Both N in `NH_3` and `Bi` in `BiH_3` have a lone pair of electrons. Since the atomic size of N is less than Bi and N is more electronegative than Bi Hence electron density is more on N in `NH_3` than on Bi in `BiH_3`. Consequently the tendency to donate its lone pair is more in `NH_3` hence `NH_3` is more basic than `BiH_3`.
