.Aiming at the research status of GaN HEMT devices,it is explored the scaling-down rules of Al GaN/GaN HEMT devices in this thesis.The main research work and achievements are as follow:1.The influence of gate length reduction(863nm~58nm)on device performance of T-shaped Al GaN/GaN HEMT devices was explored by device fabrication and measurements,and the influence of short channel effect is revealed.The Al GaN/GaN heterojunction is used to realize a series of GaN-based HEMT devices with equal source-to-drain spacing and different gate lengths.From the device test results,it can be seen that saturation drain current,transconductance and frequency characteristic of the device increases with the decrease of gate length.At the same time,the source-drain saturation voltage of device is increased and threshold voltage is shifted to the negative direction.It becomes difficult to turn off the device,and subthreshold current increases,and so the short channel effect becomes more and more significant.2.The effect of gate length and barrier layer thickness on the characteristics of GaN-based HEMT devices was studied by numerical simulation.It is found that if the thickness of Al GaN barrier layer is reduced while reducing the gate length,it is possible to reduce the adverse effects.Though thin barrier layer would reduce the saturation current of device,source-drain saturation voltage,threshold voltage,subthreshold swing and transconductance of the device will be improved.Meanwhile,thinner barrier layer leads to larger parasitic capacitance but higher cutoff frequency,which is mainly caused by the increase of the transconductance.3.The performance change of the GaN HEMTs with simultaneous verical and lateral downscaling was achieved by numerical simulation of the devices with the gate length of 50 ~ 600 nm and constant two-dimensional electron density kept for ungated Al GaN/GaN heterojunctionand kept.The results showed that saturation current and operating frequency increases with the downscaling,on-resistance and absolute value of threshold voltage decreases gradually.The variation of cutoff frequency with the device size agrees well with that reported by Jessen et al.The physical mechanism analysis shows that decrease of gate length and source-drain spacing,the increase of Al composition of Al GaN layer are beneficial to reduce on-resistance and increase saturated current.Thin barrier layer is favorable to alleviate the threshold voltage drift and sub-threshold swing increase with the decrease of the gate length.The increase of operating frequency mainly benefits from the decrease of source-drain spacing,gate length and thickness of barrier layer.Therefore,under the condition of constant 2DEG density,the simultaneous lateral and vertical scaling-down devices can avoid the influence of severe short channel effect,and keep good performance.