  Planar Hall effect (PHE) in topological insulators (TIs) is discussed as an
effect that stems mostly from conduction due to topologically protected surface
states. Although surfaces states play a critical role and are of utmost
importance in TIs, our present study reflects the need for considering the bulk
conduction in understanding PHE in TIs. Here, we demonstrate an enhancement in
PHE amplitude by three times by doubling the thickness of Bi$_2$Te$_3$ film on
Si (111). The PHE amplitude reaches $\approx$~6 n$\Omega$m in 30 quintuple
layer (QL) device as compared to $\approx$~2 n$\Omega$m in 14 QL. We find that
the PHE amplitude increases with temperature in the 30 QL Bi$_2$Te$_3$ films
grown on Si (111) and Al$_2$O$_3$ (0001). Our experiments indicate that the
contribution of bulk states to PHE in TIs could be significant.
