Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection(1) and sharp threshold swing field effect devices(2). However, initiating the impact ionization of avalanche breakdown requires a high applied electric field in a long active region, which hampers carrier multiplication with a high gain, low bias and superior noise performance(3,4). Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe/black phosphorus (BP)(5-9) heterostructures(10). We use these heterojunctions to fabricate avalanche photo-detectors (APDs) with a sensitive mid-infrared light detection (4 mu m wavelength) and impact ionization transistors with a steep subthreshold swing (<0.25 mV dec(-1)). The devices show a low avalanche threshold (<1 V), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors(1,11,12) via efficient carrier manipulation at the nanoscale.