The mechanism of resonant interband tunneling in polytype heterostructures of GaSb/AlSb/InAs gives excellent peak‐to‐valley current ratios due to the band‐gap blocking of the nonresonant current components. Using InAs as the base in a double‐barrier polytype heterostructure, it is possible to demonstrate resonant tunneling at room temperature through a quantum well as wide as 110 nm. At this width, which is about 20 times larger than that typically used in resonant tunneling diodes in the GaAs/AlGaAs system, the peak‐to‐valley ratio is 44:1 (77 K). Significant negative differential resistance is observed even for 240 nm wells. The projected device response time for a resonant tunneling transistor with a wide InAs quantum base is more than five times faster than for a GaAs device, due to the reduced base resistance.