Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers, >2 mW ( ∼ 80 K, 3.7 μm) and >0.1 mW ( ∼ 300 K, 4.3 μm). A 3.8–3.9 μm laser structure operated up to T = 180 K. At 80 K, peak power >100 mW and a slope efficiency of 48% (4.8% per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency >1 and a loss coefficient ≥ 100 cm−1. © 1998 American Institute of Physics.