| Author: | He, Jiaqi |
| Title: | Improved performance of GaN metal-insulator-semiconductor high-electron-mobility transistors towards power applications |
| Advisors: | Li, Gang (EEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Power semiconductors Power electronics Modulation-doped field-effect transistors Gallium nitride -- Electric properties Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xviii, 100 pages : color illustrations |
| Language: | English |
| Abstract: | GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) are potential candidates for the next generation of high-power electronics. To further reduce dielectric defects and suppress the gate leakage, an in-situ SiNx cap layer on the AlGaN barrier, which is grown continuously by metal-organic chemical vapor deposition (MOCVD), has been extensively studied as a means of protecting the AlGaN surface from air and process-induced damage. The passivation results in the presence of a large amount of positive fixed charges at the in-situ SiNx/AlGaN interface, inducing a higher two-dimensional electron gas (2DEG) density at the AlGaN/GaN heterojunction. This enhances the drain current but at the expense of a lower threshold voltage (Vth). To achieve energy savings and improve safety, it is necessary to employ normally-off GaN HEMTs to simplify circuit designs and for fail-safe systems. This thesis introduces the fundamentals of GaN MIS-HEMTs and covers an overview of achieving normally-off operation and low-damage fabrication processes. Moreover, several continuously improved works have been conducted to address the abovementioned issues. The first work uses an in-situ SiNx passivation to compensate for the 2DEG deficiency in the access region of ultrathin-barrier (UTB) AlGaN/GaN heterojunction. A high D2DEG of 1.50 × 1013 cm-2 and a maximum ID of 705 mA/mm was achieved. The variation of interface charges decreased the D2DEG by five times compared with the piezoelectric polarization during SiNx removal. In the case of MIS-HEMTs with oxide dielectrics, the in-situ SiNx interlayer has demonstrated its great potential in reducing the oxide traps to achieve power HEMT devices with low interface states. Furthermore, The UTB Al0.05Ga0.95N/GaN MIS-HEMTs with various Al2O3/in-situ SiNx gate dielectric stacks were fabricated. The ultralow subthreshold swing (SS) and double-sweep (testing the transfer curves up and down) Vth shift (∆Vth) was achieved by the excellent interface quality of in-situ SiNx/AlGaN. To solve the problem of the negative Vth shift induced by the positive fixed charges , the second work of pre-gate O3 treatment to realize normally-off operation on an in-situ SiNx/AlGaN/GaN platform was developed. The Vth was increased by 2 V by compensating the Al2O3/AlGaN net polarization charges with O3 treatment in the atomic layer deposition (ALD) system. This also reduced the interface traps by more than one order of magnitude. The combination of in-situ SiNx passivation and high-quality O3-treated Al2O3/AlGaN gate interface led the device to display an excellent breakdown voltage of 1498 V at a low specific on-resistance of 2.02 mΩ·cm2. Furthermore, based on the dielectric thickness-dependent experimental calculation and numerical simulation, a physical model for the positive fixed charges at the Al2O3/AlGaN interface was established. The gate stability, time-dependent, and temperature-dependent reliability were all improved. This ALD-O3 treatment provides a practical method for fabricating normally-off GaN power MIS-HEMTs by fixed charge modulation. In order to improve the positive Vth of the UTB GaN MIS-HEMTs, the third work involving two regrowth processes for the recessed-gate structure with in-situ SiNx function layer was developed. The 20 nm Al0.2Ga0.8N and 10 nm in-situ SiNx were regrown to restore 2DEG and reduce contact resistance (Rc). The partially-recessed MIS-HEMTs achieved a high Vth of more than 2.5 V with a low on-state resistance (Ron) of 5.5 Ω·mm by the gate dielectric of ALD-Al2O3 and 6.0 Ω·mm by ALD-HfO2. A Vth of 0.6 V with ΔVth below 30 mV was realized by the fully-recessed MIS-HEMTs with in-situ SiNx/Al2O3 gate stacks. The devices on the wafer also delivered interface density lower than 1012 cm−2eV−1, high field-effect mobility of 1991 cm2/V·s, and narrow Vth distribution deviations below 0.23 V. Taking advantage of in-situ SiNx with regrowth techniques shows great potential in achieving high-performance GaN normally-off MIS-HEMTs. The fourth work is the development of a high-quality etching-free regrown fishbone trenches (RFT) to improve the recessed-gate GaN MIS-HEMT performance. The innovative RFT structure and improved etching-free process, achieved a series of normally-off GaN MIS-HEMTs with high Vth and excellent dynamic properties. In addition, the RFT structure enhanced the gate breakdown voltage and time-dependent reliability without sacrificing output performance. In summary, this Ph.D. study encompasses the following aspects in a systematic self-sustainability manner. First, distinguishing influences on thin-barrier AlGaN/GaN heterojunction’s electrical properties with in-situ SiNx caps. Second, exploring the practical O3 treatment to solve the difficulty of normally-off operation on in-situ SiNx/AlGaN/GaN platform. Third, further attempt the novel regrowth processes and trench pattern design to improve the device performance of normally-off GaN MIS-HEMTs. These works will provide new insight into the development of high-power GaN MIS-HEMTs. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/12966