  The giant spin Hall effect (SHE) at room temperature is one of the most
attractive feature of topological insulators (TIs) for applications to
nano-scale spin devices. Its origin, however, remains a controversial problem.
Here, we identify the origin of the giant SHE in BiSb thin films by measuring
the spin Hall angle ${\theta}_{SH}$ under controllable contribution of surface
and bulk conduction. We found that ${\theta}_{SH}$ of a Bi$_{0.6}$Sb$_{0.4}$ TI
thin film takes colossal values (450 - 530 at 8 K, and 38 at 300 K), and is
almost governed by contribution from the topological surface states. Meanwhile,
${\theta}_{SH}$ in a Bi$_{0.2}$Sb$_{0.8}$ semi-metallic thin film without
topological surface states drastically decreases. Our results provide a
quantitative tool for analysing the origin of the giant SHE in TI thin films,
as well as a strategy for designing spin current source utilizing the surface
states of TI in high-performance nano-scale spin devices.
