Investigation On Physical Model And Device Withstand Voltage Of AlGaN/GaN Hemts

Much effort has been paid for AlGaN/GaN high electron mobility transistors (HEMTs) based on the third generation wide band gap semiconductor GaN in order to achieve high performance of frequency, power and power density due to its applied foreground of high frequency and high power fields. In this thesis,the relative problems of science and technology of fabricating high frequency and power AlGaN/GaN HEMTs such as physical model, electric field theory and device fabrication, the charge control model and effect of current collapse are investigated from the point of the view of device physics as well as the device withstand voltage and field plate technology.Firstly, an analytic model for linearizing the relation of Fermi level EF versus two-dimensional electron gas (2DEG) density ns in AlGaN/GaN HEMTs is built by simplifying the complicated nonlinear dependency of EF on ns and using the first-order Taylor polynomial of the simplified function EF(ns). The present model enhances the precision of linear approximation and the flexibility of application.Then, with using linear approximation of Fermi level versus 2DEG density and considering the trap or ion charges at insulator/AlGaN interface, an analytic linear charge control model based on metal-insulator-semiconductor (MIS) theory and Poisson equations including polarization is developed for enhancement mode as well as depletion mode AlGaN/GaN MISHEMTs. Furthurmore, considering comprehensively AlGaN barrier layer charge and polarization charge at heterointerface of GaN MISHEMT, the whole process of charge control is analyzed in detail and partitioned into four regions: I-full depletion, II-partial depletion, III-neutral region and IV-electron accumulation at insulator/AlGaN interface. Based on MIS theory and Poisson equation, the charge control relations for each region are developed so that the charge control model for the whole range of gate working voltage is built. The results show that 2DEG saturates at region II and the working voltage of gate should not exceed the 2DEG saturation voltage in order to keep the channel in control. In addition, the span of region II accounts for about 50% of the range of gate voltage before 2DEG saturates.The results of three models above all agree well with the experimental data. Secondly, the response of drain current dependence on gate step pulses of AlGaN/GaN HEMTs is investigated. Two processes, fast electron and slow electron detrapping, are proposed to describe the relaxation of trapped electrons in surface states, and the fitting formula characterizing drain current response is obtained. By fitting, the time constants associated with fast and slow electron detrapping processes are about 0.23s and 1.38s, respectively.Meanwhile, the investigation on the extraction of effective trap densities at insulator/AlGaN interfaces for GaN MISHEMTs shows that the trap densities of various insulator/AlGaN interfaces have the approximate order NTS(Al2O3) > NTS(Si3N4)≈NTS(SiO2).Thirdly, The investigation of withstand voltage test shows that two-terminal breakdown and three-terminal breakdown tests are equivalent; the gate length affects insignificantly the breakdown voltage; when the distance between gate and drain (source) is 2μm, the breakdown voltage is 60 ~ 70V and increased linearly by 54V/μm with the distance.Moreover, the modulation of the field plate on the channel electric field is investigated, and the structures of source connected field plate with long gate-drain distance,Γgate field plate, Z-type source connected field plate and hybridΓand Z-type field plates with short gate-drain distance are optimized by simulation. The dependence of breakdown voltage and frequency characteristics on the field plate structure is obtained. With the 2.2μm gate-drain distance, the breakdown voltage can reach 400V with single field plate and that can be raised to 600V with hybrid field plate structure. The dependence of frequency characteristics on field plate length and insulator thickness is strong forΓgate field plate structure, but slight for Z-type source connected field plate structure and even that on length can be ignored.Finally, 0.4μm gate length AlGaN/GaN HEMTs with gate widths of 3mm, 1mm and 100μm are designed and fabricated on SiC substrate. Under the condition of 8GHz continuous wave, a max output power density of 5.62 W/mm with efficiency of 30.8% and gain of 7.49dB is obtained for device with gate width of 100μm at VDS=28V, and a max output power of 3.05W with efficiency of 23.2% and gain of 5.85dB is achieved for device with gate width of 1mm at VDS=25V. The device with 100μm gate width has fT of 39.4GHz and fmax of 112GHz at VDS=16V. The physical models, experimental research and results of simulation optimization can be used to subsequent modeling of device characteristics and served as the theoretical or experimental reference for design and fabrication of AlGaN/GaN HEMTs.