Investigation Of Normally-off Recessed-gate AlGaN/GaN MIS-HEMT

Since the 1990s,AlGaN/GaN HEMTs have developed rapidly with their superior characteristics.Due to excellent characteristics of GaN HEMTs,such as high working frequency and temperature,high breakdown field,and low conduction losses,GaN HEMTs are well-suited in the field of high frequency and high power.However,the conventional AlGaN/GaN HEMT is depletion-mode device.For high-frequency microwaves circuits,the depletion devices will undoubtedly greatly increase the power loss of the system and have a great influence on the reliability and safety of the circuit.In addition,the power devices usually need normally-off devices to prevent device damage due to short circuit.Thus,enhancement-mode AlGaN/GaN HEMTs have gained much attention.Since the GaN-based MIS-HEMT can achieve a threshold voltage which is higher than 3V,the study of the GaN-based recessed-gate MIS-HEMT is very significant.In this paper,the characteristics of the recessed-gate and MIS are studied by simulation and experiment.And the main contents are as follows:?1?Firstly,the Silvaco TCAD simulation software is used to realize the characteristic simulation of the recessed-gate HEMT with different recessed depths.With the increase of recessed-depths,the weakening of polarization effect and the enhancement of gate control capability results in that the conduction band bottom of the device gradually increases above the Fermi level and the threshold voltage shifts positively.Then,the transfer characteristics of the conventional HEMT,the conventional MIS-HEMT,and the recessed-gate MIS-HEMT are compared,and the influence of the thickness and dielectric constant of the gate dielectric is obtained.With the increase of the dielectric thickness,the threshold voltage has a slightly positive shift due to the decrease of gate control capability.The negative shift of threshold voltage and the improvement of transconductance can obtain on account of the high dielectric constant.Based on the results obtained by the above simulation,we fabricated the conventional HEMT and recessed-gate HEMT on the same substrate for comparison evaluation before the recessed-gate MIS-HEMT.The recessed-gate MIS-HEMT with etch depth of 19nm,22nm,and 25nm and the gate dielectric of 20nm HfO₂ was fabricated,and the gate dielectric of 20nm Al₂O₃ MIS-HEMT was also fabricated.The gate leakage current of the recessed-gate MIS-HEMT is two orders of magnitude lower than the conventional HEMT.?2?As the etching depth increases,the threshold voltage of the recessed-gate MIS-HEMT shifts positively,the saturation current decreases,and the transconductance decreases.The transconductance degradation of the 25nm recessed-gate MIS-HEMT is the largest,because the interface between HfO₂/AlGaN and AlGaN/GaN is getting closer and closer,the interface charge is increased,and the etching surface roughness caused by etching damage is increased.The scattering effect of the channel electrons is significantly enhanced,resulting in a decrease in mobility and a significant decrease in transconductance.For HfO₂ and Al₂O₃,HfO₂ devices fabricated as gate dielectric have higher saturation current and transconductance due to their high dielectric constant.Two different structure E-mode MIS devices are fabricated with recessed-gate structure and in-situ N₂ plasma treatment.Partially recessed-gate MIS HEMT has realized the threshold voltage of 3.5 V,saturation current density of 726 mA/mm,peak transconductance of 210 mS/mm,and the FOM of 4.18×10⁸V²/?·cm² which is the highest value in E-mode MIS HEMT with the threshold voltage greater than 3 V.Moreover,the current collapse and breakdown voltage of partially recessed-gate MIS device and fully recessed-gate MIS device are investigated.Meanwhile,the average breakdown voltage of partially recessed-gate MIS device is 412 V,which is less than fully recessed-gate MIS device.Silvaco simulation is carried out to analyze the leakage breakdown mechanism of MIS-HEMT.And the factor causing the breakdown voltage difference between partially recessed-gate MIS device and fully recessed-gate MIS device is the difference in the width of depletion region of the two devices.This result will be helpful for the further research on the GaN recessed-gate MIS power devices.