  We study localized in-gap states and quantum spin Hall effect in Si-doped
InAs/GaSb quantum wells. We propose a model describing donor and/or acceptor
impurities to describe Si dopants. This model shows in-gap bound states and
wide conductance plateau with the quantized value $2e^2/h$ in light dopant
concentration, consistent with recent experiments by Du et al. We predict a
conductance dip structure due to backward scattering in the region where the
localization length $\xi$ is comparable with the sample width $L_y$ but much
smaller than the sample length $L_x$.
