Semiconductor device having non-uniformly thick gate oxide layer for improving refresh characteristics

To improve the refresh characteristics of a semiconductor device, a gate oxide layer comprising a first oxide layer and a second oxide layer are formed on the substrate. A portion of the second oxide layer is isotropically etched using a photoresist layer pattern. A gate is formed by sequentially forming a gate conductive layer and a hard mask layer on the second oxide layer, and sequentially etching the hard mask layer, the gate conductive layer, the second oxide layer and the first oxide layer. Due to isotropic etching of the second oxide layer, the portion of the gate oxide layer corresponding to the center portion of the channel gate is thinner than the other portion of the gate oxide layer correspond to an edge of the channel gate.