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The interband cascade laser (ICL) concept provides robust and efficient emission in the midwave infrared spectral band. While the geometry is somewhat analogous to that of a quantum cascade laser employing intersubband transitions, the ICL implementation exploits the type-II band alignment of the GaSb-based material system. A semimetallic band overlap at the boundary between the electron and hole injector regions automatically generates carriers with densities tunable by quantum confinement. Electrical injection then replenishes the carriers already present rather than creating the population inversion. In this paper, we describe and analyze the physical principles governing ICL operation, and discuss specific modifications to the active region, electron injector, hole injector, and waveguide designs that demonstrably improve the performance. The pulsed I- V and L-I characteristics of devices processed from over 50 wafers provide a statistically meaningful confirmation of the established trends.