Doping effects on the structural and electronic properties of ladderanes and ladder polysilanes have been studied using density functional theory. Two types of doping: substitution with isoelectronic atoms or heteroatoms (or radicals), have been used to design low band gap ladderanes. It is found that the B-doped [n]-ladderanes and 1,2 P-doped [n]-silaladderanes exhibit a very noticeable bent conformation, whereas the 1, 2 and 1,3 N-doped ladderanes, P-doped ladderanes, and silaladderanes keep the relatively straight ladder shapes. The isoelectronic atom doping reduces the HOMO-LUMO (H-L) gaps of [n]-ladderanes but increases those of [n]-silaladderanes with n > S. The present results show that isoelectronic atom doping is not an effective way to decrease the H-L gaps of ladderanes and silaladderanes. Heteroatom doping has a more pronounced effect than the isoelectronic atom doping. The HOMOs of heteroatom-doped ladderanes and silaladderanes are destabilized and LUMOs are stabilized, leading to significant reduction of H-L gaps. Most of the B-, N-, and P-doped [n]-silaladderanes we designed have H-L gaps below 1.5 eV. Therefore, it is expected that these silaladderanes are promising candidates of conductive or semiconductive materials. The heteroatom doping is a viable approach to reduce H-L gaps for the silaladderanes. In addition, it is found that nine different density functionals, including B3LYP, SVWN LDA, four pure GGAs, and three hybrid GGAs, as well as the time-dependent B3LYP method, all lead to the same predictions on the H-L gaps of ladderanes, silaladderanes, as well as their doped derivatives.