  We develop a comprehensive, elegant theory to explain terahertz (THz)
emission from a superlattice over a wide range of applied electric field,which
shows excellent agreement between theory andexperiment for a
GaAs/Al{0.3}Ga{0.7As superlattice. Specifically we show that increasing
electric field increases THz emission for low fields, then reduces emission for
medium fields due to field-induced wave function localization, and then
increases emission in the high field due to delocalization and Zener tunneling
between minibands. Our theory shows that Zener tunneling resonances yield high
THz emission intensities and points to superlattice design improvements.
