| dc.description.abstract | The development of Gallium Nitride (GaN)-based High Electron Mobility Transistors (HEMTs) has revolutionized the field of power electronics, offering significant advantages in terms of efficiency, switching speed, and thermal performance over traditional silicon-based devices. This Ph.D. thesis presents a comprehensive study of advanced GaN HEMT structures, with a particular focus on enhancing the performance of normally-off (E-mode) devices, which are crucial for power switching applications.
In this work, we investigate several novel device architectures and fabrication techniques aimed at overcoming the inherent challenges associated with GaN HEMTs. The primary contributions of this research are outlined in the following studies:
Extended p-GaN gate with thin AlGaN barrier HEMTs: This study investigated a normally-off p-GaN/AlGaN/GaN HEMT with the extended p-GaN. The optimized recess depth in the AlGaN barrier under the extended region of p-GaN provides improved device characteristics. The influences of recess depth in the AlGaN barrier and the extended length of the p-GaN extension on the threshold voltage (VTH), the maximum drain current (ID,MAX), and breakdown voltage (BV) were simulated and studied. The proposed transistor with a 1-μm p-GaN extension and 2-nm recess depth in AlGaN barrier shows improvement on VTH and ID,MAX without degrading the breakdown voltage compared with the device without p-GaN extension.
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O ff p GaN g ate AlGaN/GaN HEMT with a n ew s chottky s econd g ate This study presents a normally-off dual-gate AlGaN/GaN HEMT. The second gate is located between the p-GaN gate and the drain and is connected to the source. The optimized thickness and length of the AlGaN layer under the second gate next to the p-GaN significantly impact the I D,MAX and the off-state breakdown conditions. The reverse conduction characteristic is also improved because the freewheeling path of the reverse current is established between the second gate and the drain to prevent excessive voltage drop and conduction losses when the device is negatively biased. Compared with conventional HEMT, the proposed method shows a promising way to achieve normally-off GaN-based HEMTs with excellent forward and reverse conduction performance.
An AlGaN/GaN HEMT with p
GaN e xtended g ate for i mprovements on c urrent d ispersion and
b reakdown c haracteristics: This study introduces an unique p-type GaN gate AlGaN/GaN
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HEMT configuration. In this design, the p-GaN region extends toward the drain with an original gate electrode. This innovation significantly enhances the HEMT’s performance, with a 45.2% increase in breakdown voltage (BV) and a 17% higher VTH compared to conventional p-GaN gate HEMTs. The extended gate design redistributes the electric field, acting as a field plate to elevate the breakdown voltage. Furthermore, the proposed device, by reducing 17.4% of the saturation current without increasing the RRONON, possibly offers improved short-circuit capability.
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A ppseudoseudo--jjunction unction bbarrier Schottky arrier Schottky ddiode in piode in p--GaN AlGaN/GaN GaN AlGaN/GaN HEMT eHEMT epitaxial pitaxial llayers:ayers: This work investigates a pseudo-junction barrier Schottky (pseudo-JBS) diode that is created by placing an AlGaN/GaN Schottky diode in parallel with a p-GaN junction on the same epitaxial p-GaN gate AlGaN/GaN HEMT wafer. This pseudo-JBS diode employs the two-dimensional electron gas to increase the operation current, thus reducing the RRONON with high blocking voltage. The fabricated pseudo-JBS diode with anode-to cathode lengths (LAC) of 10μm shows a turn-on voltage of 1.05 V, a minimum specific RRONON (RON,MIN) of 2.53mΩ cm2, and blocking voltage of 1112 V yielding an excellent Baliga’s figure of merit of 488.7MWcm-2 on the same epitaxial p-GaN/AlGaN/GaN HEMT wafer. This study provides a promising substitute for Schottky barrier diodes without requiring extra p-GaN layer design.
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A nnew ew ggate ate ddesign esign ccombining MIS and pombining MIS and p--GaN GaN ggate ate sstructures for tructures for nnormallyormally--ooff and ff and hhigh igh oonn--ccurrent urrent ooperation: peration: This study proposes a new gate architecture that integrates both a p-GaN gate and a metal–insulator–semiconductor (MIS) structure for a normally-off AlGaN/GaN HEMT. Silvaco TCAD simulation software is used to assess the performance of the proposed design. A comprehensive analysis of the device’s transfer, output, and breakdown characteristics is carried out and compared with the conventional p-GaN gate AlGaN/GaN HEMT. The findings indicate that incorporating MIS in conjunction with the p-GaN gate leads to an augmentation in the on-state current density and a reduction in RRONON. The proposed HEMT exhibits superior attributes, with an 80% increase in drain current compared to the conventional p-GaN gate HEMT, but remains similar to VTH and BV. Consequently, the proposed HEMT demonstrates elevated current density and enhances gate control over the channel without modifying the VTH compared to the conventional p-GaN gate HEMT
Throughout this thesis, we employ a combination of experimental techniques, advanced fabrication processes, and comprehensive simulations to validate the proposed designs. Key performance metrics such as VTH stability, RON, current collapse, and BV are systematically analyzed and optimized. The findings of this research contribute to the advancement of GaN-based power devices, providing new insights into device design and fabrication. The proposed
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structures demonstrate significant potential for enhancing the performance and reliability of GaN HEMTs, paving the way for their broader adoption in high-power and high-frequency applications. This thesis concludes with a discussion of future work, highlighting potential areas for further improvement and exploration. These include the integration of advanced materials, scaling of device dimensions, and the development of new characterization techniques to fully exploit the capabilities of GaN HEMTs in next-generation power electronics. | en_US |