Devices based on a wide bandgap semiconductors also have received considerable attention. In particular, GaN based devices are considered more appropriate for higher frequency operations than SiC based ones. Although GaN has been successfully applied for blue laser development, its microwave application has encountered a number of challenging research issues, two of which are material purity and substrate material. Daimler-Chrysler, for example, uses material from Cree and silicon carbide substrate for X band high power application (10 W/mm at 50 V bias) (see Fig. 5.13). NEC has attained fmax of 90 GHz with an SiC substrate. This device, when matured, is promising for the base station applications.
Fig. 5.13. Daimler-Chrysler GaN FET.