In this work, 3-D Tri-gate and ultra-thin body planar InGaAs quantum well field effect transistors (QWFETs) with high-K gate dielectric and scaled gate-to-source/gate-to-drain (L _SIDE ) have been fabricated and compared. For the first time, 3-D Tri-gate InGaAs devices demonstrate electrostatics improvement over the ultra-thin (QW thickness, T _QW =10nm) body planar InGaAs device due to (i) narrow fin width (W _FIN ) of 30nm and (ii) high quality high-K gate dielectric interface on the InGaAs fin. Additionally, the 3-D Tri-gate InGaAs devices in this work achieve the best electrostatics, as evidenced by the steepest SS and the smallest DIBL, ever reported for any high-K III-V field effect transistor. The results in this work show that the 3-D Tri-gate device architecture is an effective way to improve the scalability of III-V FETs for future low power logic applications.