| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 104 === The applications of gallium nitride high electron mobility transistors (GaN HEMTs) have become more and more important in recent years. Due to the outstanding material properties including wide-band-gap and high electron mobility, GaN HEMTs are widely applied to high voltage electronics and high efficiency power conversion systems. The two dimensional electron gas (2DEG) formed in heterojunction ensures the large operating output current and low on-resistance of the device. However, the leakage current and current collapse phenomenon concerning to the material defects reduce the power conversion efficiency of the device in high speed switching. In this research, enhancement-mode (E-mode) AlGaN/GaN HEMTs are demonstrated; the electrical characteristics and dynamic characteristics are also investigated.
In this thesis, dynamic output characteristics are analyzed between the conventional p-GaN HEMT and the HEMT with plasma enhanced chemical vapor deposition (PECVD) SiO2 as the passivation. The current collapse can be suppressed effectively in E-mode HEMT with SiO2 due to the extra electron-accumulating space created by passivation. Electrons that used to accumulate in p-GaN capping layer and deplete 2DEG carriers can be neutralized by electron-accumulating space formed in SiO2.
In order to construct metal-insulator-semiconductor (MIS)-HEMTs with optimized performance, high quality of layer interface is the critical factor of current collapse suppression and gate leakage minimization. We propose surface treatments including fluorine plasma, argon plasma and nitrogen plasma to improve interface quality.
Based on our previous experience of developing AlGaN/GaN HEMTs, we construct MIS structures with atomic layer deposition (ALD) Al2O3. The Al2O3 can passivate the surface defects formed by plasma bombardment and suppress current collapse in long pulse mode. Also, we investigate the impact of epi structures on electrical characteristics and the phenomenon of current collapse. The power conversion efficiency in p-GaN MIS-HEMTs can be effectively improved by double heterostructure.
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