Diamond surface channel FET with f/sub max/ above 30 GHz

Diamond FETs are attractive candidates for power applications mainly due to the high breakdown field despite the fact that doping of diamond is only possible for p-type FET devices, however with a high sheet charge density above 10/sup 13/ cm/sup -2/ and mobilities of 100 to 350 cm/sup 2//Vs. In this case a hydrogen induced surface channel FET structure was used, in which a sub-surface hole channel is created by hydrogen plasma treatment. In the micrometer gate length regime such structures have shown excellent breakdown characteristics and have thus been used to evaluate the power handling capability of diamond FETs. In this study the gate length performance between 5.0 μm and 0.2 μm was investigated, using an experimental self-aligned symmetric gate technology, where the gates were realized by a 3-level e-beam lithography process. The DC output characteristics of the 0.2 μm gate length device are displayed. The surface channel FETs are enhancement mode devices. The gate built-in potential of 0.9 V (Al) is sufficient to pinch off the sheet charge of 1.35×10/sup 13/ cm/sup -2/. The maximum output current approx. 200 mA/mm at a (forward) gate bias of -2.4 V despite the high drain saturation voltage of -5 V indicating the influence of parasitic resistances. The maximum transconductance g/sub m/. reaches 107 mS/mm with the g/sub m//g/sub DS/ voltage gain still being 10/sup 2/. Also shown is the bias point for maximum f/sub T/. The cut-off frequencies f/sub T/ and f/sub max/(MAG) and f/sub max/(U) were directly extracted from the gain plots as shown. The values for fT = 11.8 GHz and f/sub max/(MAG) = 30.6 GHz and f.(U) = 33.0 GHz are the highest values obtained for diamond FET devices up to now. The high f/sub max//f/sub T/ ratio is mainly due to the high voltage gain.