High-performance InP-based enhancement-mode HEMTs using non-alloyed ohmic contacts and Pt-based buried-gate technologies

High performance InP-based InAlAs/InGaAs enhancement-mode HEMT's are demonstrated using two improved approaches to device structure design and fabrication, i.e., nonalloyed ohmic contacts and Pt-based buried-gate technologies, to reduce the source resistance (R/sub S/). With specially designed cap layer structures, nonalloyed ohmic contacts to the device channel were obtained providing contact resistance as low as 0.067 /spl Omega//spl middot/mm. Furthermore, in device fabrication, a Pt-based buried-gate approach is used in which depletion-mode HEMTs are first intentionally fabricated, and then, the Pt-based gate metal is annealed at 250/spl deg/C, causing the Pt-InAlAs reaction to take place under the gate electrode so that Pt sinks into InAlAs and depletes the channel. As a result, the depletion-mode HEMTs are changed to enhancement-mode, while the channel region between the source and gate electrodes remain undepleted, and therefore, the small R/sub S/ of 0.2 /spl Omega//spl middot/mm can be maintained. Excellent maximum transconductance of 1170 mS/mm was obtained for a 0.5-/spl mu/m-gate device. A maximum current-gain cutoff frequency f/sub T/ of 41.2 GHz and maximum unilateral power-gain cutoff frequency f/sub max/ of 61 GHz were demonstrated for a 0.6-/spl mu/m-gate enhancement-mode HEMT.