  Initiating impact ionization of avalanche breakdown essentially requires
applying a high electric field in a long active region, hampering
carrier-multiplication with high gain, low bias and superior noise performance.
Here we report the observation of ballistic avalanche phenomena in sub-MFP
scaled vertical indium selenide (InSe)/black phosphorus (BP) heterostructures.
The heterojunction is engineered to avalanche photodetectors (APD) and impact
ionization transistors, demonstrating ultra-sensitive mid-IR light detection (4
{\mu}m wavelength) and ultra-steep subthreshold swing, respectively. These
devices show an extremely low avalanche threshold (<1 volt), excellent low
noise figures and distinctive density spectral shape. Further transport
measurement evidences the breakdown originals from a ballistic avalanche
phenomenon, where the sub-MFP BP channel enables both electrons and holes to
impact-ionize the lattice and abruptly amplify the current without scattering
from the obstacles in a deterministic nature. Our results shed light on the
development of advanced photodetectors and efficiently facilitating carriers on
the nanoscale.
