  Over the last decade, progress in wide bandgap, III-V materials systems based
on gallium nitride (GaN) has been a major driver in the realization of high
power and high frequency electronic devices. Since the highly conductive,
two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on
built-in polarization fields (not doping) and is confined to very small
thicknesses, its charge carriers exhibit much higher mobilities in comparison
to their doped counterparts. In this study, we show that this heterostructured
material also offers the unique ability to manipulate electrical transport
separately from thermal transport through the examination of fully-suspended
AlGaN/GaN diaphragms of varied GaN buffer layer thicknesses. Notably, we show
that ~$100$ nm thin GaN layers can considerably impede heat flow without
electrical transport degradation, and that a significant improvement (~$4$x) in
the thermoelectric figure of merit ($\it zT$) over externally doped GaN is
observed in 2DEG based heterostructures. We also observe state-of-the art
thermoelectric power factors ($4-7\times$ $10^{-3}$$\,Wm^{-1}K^{-2}$) at room
temperature) in the 2DEG of this material system. This remarkable tuning
behavior and thermoelectric enhancement, elucidated here for the first time in
a polarization-based heterostructure, is achieved since the electrons are at
the heterostructured interface, while the phonons are within the material
system. These results highlight the potential for using the 2DEG in III-V
materials for on-chip thermal sensing and energy harvesting.
