| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 105 === 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.
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.
In this research, p-GaN gate double heterostructure HEMTs with various gate field plate lengths are fabricated and we investigate the influence of field plates on the current behavior of p-GaN gate HEMTs. Transfer curves of the devices indicate field plates extended toward the source play some role in suppressing channel current, and the threshold voltage tilt toward even higher voltage in double heterostructure. P-GaN gate HEMTs with field plates show three stages current raises, and exhibit much higher threshold voltage than the one without field plate (i.e. Vth=5.90V ). We propose a Threshold Enhancement model to explain the mechanism of three stage current behavior.
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