In this paper we report the first self-aligned vertical implant-diffused (VID) and lateral implant diffused (LID) MOSFET test structures in 4H-SiC by using the differing diffusivities of implanted boron and nitrogen in silicon carbide. Boron diffusion was studied with and without nitrogen co-implants. The optimal diffusion conditions resulted in a 1 mum lateral diffusion width. A blocking voltage of 300 V was achieved in these test structures with a blocking layer thickness of only about 2 mum.