A new transistor effect is demonstrated in a 120 nm base, bipolar quantum‐well, resonant‐tunneling transistor (BiQuaRTT). In this BiQuaRTT, a strong, multiple negative differential resistance (NDR) characteristic is obtained at room temperature with high‐current gain (≳50). The effect is shown to be the consequence of an asymmetric, quantum‐well‐base heterostructure whose shape is controlled by the base/collector bias. Changes in the quantum‐well shape lead to large modulations of the transmission coefficient for quasi‐thermalized minority electrons crossing the quantum‐well base. In this letter, we describe the transport characteristics of these transistors, including also temperature and magnetic field dependence.