  In the present paper, we consider the initial value problem for the bipolar
Vlasov-Poisson-Boltzmann (bVPB) system and its corresponding modified
Vlasov-Poisson-Boltzmann (mVPB). We give the spectrum analysis on the
linearized bVPB and mVPB systems around their equilibrium state and show the
optimal convergence rate of global solutions. It was showed that the electric
field decays exponentially and the distribution function tends to the absolute
Maxwellian at the optimal convergence rate $(1+t)^{-3/4}$ for the bVPB system,
yet both the electric field and the distribution function converge to
equilibrium state at the optimal rate $(1+t)^{-3/4}$ for the mVPB system.
