Magneto‐oscillations due to a minority carrier two‐dimensional electron gas (2DEG) in the base of a bipolar quantum well resonant tunneling transistor at liquid helium temperatures are reported, we believe, for the first time. These are attributed to a high‐density 2DEG formed in the 20 nm undoped setback layer between the base and emitter. The 2DEG density has a nonmonotonic dependence on the emitter/base junction bias, increasing to a maximum at flatband conditions, then decreasing at higher bias. Associated lateral electron diffusion dominates recombination such that the current gain can increase 1000 times in a 6 T magnetic field. ©1996 American Institute of Physics.