Split-gate ambipolar organic transistor technology has been proposed as a solution for simple and low-cost fabrication of complementary electronics. In a split gate device, the polarity of the transistor can be controlled. Depending on the voltage bias of an additional control gate, a unipolar type OTFT (p- or n-) can operate, exhibiting a large on/off current ratio. However, conventional split-gate ambipolar organic thin-film transistors suffer from operational instability including a large I-V hysteresis and high bias stress effects due to a charge trapping at the dielectric/semiconductor interface. Here we demonstrate that such issues can be solved with top-gate device geometry and non-planar split-gate architecture. The proposed device operates in a controllable unipolar n(or p)-type mode with more robust electrical characteristics including hysteresis-free I-V characteristics as well as higher bias stress stability than previous split-gate ambipolar devices. Furthermore, we also demonstrate that the device has higher hole and electron carrier mobilities.