A method of making a non-volatile MOS semiconductor memory device includes a formation phase, in a semiconductor material substrate, of isolation regions filled by field oxide and of memory cells separated each other by said isolation regions The memory cells include an electrically active region surmounted by a gate electrode electrically isolated from the semiconductor material substrate by a first dielectric layer; the gate electrode includes a floating gate defined simultaneously to the active electrically region. A formation phase of said floating gate exhibiting a substantially saddle shape including a concavity is proposed.