We investigated stability and the electronic structure of extended defects including antisite domain boundaries and stacking faults in the kesterite-structured semiconductors,
Cu
2
ZnSnS
4
(CZTS) and
Cu
2
ZnSnSe
4
(CZTSe). Our hybrid density functional theory calculations show that stacking faults in CZTS and CZTSe induce a higher conduction band edge than the bulk counterparts, and thus the stacking faults act as electron barriers. Antisite domain boundaries, however, accumulate electrons as the conduction band edge is reduced in energy, having an opposite role. An Ising model was constructed to account for the stability of stacking faults, which shows the nearest-neighbor interaction is stronger in the case of the selenide.