  We study the origin of photocurrent generated in doped multilayer BP
photo-transistors, and find that it is dominated by thermally driven
thermoelectric and bolometric processes. The experimentally observed
photocurrent polarities are consistent with photo-thermal processes. The
photo-thermoelectric current can be generated up to a $\mu$m away from the
contacts, indicating a long thermal decay length. With an applied source-drain
bias, a photo-bolometric current is generated across the whole device,
overwhelming the photo-thermoelectric contribution at a moderate bias. The
photo-responsivity in the multilayer BP device is two orders of magnitude
larger than that observed in graphene.
