Date of Award
Open Access Dissertation
Wide bandgap (WBG) AlGaN/GaN and ultrawide bandgap (UWBG) AlGaN/AlGaN III-nitride high electron mobility transistors (HEMTs) have come a long way since their initial demonstration and are desired for a multitude of applications in deep-scaled high-frequency, high-voltage, high-current, and high-temperature power electronics. Although AlGaN/GaN HEMTs have recently become state-of-the-art in consumer electronics chargers and amplifiers, the performance of the devices is limited by severe self-heating which significantly reduce their efficacy in demanding applications that require high-current density. One strategy to reduce this self-heating effect in GaN HEMTs is to use high thermal conductivity SiC or bulk AlN substrates. While this approach is effective for lateral WBG devices such as GaN HEMTs, this approach fails for scaling of high-power devices in both UWBG and vertical transistors due to: a) the large series electrical resistance of the substrates presenting a large thermal load from Joule heating and b) the large thermal resistance (Rtℎ) and heat capacity of the substrates leading to unacceptably high temperature rise, causing device failure under extreme operating conditions, the very conditions for which the intrinsic properties of III-nitrides are desired. In this dissertation, we demonstrated an approach to completely eliminate the substrate parasitic for better thermal management of HEMTs. We accomplished this using laser liftoff (LLO) of HEMT structure and bonding to a heat sink with minimal total thermal resistance. In this work, we developed a novel LLO technique for GaN HEMTs on sapphire and demonstrated 193 nm ArF excimer LLO transfer of AlGaN/GaN HEMTs fabricated with integrated >10 µm thick epitaxial AlN heat spreading buffer layer to a copper heat sink for the first time resulting in highest drain current density y (IDS) and mobility (𝜇𝑛) reported-to-date for LLO AlGaN/GaN HEMTs. This improvement is due to both the inclusion of the AlN heat-spreader and elimination of the substrate thermal parasitic. The same LLO technique can also be applied to UWBG AlGaN HEMTs to show the applicability of the technique in next generation deep-scaled power electronics.
LLO technique was also applied for the first time to realize flexible AlGaN/GaN HEMTs on low-cost copper tape on polyethylene terephthalate (PET). LLO transfer from sapphire to copper tape resulted in minimal damage to the GaN HEMTs, in stark contrast to existing techniques for transfer from silicon (Si) substrate to different flexible substrates that resulted in irreversible degradation in current handling by as much as 60%. The heterojunction remained structurally intact after LLO, leading to preserved electron mobility and carrier concentration. This flexible GaN HEMT exhibited a gauge factor of 3x104 , much better than state of the-art strain sensors, while having the capacity to be integrated with processing circuitry through the GaN platform. The piezoelectric properties of the AlGaN/GaN interface resulted in an increase in the channel electron concentration under tensile stress, in excellent agreement with the theoretical values for this interface. This demonstration shows the utility of low-cost flexible GaN circuits in applications such as microphones for hearing aids, piezoelectric stethoscopes, and integrated speakers in smartphones.
Alam, M.(2022). Applications of Laser Liftoff Technique for Wide Bandgap Power and Flexible Electronics. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/7112