A semiconductor transistor includes a first lightly doped-drain region disposed in a drain region of a semiconductor substrate; a first heavily doped region disposed in the first lightly doped-drain region; and a gate located on the channel region; a gate oxide layer between the gate and the channel region; and a first insulating feature disposed in the first lightly doped-drain region between the channel region and the first heavily doped region. The gate overlaps with the first insulating feature. The thickness of the first insulating feature is greater than that of the gate oxide layer.