Laser lifted off III-Nitride and GaN circuits to enable next generation chargers, electric vehicle drives, and wearable electronics

AlxGa1-xN semiconductors have extreme properties such as high heat removal, high temperature operation, and high current/voltage handling that are ideal for high power compact electronics in electric vehicles. These extreme properties make them excellent for low power applications such as wearables beyond traditional silicon as validated by the emergence of GaN chargers in consumer electronics.

Further scaling of device performance with Al content is currently limited by the wafers on which AlxGa1-xN active layer coatings are produced. By removing the wafer and transferring ultrathin active coatings to application specific engineered surfaces, can the full promise of AlGaN’s superior properties be realized in ultra-compact devices? This question will be answered using a patent pending laser liftoff (LLO) AlGaN transfer developed by our team to remove the growth wafer.

If successful, our work would transform power transistors for electric vehicles and power grid applications and could lead to the first practical wearable III-Nitride devices from this Nobel winning material system that led to the 2014 Physics prize for the blue LED. Part of this work will be performed by Engineering undergraduates in collaboration with the nation’s best department of Exercise Science and could enable tech transfer of what could be the first application of flexible III-N in healthcare.

This work will be performed in the following 3 thrusts.

1. Characterizing steady state and transient thermal performance of AlGaN channel transistors by transferring/soldering to Cu heat sinks. This will eliminate the series thermal/electrical resistances of the substrate, reducing thermal time constants from the ms range to the us range.

These are suitable for kHz-MHz switching applications without significant temperature rise, a crucial requirement in deep-scaled power electronics that can reach the ideal performance codified in the Baliga Figure of merit.

2. Piezoelectric sensing on flexible substrates by transferring depletion mode high-electron mobility transistors (HEMTs) to flexible polyethylene terephthalate (PET). By flexing the AlGaN heterojunction, piezoelectric charge in the channel changes, an effect amplified by the transistor Given the ~1A/mm drive we have obtained for LLO transferred AlGaN/GaN transistors, and PET’s known high critical breakdown field >5MV/cm, we enable new functionalities previously not possible in flexible electronics.

We will mount this device on human participants to evaluate the viability of wearable III-N heart rate sensors as a piezoelectric stethoscope.

3. Integration of visible III-N emitters and photodetectors in a single package using a LLO pick and place approach to demonstrate a flexible III-N photonic circuit. This will be used to develop an understanding of bandwidth limiting defects induced by LLO and packaging in low-leakage circuits.

Defect studies will also inform the other case studies to determine the ultimate bandwidth and power handling capabilities of AlGaN devices. A key issue is that of strain relief induced damage from the highly traumatic laser liftoff and transfer process. This is a problem that we have minimized through our preliminary enabling research, although it is a key problem that will limit the viability of this technology.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.