In an attempt to enhance the operation temperature of GaAs-based single-electron transistors (SETs), a novel Schottky in-plane gate (IPG) SET based on voltage-controlled depletion of two-dimensional electron gas (2DEG) was proposed, fabricated and characterized. For device fabrication, EB lithography and the in-situ electrochemical process were applied to a MBE-grown AlGaAs/GaAs 2DEG wafer, and narrow Pt IPGs with 200 nm length were realized on the side walls of 2DEG. The fabricated SETs clearly showed Coulomb oscillations with a Coulomb gap and Coulomb staircase at low temperatures. Coulomb oscillation was observed up to 20 K which is much higher than the operation temperature of split-gate devices having similar dimensions of several hundred nm. The small capacitance values as well as the observed strong bias dependences of device characteristics are explained in terms of the depletion characteristics of 2DEG with the Schottky IPG. The current leakage within the Coulomb gap is compatible with the cotunneling theory.