Owing to the large band gap (EgGaN=3.4eV) and high electron saturation velocity, GaN based high electron mobility transistors (HEMTs) are attractive for high voltage and high speed switching. In this work, three aspects of such transistors are studied. The role of polarization in leakage currents in gate stacks, and consequences for high-voltage switching is identified. High-performance GaN HEMTs on silicon by RF-plasma MBE growth, with low-resistance MBE-regrown ohmic contacts is demonstrated. The role of isotope and disorder engineering is demonstrated as a fundamentally new method to boost the speed of GaN HEMTs.
Ultrathin epitaxial barriers (such as AlN or InAlN) result in substantial leakage currents preventing the capability to block high drain voltages, and dielectrics like ALD Al2O3 can substantially mitigate this problem. In this work we present a comprehensive electrical characterization/interface study of Al2O3/ (In)AlN/GaN MOS-HEMTs. We find the presence, and propose an origin of benign donor-type interface (ALD/III-nitride) charges that are closely linked to the polarization charges of the underlying metal polar nitride substrate. These findings are expected to accelerate the choice of optimal gate stacks for nitride HEMTs and further our understanding of ALD/III-nitride interfaces with useful consequences for high-performance devices. We analyze the gate leakage currents in InAlN/AlN/GaN transistors and propose and demonstrate mechanisms to accurately capture the trap-assisted and tunneling dominated current transport in these heterostructures.
Large-area and low-cost Silicon wafers are attractive substrates for GaN RF and power electronics. We address the key challenges and find solutions for the RF-MBE growth of GaN HEMTs on Si (111). By developing low-leakage buffer layers, and employing raised source/drain regrown ohmic contacts by MBE, high performance HEMTs are realized. It is expected to motivate further ideas for integration of GaN with Silicon that go beyond RF and power electronics into the regime of low power digital logic by exploiting the unique polarization properties of GaN. Along those lines, polarization engineered III-nitride heterostructures where 2D electron and hole gases coexist are demonstrated.
The measured electron saturation velocity in the fastest GaN transistors is still far below the expected . A phonon mode density bottleneck was identified as possible root cause. Theoretical calculations subsequently predicted that by using a mixture of disordered isotopic alloy Ga( 14N0.5 15N0.5), it is possible to modify the density of LO phonon modes and consequently speed up GaN HEMTs. In this work we experimentally demonstrate that by controlling the concentrations of the subatomic particle - neutrons in Nitrogen atoms by MBE, it is indeed possible to improve the saturation velocity, providing a means to go beyond what is conventionally thought possible in semiconductor physics.