A semiconductor device has an active region formed on a semiconductor substrate. The opposite sides of the active region are defined by channel stoppers. The semiconductor device is made by a manufacturing process which prevents the active region from narrowing responsive a spreading of the channel stopper into the active region. This method comprises the steps of forming a first oxide layer by oxidizing a surface of the semiconductor substrate; forming a nitride layer over the first oxide layer; forming a second oxide layer having a predetermined thickness over a predetermined portion of the first layer where the active region is to be formed, the second layer being formed by removing the nitride layer over the predetermined portion and then oxidizing; removing the nitride layer; implanting an impurity for forming the channel stoppers by using the second oxide layer as a mask; removing the second oxide layer; forming a gate oxide layer by oxidizing over the active region; and forming an electrode over the gate oxide layer.