Some of the best high-temperature commercial devices are now GaN field-effect transistors (FETs) on silicon substrates. However, these devices cannot meet requirements for space applications requiring high radiation hardness and for operations at temperatures as high as 600 ^oC. High temperature and radiation hard applications stimulated interest in developing transistors using ultrawideband gap materials including AlN, AlGaN with a high molecular fraction of aluminum, gallium oxide, diamond, boron nitride, and their heterojunctions. A wider bandgap and, therefore, larger energy required to produce an electron-hole pair and larger energy gap discontinuities in heterostructures formed by these materials make them both more tolerant to radiation and more capable of operating at higher temperatures. Insulated gate (Metal Insulator Heterostructure FET (MISHFET) structures with high K-materials implemented in the AlGaN materials system, the power FINFET configurations implemented in GaN and diamond, and gate edge and channel engineering approaches are key technologies for ultra-wide bandgap semiconductor applications. Using all AlGaN materials is now a proven approach to compete with GaN. Measured and predicted materials properties of BN and diamond promise an even better performance but the power device applications of these materials and their heterojunctions have not yet been sufficiently explored. I will review the material parameters of ultra-wideband gap semiconductors and specific device designs linking them to the expected radiation hardness and high-temperature performance and to improving the reliability and lifetime of ultra-wideband gap transistors.