Nitrogen atom has the smallest size and a very high electron density aroun the nitrogen. Therefore, its electron releasing tendency or the basic strength is the maximum. Down the group, there is a gradual increase in atomic size and decrease in the electron density on the central atom. Consequently, the electron releasing tendency or basic strengths of the hydrides decrease in the order as given below

NH₃ > PH₃ > AsH₃ > Sb H₃ > BiH₃

Therefore, NH₃ is basic while BiH₃ is only feebly basic.