A new vertical power MOSFET structure called rectangular-grooved MOSFET (RMOS) is proposed, in which the vertical channels are provided along the sidewalls of the rectangular grooves formed by a reactive ion-beam etching (RIBE) technique. The structure is characterized by reduced ON -resistance and high packing density. The tionship between the ON-resistance and the packing density in the new structure is calculated. It is demonstrated that the structure essentially possesses a lower ON -resistance per unit area than VMOS and CMOS structures. Experimental results are also described in detail.