Tajalli, Alaleh (2018) Characterization and Study of Reliability Aspects in GaN High ElectronMobility Transistors. [Ph.D. thesis]
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Abstract (italian or english)
GaN-based high electron mobility transistors (HEMTs) have excellent performance for power applications. Indeed, characteristics such as the high breakdown electric filed (3.3 MV/cm), the low ON-Resistance (RON) and the good thermal dissipation make the GaN-based diode and transistor a good potential for high frequency and power applications. The other outstanding feature of GaN-based HEMTs is the high electron mobility (1200 cm2/V.s) of the 2-dimensional electron gas (2DEG), formed at the interface between AlGaN and GaN, which leads to a low channel resistance and a high current density.
This thesis presents an overview of the most relevant trapping and degradation mechanisms that limit the performance and lifetime of GaN-based transistors for power electronics applications. To that end, pulsed I-V and drain current transient measurements are employed in order to investigate the trapping effects. The degradations of AlGaN/GaN MIS-HEMTs submitted to the gate step-stress experiments are investigated in the first part of this thesis. The results, that are obtained by a combined electrical and optical characterization over the different voltages, are discussed in chapter 2 which indicate the existence of a field- and hot-electron induced phenomena as the AlGaN/GaNMIS-HEMTs degradation mechanism.
A specific discussion is devoted to investigate the proton irradiation effect on the dynamic-Ron in HEMTs and is presented in chapter 3. It is shown that the proton irradiation is an effective and controllable method to reduce the dynamic-Ron in AlGaN/GaN HEMTs. Indeed, it is shown that samples that are submitted to a proton irradiation at high fluences (1.5£1014 cm– 2, 3MeV) exhibit a complete suppression of dynamic-Ron (complete voltage range, 150°C). This chapter further continuous to describe the voltage and temperature-dependent pulsed I-V characteristics of 650 V-rated transistors. It also points out the physical origin of dynamic RON in these devices.
Furthermore, owing to the positive and stable threshold voltage, the low on-resistance and the high breakdown field, the p-GaN gate GaN-based transistors are commonly accepted as promising devices for application in power converters. To that end, chapter 4 deals with the mechanisms that limit the dynamic performance and the reliability of normally-off GaN-based transistors. This chapter proposed the suppression of threshold voltage instability by a suitable passivation on the p-GaN sidewall. The improved reliability of device highlights that hole trapping mostly takes place on the sidewalls.
Finally, in chapter 5, a low leakage current and a state-of-the-art vertical breakdown voltage of above 1400 V a carbon-free GaN-on-Si device are demonstrated. These characteristics are achieved thanks to a thick and excellent crystal quality of GaN buffer. Indeed, low trapping effects are observed all the way to 1200 V with a low dependency of the substrate bias on the current density. The first demonstration of trap-free at such high voltage with this material system, could paves the way for 1200 V applications with GaN-on-Si resulting in a lower Ron and thus higher efficiency as compared to SiC and Si devices.
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