An electric field effect transistor of high breakdown voltage and a method of manufacturing the same are disclosed. A recessed portion is formed at the channel region and is filled by a protective oxide layer. Lightly doped source/drain regions are formed under the protective oxide layer. The protective oxide layer protects the lightly doped source/drain regions. Accordingly, the protective oxide layer prevents the electric field from being concentrated to a bottom corner portion of the gate structure. In addition, the effective channel length is elongated since an electric power source is connected to heavily doped source/drain regions from an outside source of the transistor, instead of being connected to lightly doped source/drain regions.