Band edge structure on k • p perturbation theory consider a non-degenerate orbital ψnk at k = 0 in time band n of a cubic crystal. Use second-order perturbation theory to find the result where the sum is over all other orbital’s ψjk at k = 0. The effective mass at this point is this problem somewhat difficult. The mass at the conduction band edge in a narrow gap semi conductor is often dominated by the effect of the valence band edge, whence where the sum is over the valence bands; Eg is the energy gap. For given matrix elements, small gaps lead to small masses.