20 Décembre – Thesis defense - Kalparupa Mukherjee

11 h Amphi Jean-Paul Dom - Laboratory IMS / building A31 (Talence campus)

Investigation into trapping mechanisms and impact on performances and reliability of GaN HEMTs through physical simulation and electro-optical characterization.

Gallium Nitride has emerged as a terrific contender to lead the future of the semiconductor industry beyond the performance limits of silicon.
The immense potential of the AlGaN/GaN HEMT device is derived from the high density, high mobility electron gas formed at its hetero-structure. However, frequent subjection to high electric field, temperature and stress conditions makes the device vulnerable to reliability issues that restrict its efficiency and life time. A dominant contributor to several parasitic and reliability issues are traps present within the semiconductor structure which restrict the channel density and aggravate the device static and dynamic response. As the GaN industry addresses an increasing demand for superior devices, reliability analysis is of critical importance. There is a necessity to enable advancements in trap inhibition which would allow the realization of stronger, efficient devices.
The motivation of this work is to recognize distinct ways in which various traps affect the performance and reliability metrics of GaN 0.25 µm HEMTs through a study of devices of the GH-25 process optimized for high power applications up to 20 GHz. The investigation employs physical TCAD simulations to provide insight and perspective to electrical and optical characterizations. Detailed analysis into independent and interrelated effects is performed to identify the relative impact of traps in circumstances presenting notable deviations from the ideal device response.
The methodology to develop a representative TCAD model derived closely from internal physics is described with special focus on the sensitive gate leakage characteristic which reflects the influence of fundamental physical processes as well as parasitic effects commonly encountered in GaN HEMTs. Targeted simulations provide a pivotal link between the observation of a reliability issue and its underlying origin in trapping phenomena. Establishing associations between the spatial location of traps and the degradations they could trigger is an important objective of this thesis.
Several simulation strategies that explore trapping behavior in various steady state and transient environments are discussed which allow detailed perception into the manner and extent to which trap attributes affect operational considerations. Approaches to distinguish disparate trap interactions are also described. The central case study in this thesis is an abstruse parasitic leakage phenomenon, identified in the GH 25 process as a consequence of aging stress. Referred to as the “belly shape”, it presents an interesting example of how the developed strategies can be applied to discern the causality, impact and evolution of the responsible traps. In order to take a deeper look into trapping modes, further aging and LASER characterizations are performed to alter the general occupational dynamics and observe the modulation of trap control over device response.

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