A new InGaP/GaAs resonant-tunneling transistor (RTT) has been fabricated successfully and demonstrated. A 5-period InGaP/GaAs superlattice is used as a confinement barrier for holes and an RT route for electrons. A transistor action with a common-emitter current gain up to 220 and an offset voltage of 85 mV are obtained. Due to the RT effect within the 5-period superlattice near the emitter-base p-n junction region, the N-shaped negative-differential-resistance (NDR) phenomena are observed at room temperature. Furthermore, the N-shaped NDR is found both in the saturation and forward-active region. The widely operating regime of NDR may provide the potential for practical applications.