A fully depleted castellated-gate MOSFET device is disclosed along with a method of making the same. The device has robust I/O applications, and includes a semiconductor substrate body having an upper portion with an upper end surface and a lower portion with a lower end surface. A source region, a drain region, and a channel-forming region between the source and drain regions are all formed in the semiconductor substrate body. trench isolation insulator islands surround the source and drain regions as well as the channel-forming region. The channel-forming region is made up of a plurality of thin, spaced, vertically-orientated conductive channel elements that span longitudinally along the device between the source and drain regions. A gate structure is also provided in the form of a plurality of spaced, castellated gate elements interposed between the channel elements. The gate structure also includes a top gate member which interconnects the gate elements at their upper vertical ends to cover the channel elements. Finally, a dielectric layer is provided to separate the conductive channel elements from the gate structure.