  A graphene $pn$ junction is studied theoretically in the presence of both
intrinsic and Rashba spin-orbit couplings. We show that a crossover from
perfect reflection to perfect transmission is achieved at normal incidence by
tuning the perpendicular electric field. By further studying angular dependent
transmission, we demonstrate that perfect reflection at normal incidence can be
clearly distinguished from trivial band gap effects. We also investigate how
spin-orbit effects modify the conductance and the Fano factor associated with a
potential step in both $nn$ and $np$ cases.
