A novel functional InGaP/GaAs/InGaAs step-compositional-emitter heterojunction bipolar transistor (HBT) is fabricated and demonstrated. Due to the avalanche multiplication and discontinuous confinement effects for electrons in the InGaAs quantum well and at InGaP/GaAs heterojunction, respectively, an interesting triple-route S-shaped negative-differential-resistance switch is observed under inverted operation mode at room temperature. In addition, the excellent transistor performances including a high current gain of 220 and a low offset voltage of 60 mV are achieved under normal operation mode. Consequently, the device could be used for signal amplifier under normal operation mode and multiple-valued logic circuit application under inverted operation mode. (C) 2004 Elsevier Ltd. All rights reserved.