Ultra-scaled GaN high electron mobility transistors (HEMTs) are attractive candidates for high-power electronics with speed approaching THz, due to its wide bandgap, high electron velocity and high channel charge concentration. To realize these potentials, transistor technologies that offer low parasitic resistances and capacitances have to be developed, in addition to design of the intrinsic device layer structures. To this end, this thesis describes the development of non-alloyed ohmic contacts regrown by molecular beam epitaxy (MBE) and the resultant HEMTs at Notre Dame. This effort placed Notre Dame as the third group in the world to successfully demonstrate high performance GaN HEMTs with non-alloyed regrown ohmics after UCSB and HRL. The developed technology was subsequently transferred with success to TriQuint Semiconductor, Inc. Compared to the conventional alloyed ohmic contacts, non-alloyed MBE regrown ohmic contacts have several advantages: 1) lower contact resistance, 2) source/drain definition with uncompromised edge acuteness, and 3) unpinned HEMT surface, which is discovered by our group and described in this thesis.
The fabrication processes for the regrown contacts were first developed while the MBE regrowth was performed by Dr. JenaåÁåøs MBE group members. Following liftoff of the regrowth mask, the gate electrode was placed using electron beam lithography. Several generations of metal-face lattice-matched InAlN/AlN/GaN HEMTs without gate recess were fabricated and tested. The highlights of these efforts include: a minimum total contact resistance of 0.1 ohm-mm with a regrowth interface resistance of 0.02 ohm-mm; an output drain current of 1.8 A/mm, an extrinsic transconductance of 544 mS/mm, ft/fmax of 216/80 GHz on a 60-nm InAlN HEMT with non-alloyed ohmic contacts and without passivation. Temperature dependence behavior of ohmic contacts and HEMTs was also investigated to uncover the underlying physical processes limiting their performance. It was found that the regrowth interface resistance obtained in this work is very close to the quantum resistance limit. It was also discovered that for these HEMTs with non-alloyed ohmic contacts, the conventional dielectric passivation actually decreases the device speed, opposite to the observations for conventional HEMTs with alloyed ohmic contacts, which in turn suggests traps states generated during ohmic alloying are responsible for the dispersion effect plaguing the HEMTs.
To further improve device performance, self-aligned HEMT processes are being developed at Notre Dame. The preliminary results of a gate-first self-aligned symmetric HEMT process development are discussed, as well as simulated device performance.