We report on a new nanoelectronic planar three-terminal device, fabricated from III/V semiconductor-based heterosystems. Utilizing the benefits of selfgating and in-plane gates, the tunable three-terminal device presented exhibits strong non-linear input- and transfer-characteristics, both, at liquid Helium and at room temperature. For a given side-gate voltage, the devices input characteristics closely resemble that of a conventional diode, although it is fabricated by a single post-growth patterning process only, i.e., etching of deep trenches. We present a simple model, based on an equivalent circuit, which well reproduces the experimental findings. Possible applications are discussed. (C) 2005 Elsevier Ltd. All rights reserved.