| Based on the excellent physical properties of wide bandgap, high critical breakdown electric field and thermal conductivity, etc., silicon carbide (SiC) is considered to be one of suitable materials for the new generation power devices, which has broad application prospects in the high-voltage, high-frequency and high-temperature conditions. As an important power switching device,4H-SiC bipolar transistor (BJT) has advantages in low on-resistance, fast switching speed and high reliability, which is more suitable for ultra-high voltage (> 5000V), large conduction current and extreme temperatures (> 500℃) applications. In recent years, breakthrough progress on 4H-SiC BJT has been achieved in the world. However, in domestic, there is big delay in epilayer growth, structure design and device fabrication. In this dissertation, both simulation and experimental researches are performed to improve the device performance. The main contributions are presented as follows:1. The simulation models for 4H-SiC BJT are improved. An interface model between 4H-SiC and silicon dioxide, which could influence the surface recombination current in base region, has been established. Based on our previous experimental results, the model is verified. Then, a new deep level defect model for the emitter-base junction is also proposed. Comparing presented model with the published results from Kyoto University, the energy level and the capture cross-section are extracted. Two main operational modes for 4H-SiC BJT as power switching device are investigated. The main parameters for device characterization, including current gain, on-resistance, breakdown voltage and switching time, are studied and analyzed systematically.2. Non-ideal effects in 4H-SiC BJTs are investigated. Based on the test results of our previous device with double Gaussian-doped base, the influence of emitter width on the current crowding effect is analyzed firstly. It is shown that the non-uniform of potential distribution in base region is the main factor causing current crowding in emitter. Then, the Early effect is analyzed, comparing the device characteristics with double base and single base. Finally, base punch-through effect in base region and electric field crowding effect at the bottom of the isolation trench is introduced. The design rules for epilayer parameters in 4H-SiC BJT is presented, including emitter region, base region and collector region. In addition, a new termination structure named JTE Rings is proposed to promote the reverse characteristic. Results show that the optimal implantation window of JTE Rings termination is three times larger than the conventional JTE termination without additional process step.3 Three new structure are proposed and verified by simulation to improve the current gain of 4H-SiC BJT. First, BJT with V-shape trench (VT) in active region is presented. By introducing a groove type of metal-high k dielectric-silicon carbide (MIS) structure into the active region along the base-emitter sidewall which is formed with the process of isolation etching, a large electric field appears at the interface between high-k dielectric and bulk material by analyzing the potential distribution in forward mode, thus accelerating the electron transport to be beneficial to current gain. Based on a doping concentration of 4×1017cm-3 and thickness of 0.6μm base region, the current gain of VT-BJT is 1.66 times and 1.79 times the current gain of conventional BJT without and with the influence of deep level defects in base-emitter junction, respectively. Second,4H-SiC Work-Function Dependent (WFD) bipolar transistor is proposed. A p-type schottky contact is introduced to replace the conventional emitter region. With the minority carrier injection in emitter schottky contact, simulated ultra-high current gain could be obtained by proper choice of the metal work-function. Third,4H-SiC monolithic Darlington transistor is studied. Its input and output characteristics are simulated, while the influence of different area ration between the drive BJT and output BJT on the current gain is analyzed.4. Research on SiC minority carrier lifetime and its impact on BJT performance has been performed. Based on analyzing the current investigation of SiC minority carrier lifetime, it is suggested that C vacancy is the main lifetime killer in SiC materials. To enhance the minority carrier lifetime, thermal oxidation experiment is executed. The results from microwave photoconductive test show that the lifetime increases with the oxidation time increasing. C free impurities generated at SiC/SiO2 interface during the oxidation could supplement the C vacancy, thus improve the minority carrier lifetime. The influence of emitter lifetime and base lifetime on the device performance are investigated. A long lifetime in both emitter region and base region is helpful for improving the current gain. However, the storage time in turn-off characteristic will increase when elevating the base electron lifetime. To solve this problem, a suitable base electron lifetime should be tradeoff between switch time and current gain.5. Experiment design and device fabrication has been implemented. The process and layout for 4H-SiC BJT and related bipolar devices are carried out, while the key processes are experimentally studied. To form the emitter and base ohmic contact simultaneously, n-type 4H-SiC ohmic contact with Ti/Al metal system is chosen. The low specific contact resistance of 6×10-5Ω·cm2 has been achieved with 3 minutes rapid thermal annealing at 1000℃, which is suitable to the device requirement. The implantation experiment for JTE Rings termination is also executed. The secondary ion mass spectroscopy show that the implantation results coincide with the expected design. Finally, High performance 4H-SiC BJTs and Darlington transistors are successfully fabricated. Using the structure with 20-μm thick and 3×1015cm-3 doped collector region,0.6μm thick and 4.5×1017cm-3 doped base region, and 0.8μm thick and 2.0×1019cm-3 doped emitter region.4H-SiC BJT with maximum current gain of 63.6 is achieved. The open base blocking voltage is exceeded to 2000V, which is limited by the test equipment. The non-isolated Darlington transistor shows maximum current gain of 313 and blocking voltage over 1900V, while the isolated Darlington transistor shows maximum current gain of 948 and blocking voltage 750V, which are first results reported in domestic.
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