A manufacturing method of a field-effect transistor of trench gate type, comprising: - formation of at least one trench (11, 12, 13) in an active semi-conductive layer of a first type of conductivity of a substrate comprising two opposite faces called front face and rear face, - primary implantation of ions having a second type of conductivity so as to implant each trench of the substrate to form an active gate region, - deposit of a layer of poly-crystalline silicon of the second type of conductivity on the implanted active gate region, - partial oxidation of the layer of poly-crystalline silicon to obtain an electrically insulating film of oxidized poly-crystalline silicon on a sub-layer of non-oxidized poly-crystalline silicon, the sub-layer of poly-crystalline silicon and the implanted area forming an active gate region, and - metallization of the substrate on its front face to form an active source region, and - metallization of the substrate on another face to form an active drain region, said method being characterized in that the formation step comprises the sub-steps of: - depositing a primary mask on the front face of the semiconductive substrate, the primary mask including a principal opening and two subsidiary openings (42, 43), the dimensions of the principal opening being greater than the dimensions of the subsidiary openings, - primary etching of the substrate through the principal opening to form a principal trench, and through the subsidiary openings to form two subsidiary trenches (12, 13), the primary implantation step being done through the primary mask, and in that the method further comprises: - a deposit step of a secondary etching mask on the front face of the substrate following the primary implantation step, said secondary etching mask including a secondary etching opening extending above the principal trench, - a secondary etching step of the substrate through the secondary etching opening to form a secondary trench in the principal trench. - formation of at least one trench (11, 12, 13) in an active semi-conductive layer of a first type of conductivity of a substrate comprising two opposite faces called front face and rear face, - primary implantation of ions having a second type of conductivity so as to implant each trench of the substrate to form an active gate region, - deposit of a layer of poly-crystalline silicon of the second type of conductivity on the implanted active gate region, - partial oxidation of the layer of poly-crystalline silicon to obtain an electrically insulating film of oxidized poly-crystalline silicon on a sub-layer of non-oxidized poly-crystalline silicon, the sub-layer of poly-crystalline silicon and the implanted area forming an active gate region, and - metallization of the substrate on its front face to form an active source region, and - metallization of the substrate on another face to form an active drain region, said method being characterized in that the formation step comprises the sub-steps of: - depositing a primary mask on the front face of the semiconductive substrate, the primary mask including a principal opening and two subsidiary openings (42, 43), the dimensions of the principal opening being greater than the dimensions of the subsidiary openings, - primary etching of the substrate through the principal opening to form a principal trench, and through the subsidiary openings to form two subsidiary trenches (12, 13), the primary implantation step being done through the primary mask, and in that the method further comprises: - a deposit step of a secondary etching mask on the front face of the substrate following the primary implantation step, said secondary etching mask including a secondary etching opening extending above the principal trench, - a secondary etching step of the substrate through the secondary etching opening to form a secondary trench in the principal trench.