We present the first low temperature (5K-300K) optoelectronic characterization of scalable, waveguide- coupled photodetectors based on III-V heterojunctions integrated on a silicon photonics platform. The effect of traps become apparent at temperatures below -140K and we explore for the first time their excitation via both electrical and optical power. A reliability challenge is facet control in conventional Si (100) oriented wafers, so we also develop comparable photodetectors on SOI with (110) crystal orientation. This allows us to further scale the active region to -0.003λ3, for λ=1550 nm and simultaneously maintaining a responsivity in the range 10%-40% down to 5K under direct illumination and improve performance variability by 3 orders of magnitude compared to the (100) SOI counterparts. This work shows the strong promise of this technology for future quantum optical links and advanced low-temperature optoelectronics and optical computing.