  The surface states of three-dimensional topological insulators posses the
unique property of spin-momentum interlocking. This property gives rise to the
interesting inverse Edelstein effect (IEE), in which an applied spin bias $\mu$
is converted to a measurable charge voltage difference $V$. We develop a
semiclassical theory for the IEE of the surface states of
$\text{Bi}_2\text{Se}_3$ thin films, which is applicable from the ballistic
regime to diffusive regime. We find that the IEE efficiency ratio
$\gamma=V/\mu$ exhibits universal dependence on sample size, and approaches
$\pi/4$ in the ballistic limit and $1$ in the diffusive limit.
