We present tunneling field-effect transistors fabricated from InP-GaAs heterostructure nanowires with an n-i-p doping profile, where the intrinsic InP region is modulated by a top gate. The devices show an inverse subthreshold slope down to 50 mV/dec averaged over two decades with an on/off current ratio of approximately 10(7) for a gate voltage swing (V(GS)) of 1 V and an on-current of 2.2 μA/μm. Low-temperature measurements suggest a mechanism of trap-assisted tunneling, possibly explained by a narrow band gap segment of InGaAsP.