Simulation Study Of N-polar GaN-based HEMTs

Nitrogen-polar GaN based high electron mobility transistors(HEMTs)have advantages of low ohmic contact resistance and strong 2D electron gas(2DEG)confinement compared with traditional Ga-polar GaN based HEMTs.It has great potential in applications such as high-power microwave power transistors,CMOS logic circuits,ultra-high speed digital signal integrated circuits and systems.At present,N-polar GaN-based HEMT is still in the early stage of development.Some key fundamental issues need to be solved,such as the high-quality N-polar III-nitride materials epitaxy technology,mechanism of charges directional accumulation and electric field distribution in N-polar GaN/AlGaN heterostructures,etc.In order to analyze the mechanism of electric field distribution and charge directional accumulation in N-polar GaN/AlGaN heterostructures,the relationship between 2DEG in channel and N-polar HEMTs device structure parameters,the influence of different HEMT structures on DC characteristics and the influence of accumulated charges on the RF performance should be further studied.In this work,the issues mentioned above are studied by N-polar GaN/AlGaN HEMTs simulation.The main research contents are as follows:1.Study on DC characteristics of N-polar GaN-based HEMT.The effects of the donor impurity doping concentration in gradient AlGaN layer and the thickness of AlN interlayer on the 2DEG density of N-polar HEMTs were systematically analyzed.A N-polar HEMT device with good DC performance is obtained after optimizing the structure parameters.The saturation output current density of a 2-?m-gate device is0.85 A/mm at zero gate bias.2.N-polar GaN-based heterojunctions charge distribution model.Firstly,the electric field distribution and charge accumulation of ideal N-polar GaN/AlGaN/GaN heterostructures are systematically analyzed.The behavior and formation mechanism of 2DEG and 2DHG(2-dimensional hole gas)in the structure are expounded.We propose a N-polar HEMT structure of GaN/AlGaN/AlN heterostructure for the practical application and establish the charge distribution model based on the simulation results.The simulation results show that the 2DHG at the negative polarization interface can be effectively eliminated and the higher-density channel 2DEG can be obtained without intentional doping.In addition,the GaN/AlGaN/AlN heterostructure was further optimized to improve the electron barrier height of the conduction band and enhance the limitation of 2DEG.A novel GaN/AlN/AlGaN/AlN heterostructure was obtained which can be used in N-polar HEMT devices.3.Study on RF characteristics of N-polar GaN HEMT.The simulation results show that the 2DHG generated at AlGaN/GaN interface has a negative effect on the frequency performance of N-polar HEMT devices.To solve this problem,we proposed a novel GaN/AlN/AlGaN/graded Al_xGa_1-xN/AlN HEMT device structure based on the charge model.This device can effectively eliminate the 2DHG induced in the traditional HEMT structure and improve the RF characteristics of device.In addition,the strong polarization electric field and wide bandgap of AlN barrier in the proposed structure can induce a high-density 2DEG of 5.7×10¹³ cm^-2and more over improves the limitation of 2DEG.The 100-nm-gate HEMT has a saturation output current density of11.5 A/mm at 0 V gate bias.The peak transconductance g_m of that device is 1.36 S/mm and the cutoff frequency f _T is 220 GHz under the test condition of leakage voltage of 3V.