  Graphene shows a strong promise for detection of terahertz (THz) radiation
due to its high carrier mobility, compatibility with on-chip waveguides and
transistors, and small heat capacitance. At the same time, weak reaction of
graphene's physical properties on the detected radiation can be traced down to
the absence of band gap. Here, we study the effect of electrically-induced band
gap on THz detection in graphene bilayer with split-gate p-n junction. We show
that gap induction leads to simultaneous increase in current and voltage
responsivities. At operating temperatures of ~25 K, the responsivity at 20 meV
band gap is from 3 to 20 times larger than that in the gapless state. The
maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50
kV/W, while the noise equivalent power falls down to 36 fW/Hz^0.5. These values
set new records for semiconductor-based cryogenic terahertz detectors, and pave
the way for efficient and fast terahertz detection.

Mesoscale and Nanoscale Physics