  Photodetectors are key optoelectronic building blocks performing the
essential optical-to-electrical signal conversion, and unlike solar cells,
operate at a specific wavelength and at high signal or sensory speeds. Towards
achieving high detector performance, device physics, however, places a
fundamental limit of the achievable detector sensitivity, such as responsivity
and gain, when simultaneously aimed to increasing the detectors temporal
response, speed, known as the gain-bandwidth product (GBP). While detectors GBP
has been increasing in recent years, the average GBP is still relatively modest
(~10^6-10^7 Hz-A/W). Here we discuss photodetector performance limits and
opportunities based on arguments from scaling length theory relating
photocarrier channel length, mobility, electrical resistance with optical
waveguide mode constrains. We show that short-channel detectors are synergistic
with slot-waveguide approaches, and when combined, offer a high-degree of
detector design synergy especially for the class of nanometer-thin materials.
Indeed, we find that two dimensional material-based detectors are not limited
by their low mobility and can, in principle, allow for 100 GHz fast response
rates. However, contact resistance is still a challenge for such thin
materials, a research topic that is still not addressed yet. An interim
solution is to utilize heterojunction approaches for functionality separation.
Nonetheless, atomistically- and nanometer-thin materials used in such
next-generation scaling length theory based detectors also demand high material
quality and monolithic integration strategies into photonic circuits including
foundry-near processes. As it stands, this letter aims to guide the community
if achieving the next generation photodetectors aiming for a performance target
of GBP = 10^12 Hz-A/W.
