A novel GaAs quantum‐well (QW) negative differential resistance (NDR) device, prepared by molecular‐beam epitaxy (MBE), has been fabricated and demonstrated. TheS‐shaped NDR characteristics is attributed mainly to the impact ionization effect. First, the theoretical analysis based on a triangular barrier model is employed to study the influence of structure parameters on the barrier height. Then, the experimentalS‐shaped NDR behaviors are observed and analyzed for two‐terminal operation and three‐terminal operation, respectively. The successful usage of three‐terminal control gives flexible and wide application in switching circuits. Furthermore, the studied device exhibits a considerable reaction to illumination and temperature variation. Therefore, the studied NDR device may be used as a sensitive switch under excitation of light illumination and temperature variation.