Research On Nitride Semiconductor Enhancement-mode Hemts And The Implementation Methods

Gallium nitride based high electron mobility transistors?HEMTs?is widely used in microwave,high power and other fields by the reason of its high current density,high output power,high working frequency,excellent resistance to irradiation and etc..Because of the influence of spontaneous polarization and piezoelectric polarization,AlGaN/GaN HEMTs are natural depletion-mode devices,which limit their potential to be applied in more areas.In this article,enhancement-mode?E-mode?AlGaN/GaN HEMTs are fabricated and studied.While new fabrication process for E-mode devices is developed,and the working mechanism of E-mode devices are analyzed.Anyway,the article aims to make contributions to the application and popularization of GaN based E-mode devices.And the main research work and results are as follows:The fabrication and study of the E-mode double heterojunction AlGaN/GaN HEMT.Two different GaN channel layer AlGaN/GaN double heterojunction materials were grown by MOCVD and then fabricated the E-mode devices by RIE fluorine injection technology.According to the direct current?DC?measurement,compared with E-mode AlGaN/GaN HEMT with thick GaN channel layer,E-mode device with thinner GaN channel layer had more positive threshold voltage,smaller schottky leakage current and lower DIBL value.Analyzing the trap effect,E-mode device with thin GaN channel layer had higher density of deep level trap states.Additionally,on the basis of the annealing experiment,the thinner the GaN channel layer was,the weaker the temperature stability of the E-mode device would be.The threshold voltage model for AlGaN/GaN nanowire channel HEMT was built and the fabrication of E-mode nanowire channel HEMT.Firstly,nanowire channel HEMTs with different channel widths were fabricated.By studying the threshold voltage of devices,with the reduction of the nanowire channel width,the threshold voltage were more positive and the output current were decreased.Meanwhile,the frequency characteristics of the device would also be degraded.And then,by combining the depletion effect of the sidewall gate and the change of the polarization effect,the threshold voltage model of AlGaN/GaN nanowire channel HEMT was built.As well the nanowire channel HEMT with 60 nm nanowire channel width was designed,which realized the E-mode device with the threshold voltage of+0.7 V.The current density of which was 65 mA/mm at V_GS=3 V and the peak transconductance of which was 35 mS/mm.In terms of RF,the device's f_T was 2GHz and f_max was 10 GHz.The development of oxygen plasma treatment process of ICP and the study of E-mode AlGaN/GaN device combining with the recessed-gate structure and oxygen plasma treatment.First of all,the recessed-gate HEMT with different barrier layer thickness and the HEMT with different oxygen plasma treatment conditions were studied.According to the study,oxygen plasma treatment had not only oxidation effect but also etching effect on the barrier layer.E-mode AlGaN/GaN HEMT combined with the recessed-gate structure and oxygen plasma treatment was designed with a threshold voltage of+0.5 V,current density of 410 mA/mm at V_GS-V_TH=2 V and a DIBL of 5 mV/V.Additionally,compared with the conventional HEMT,the E-mode HEMT showed good breakdown and RF characteristics.And then,the low-speed etching condition of the recessed-gate was developed with the etching rate of 3 nm/min and as well the fully recessed-gate AlGaN/GaN HEMT was fabricated with a+1.5 V threshold voltage and 0.85 MV/cm breakdown field strength,which was a high value in the fully recessed-gate devices.Lastly,the E-mode fully recessed-gate AlGaN/GaN MIS-HEMT was fabricated under the low-speed etching condition.In this case,the gate voltage could reach 10 V and the threshold voltage of device was+5 V.At the same time,its current density at V_GS=10 V was as high as 800 mA/mm.The fabrication of three new structure E-mode HEMTs.?1?.The fluorine plasma treatment process based on ICP was developed,and the devices with fluorine plasma treatment were studied.In the study,the fluorine plasma treatment based on ICP made the device change from the D-mode to the E-mode by fluorine injection and etching effect on the barrier layer.?2?.The E-mode nanowire channel HEMTs were fabricated by O₂ plasma treatment or N₂O plasma treatment,and the nanowire channel width was 100 nm.The results showed that compared with E-mode nanowire channel HEMT with O₂ plasma treatment,E-mode nanowire channel HEMT with N₂O plasma treatment had better characteristics of device,with lower trap concentration,and larger the output current.Also DIBL was as low as 2 mV/V,and SS value was 70 mV/decade.In the frequency characteristic,the f_T reached 19 GHz and f_max came up to 58 GHz.?3?.E-mode nanowire channel HEMT was fabricated by CF₄ plasma treatment based ICP with 200 nm nanowire channel width.The current density of the device at V_GS-V_TH=2 V was 460 mA/mm and the peak transconductance reached up to 370 mS/mm.Meanwhile,the on/off ratio of the device was 10⁹,and the DIBL value reached 2 mV/V.Also the breakdown voltage was 138V which was 25%higher than the conventional HEMT.In the terms of RF,f_T of device got up to 22 GHz and f_max reached 60 GHz,which was the highest value in the E-mode nanowire channel AlGaN/GaN HEMT.Disadvantages in this study that need to be improved and further researched:?1?The double heterojunction HEMT with thin GaN channel layer shows lower current density and higher trap density,so the process of material growth and device fabrication still need to be optimized;?2?The low speed recessed-gate etching technology still has some influence on the channel electron mobility,which needs further optimization;?3?The interface of fully recessed-gate MIS-HEMT still exist more interface states,which needs to be further reduced by other ways and to improve the reliability of devices;?4?The gate control capability of E-mode nanowire channel devices is strong,which reveals huge potential of applications in integrated circuits.In this case,its mechanism and design rules need a more in-depth study.