The Novel Structure And Experimental Study For 4H-SiC Microwave Power MESFETs

Silicon carbide (SiC) is an attractive wide band-gap semiconductor material for high-power, high-voltage, high-frequency and high-temperature applications due to its superior properties, such as the wide bandgap, high critical electric field, high thermal conductivity and high electron saturation velocity, and the relatively mature material growth and device fabrication technology. 4H-SiC metal semiconductor field-effect transistors (MESFETs) are emerging as a promising technology for high power microwave applications such as transmitters for commercial and military communications. On account of the recent progress in device process and the technology for producing high quality SiC substrates and epitaxial films, impressive performances for SiC MESFETs has been reported. However, although these performances were very promising, two limitations have appeared, i.e., the self-heating and trapping effects.This dissertation majors on studying the power and frequency characteristics of the 4H-SiC MESFETs. A new structure of 4H-SiC MESFETs based on the source field plate technology is proposed in this thesis. Meanwhile, a three-dimensional (3D) electro-thermal analytical model to accurately predict the temperature distribution in multi-finger SiC MESFETs has been developed. And a large periphery SiC MESFETs with multi-recess gate was fabricated. The detail contributions of the dissertation are listed as followings:(1) Novel 4H-SiC MESFETs with the source field plate: The proposed structure, which is the source metal extension over the gate to the drift region, not only improve the breakdown voltage, but also eliminate the drawback of low gain characteristics relatively resulted from additional feedback capacitance associated with the field plate electrode. The improvement of the breakdown voltage is due to the facte that the depletion layer formed under the source field plate electrode resulted in the reduction of the electric field strength at the gate edge toward drain. Moreover, the field plate to channel capacitance becomes a drain-source capacitance, which could be absorbed in the output tuning network, thereby reducing gate-to-drain capacitance and improving the gain characteristics. As compared to the conventional structures, the MESFETs with the source field plate show an approximately 66% increase in breakdown voltage, which is responsible for the 73% improvement in the power density. At the same time, the proposed device, which is compatible with conventional SiC MESFETs technology, can afford a novel option to improve the performance of the high-power SiC MESFET devices.(2) Electro-thermal analytical model for multi-finger 4H-SiC power MESFETs: A three-dimensional (3D) electro-thermal analytical model to accurately predict the temperature distribution in multi-finger SiC-MESFETs has been proposed by solving the 3D linear heat transfer equation in solid material. The results of the analytical and numerical investigation of self-heating effects have also been presented. The analytical results are well supported by the two-dimensional electro-thermal simulation results obtained by Atlas. The models give an explicit influence on temperature distribution in terms of the structure parameter and operation condition, such as the gate-to-gate pitch, the thickness of the substrate and the source-drain bias. The obtained results can be used for optimization of the thermal design of the multi-finger 4H-SiC power MESFETs.(3) 4H-SiC MESFETs with a multi-recess gate: A large periphery SiC MESFETs with multi-recess gate to increase the output power and drain efficiency was fabricated. The gate recess reduces the peak electric field at the gate, enabling both higher operating breakdown voltage and reduced dispersion, leading to higher output power densities and power-added efficiency (PAE). Numerical simulations have indicated similar effects for SiC MESFETs with a multi-recess gate. Packaged devices with 5 mm gate periphery of these transistors demonstrated an output power of 13.5 W with a linear gain of 11.3 dB and a power-added efficiency of 50% under pulse operation at 2 GHz. These results are improved compared to conventional MESFETs fabricated in this work using the same process. The influences of the key processes, such as the omic contact, the recess etch and the air bridge, on the performance of SiC MESFETs also study in this thesis. The typical specific contact resistance on n+ epilayer extracted from transmission line method (TLM) is deduced to be about 1.05×10~6Ω.cm~2. The gate-drain breakdown voltage measured is over 100 V for proposed devices and about 50 V for conventional devices.Finally, this dissertation also studies the influence of the surface state effects and the gate-source scaling effects in SiC MESFETs. The mechanism by which acceptor-type traps effect the transconductance and drain current changes has been discussed. The simulation results show that transconductance exhibits negative frequency dispersion behavior, which is caused by the charge exchange via the surface states existing between the gate-source and gate-drain terminals. The current degradation behavior and the threshold voltage shift are also observed due to acceptor-type traps, acting as electron traps, in MESFET devices. Two-dimensional DC and small-signal ac analyses show that a downscaling of the gate-source distance can improve device performance, enhancing drain current, transconductance and maximum oscillation frequency. The variations of gate-to-source capacitance, gate-to-drain capacitance, cut-off frequency and maximum oscillation frequency with respect to the change in gate-source length have also been studied in detail. The obtained results can be used for a design guideline for the layout of 4H-SiC MESFETs.