Theory of negative differential conductivity in a superlattice miniband

Bloch equation conductivity perpendicular to the layers of a superlattice (period d) is evaluated using an extension of the balance-equation approach [X. L. Lei and C. S. Ting, Phys. Rev. B 32, 1112 (1985)] to narrow-band transport. The perpendicular peak drift velocity vₚ and the critical field Ec, at which the drift velocity peaks, are analyzed as functions of miniband width. Our theoretical prediction that Ecd increases with decreasing miniband width agrees well with the data of Sibille et al. [Phys. Rev. Lett. 64, 52 (1990)], even for the samples of narrowest miniband width in their experiment.