  The rectification of electromagnetic waves to direct currents is a crucial
process for energy harvesting, beyond-5G wireless communications, ultra-fast
science, and observational astronomy. As the radiation frequency is raised to
the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics
becomes challenging and requires alternative rectification protocols. Here we
address this challenge by tunnel field-effect transistors made of bilayer
graphene (BLG). Taking advantage of BLG's electrically tunable band structure,
we create a lateral tunnel junction and couple it to an antenna exposed to THz
radiation. The incoming radiation is then down-converted by the tunnel junction
nonlinearity, resulting in high-responsivity (> 4 kV/W) and low-noise (0.2
pW/$\sqrt{\mathrm{Hz}}$}) detection. We demonstrate how switching from
intraband Ohmic to interband tunneling regime can raise detectors' responsivity
by few orders of magnitude, in agreement with the developed theory. Our work
demonstrates a potential application of tunnel transistors for THz detection
and reveals BLG as a promising platform therefor.
