films 19 on the silicon substrate 11 the resultant structure is shown in fig4 f next ion implantation of phosphorus is performed using the polysilicon as a mask to form n regions 20 the dose of phosphorus is about 5 u00d7 10 12 cm 2 and the implantation energy is 20 kev to implant ions in the shallow ranges the resultant structure is shown in fig4 g next and oxide sio 2 film is deposited by cvd to a thickness of 300 to 500 nm and anisotropic etching in the form of reactive ion etching is performed to form spacers 21 on the sides of the polysilicon electrode the width of the spacers 21 is about 0 2 to 0 3 micrometer ion implantation of arsenic as is then performed using the polysilicon and the spacers 21 as a mask to form n regions 22 the dose of arsenic is 5 u00d7 10 15 cm 3 the resultant structure is shown in fig4 h subsequently an inter layer insulating film 23 is formed a contact hole is opened a metal wiring conductor 24 of aluminum is formed and finally a protective film 25 is formed to complete the process fig5 shows the relationship between the impurity concentration at the surface of the n regions and the maximum lateral electric field here the gate oxide film is 25 nm thick the gate length is 1 2 micrometers the drain source voltage v ds is 6v and the gate source voltage v gs is 3v it is seen that to minimize the electric field the impurity concentration of.