Field effect transistors (FETs) in diamond should outperform FET structures on other wide bandgap materials like SiC and GaN in high power/high temperature applications due to the ideal diamond materials properties. However, the technology of these structures proved difficult leaving two device concepts to investigate: (1) the boron δ-doped p-channel FET and (2) the hydrogen induced p-type surface-channel-FET. The δ-channel-FET approach follows a traditional design path of power FET structures. Here, simulation results have enabled the extrapolation of a maximum RF output power to 27 W/mm, a value which is indeed higher than for any FET based on III-Nitrides or SiC. However, due to the narrow technological parameter window, fabricated δ-channel-FETs are still well behind expectations. In contrast, concerning the surface-channel-FET the physical/chemical nature of its channel remains still under discussion. Nevertheless, results obtained with this FET concept yielded a VDmax > 200 V (