Developing Experimental Approaches to Gain Physical Insights into High Electric Field and Hot Electron Reliability of AlGaN/GaN HEMTs

Abstract

Next-generation power conversion systems, designed to achieve environmental and economic sustainability, are required to be efficient and smaller in size. Advances in silicon-based power devices, which are the major driving force in advancing power conversion technology, have successfully accomplished the task till now. However, with increased technological maturity and advances, silicon technology has reached its performance and scalability limits in the low- and medium-power segments. This has driven the research toward devices based on wide-band gap semiconductors due to their inherent capability to handle higher critical electric fields. AlGaN/GaN high electron mobility transistors (HEMTs) have recently emerged as the most promising device of choice in the power semiconductor market. While devices with promising performance figures have already been demonstrated, their reliability is still not well understood. This is attributed primarily to a completely different reliability landscape as compared to silicon-based devices and a limited understanding of the involved physical mechanisms. Out of different operating regimes, the semi-ON state, in which the channel is partially on, has emerged as one of the most critical regimes. Semi-ON state is commonly encountered during the switching operation when the device attempts to switch from OFF-state to ON-state or vice-versa. The simultaneous presence of considerable channel electron density and high electric field results in the generation of highly energized electrons, commonly referred to as hot electrons. A complex interplay of high electric field effects, hot electron trapping, and self-heating can trigger device instabilities and material degradation, which can subsequently lead to device failure. The work presented in this thesis, employing in-house developed complex measurement techniques integrating electro-opto-thermal excitation and computational analysis techniques, carries out physics-based exploration of the AlGaN/GaN HEMTs in this critical semi-ON stress regime. This work reveals a semi-ON state stress-induced gate degradation mechanism that led to early device failure. A critical semi-ON-state drain stress voltage is identified above which the gate current in Schottky-gated AlGaN/GaN HEMTs increase significantly and degrades permanently. Through detailed analysis of channel electric field, hot electron distribution, and electron temperature, under different drain bias stress, the drain edge of the transistor is revealed to emerge as a hot electron hot spot. Hot electron–buffer trap interaction-induced thermoelastic stress buildup near the drain edge and subsequent defect formation in the GaN buffer is established to explain the observed performance degradations. Finally, these processes are shown to lead to catastrophic failure of the device for longer stress times by the formation of cracks and pits in the GaN buffer. Given that the high voltage GaN device design focuses only on the field management near the gate edge, the finding of the onset of device degradation by shifting of the electric field peak near the drain edge is critical for device design. Furthermore, long-term semi-ON stress revealed current conduction instabilities in the device. A unique increase in drain current was found to be accompanied by confinement of hot electron distribution along the center of the device width. This would result in a high current flowing through a very confined region along the device width, which can have severe reliability consequences. Physical insights are provided into the mechanism with the help of in-situ thermoreflectance measurements and detailed computations. Thermo-reflectance-based temperature monitoring showed a non-uniform temperature distribution along the device width. Detailed computations, considering the non-uniform temperature distribution, established heating-induced non-uniform hole emission along the device width and their subsequent lateral redistribution to be responsible for the experimentally observed current increase and hot electron confinement.

Furthermore, the processes governing the device’s response to cyclic nanosecond high voltage semi-ON state stress pulses and its dependence on channel field engineering are also investigated. In response to stress pulses, the on-resistance of the device was found to change, resulting in a dynamic on-resistance evolution. Physical mechanisms governing this dynamic on-resistance evolution in response to cyclic nanosecond high-field (OFF-state) and hot-electron (semi-ON state) stress pulses were explored. The dynamic on-resistance was found to exhibit a dependence on the number of stress pulses (N), with ΔRON initially increasing, followed by a reduction in the rate of increase as N is increased. A physical mechanism based on pulse-to-pulse modulation of driving forces of trapping, governed by surface and buffer trap charging accumulated over the preceding stress pulses, is proposed to explain the observations. Well-calibrated computations are then used to provide further physical insights and to justify the proposed mechanism. Finally, the proposed mechanism is experimentally validated by modulating the trapping/de-trapping rates and studying their influence on the ΔRON evolution. Furthermore, it is revealed that for the device designs exhibiting a shift in the electric field peak to drain in response to the cyclic stress pulses, an additional increase in dynamic on-resistance is observed as compared to devices having field confined near the field plate edge. Factors accelerating this field shift to drain edge, including drain bias, channel current, and stress pulse width (PW), were found to accelerate the increase in dynamic on-resistance. Furthermore, the devices exhibiting such an increase in dynamic on-resistance showed a slower recovery of the ON-resistance when compared to devices with field peak only near the gate/field plate edge. Physical insights were developed using detailed experimentation and well-calibrated computations, which were then experimentally validated by studying the dependence of the ΔRON behaviour on: 1) passivation thickness-induced electric field profile modulation, and 2) substrate temperature-induced trapping/de-trapping rate modulation. To complete the analysis in the semi-ON regime, the hot electron induced self-heating behavior of the device and the impact of self-heating on hot electron and channel field distribution are also analyzed. Investigations revealed a unique increase in hot electron population and energy near the drain edge as the lattice temperature of the device was increased, contrary to an expected reduction in hot electron population. Well-calibrated computations revealed a unique surface and buffer trap charging–discharging phenomenon led electric field and electron density modulation to be responsible for the observed phenomenon. A physical mechanism based on the experimental and computational observations is also proposed and subsequently validated using dedicated experiments. We also investigated how the presence of unique phonon-assisted hot electron transitions during semi-ON state operation of AlGaN/GaN HEMTs could result in distinct self-heating behavior of the device. This establishes the importance of considering hot electron-triggered transitions, such as hot-electron-defect state recombination, inter-valley transitions, in addition to conventionally considered self-heating sources, while estimating heating effects in these devices. The phenomena are probed through detailed experimentation involving devices with different field plate lengths, electro-luminescence microscopy and spectroscopy, and thermal imaging analysis.