  An ultraviolet (UV) photodetector employing the two-dimensional electron gas
(2DEG) formed at the AlGaN/GaN interface as an interdigitated transducer (IDT)
is characterized under optical stimulus. The 2DEG-IDT photodetector exhibits a
record high normalized photocurrent-to-dark current ratio (NPDR,
$6\times10^{14}$). In addition, we observe a high responsivity ($7,800$ A/W)
and ultraviolet-visible rejection-ratio ($10^{6}$), among the highest reported
values for any GaN photodetector architecture. We propose a gain mechanism to
explain the high responsivity of this device architecture, which corresponds to
an internal gain of $26,000$. We argue that the valence band offset in the
AlGaN/GaN heterostructure is essential in achieving this high responsivity,
allowing for large gains without necessitating the presence of trap states, in
contrast to common metal-semiconductor-metal (MSM) photodetector architectures.
Our proposed gain mechanism is consistent with measurements of the scaling of
gain with device channel width and incident power. In addition to high
performance, this photodetector architecture has a simple two-step fabrication
flow that is monolithically compatible with AlGaN/GaN high electron mobility
transistor (HEMT) processing. This unique combination of low dark current, high
responsivity and compatibility with HEMT processing is attractive for a variety
of UV sensing applications.
