Simulation Of Enhancement-mode Recessed-gate AlGaN/GaN HEMT Devices

GaN, wide bandgap semiconductor, has excellent performance in physical chemistry and electric properties. Just because of this reason, it is considered the third generation semiconductor materials, likes the first generation element semiconductor materials Si and the second generation compound semiconductor materials such as GaAs, GaP and InP. Compare with recently semiconductor materials, GaN has some semiconductor features (large direct bandgap, high saturation drift velocity, large conduction band discontinuities, high thermal stability and strong piezoelectric and spontaneous polarization) which indicates it can be applied in high-temperature, high-frequency and high-power field. Especially, AlGaN/GaN devices have far outweigh any semiconductor materials in microwave high power and high-temperature occasion.However, all of research focuses on depletion-mode GaN devices for the usage in high-voltage switching and driving of high speed circuit. It ensures that only if we take positive gate voltage that get the operating current in above mentioned occasion. So the research and design has an important issue and worth for depletion-mode GaN devices.This paper analyzes the basic operation principle and physical model such as the drift diffusion transport model, carrier mobility model, SRH generation-recombination model and selberherr's impact ionization model. Considering device model and material properties, we simulate the depletion-mode devices using device simulation tools ATLAS. By means of numerous 2D device simulation, juxtaposes the different effect which caused by different structure parameters. Meanwhile, find out that AlGaN layer doping, Al barrier layer constituent and Schottky barrier height can influence devices performance. From simulation, it thinks that we can increase the channel electron concentration, improve the output saturation current Ids and threshold voltage Vth via increasing doping level of AlGaN layer, improving the ratio of Al barrier layer constituent, decreasing Schottky barrier height.Based on above mentioned simulation result, we take the simulation for recessed-gate enhancement-mode HEMT devices with same structure, different etching depth and analyze the effect on HEMT devices properties with different recessed-gate depth. For example in the occasion of different recessed gate depth (0nm,5nm, 10nm). Then originate the relationships of saturation current, maximum transconductance, threshold voltage and frequency characteristic with the recessed-gate depth changing. Higher recessed-gate depth, then we can get lower saturation current; lower maximum transconductance and the threshold voltage move positive direction, but ft and fmax reduce a little. When generate 5nm SiO2 isolation layer between gate and AlGaN barrier layer, simulate for this MOS enhancement mode HEMT devices and find that the saturation current will lower, the maximum transconductance higher and threshold voltage move positive direction with increasing etching depth comparing with traditional enhancement mode HEMT devices. Because of SiO2 isolation layer existing, the threshold voltage modulus of MOS HEMT devices is higher than traditional. At the same time, MOS HEMT devices' saturation current is higher than latter under certain gate voltage condition.Finally, the threshold voltage of MOS HEMT devices is lower than latter when the etching depth is less than 13nm. Nevertheless threshold voltage and saturation current of MOS HEMT are higher than latter but maximum oscillation frequency and cutoff frequency lower when the etching depth is higher than 13nm.