High-performance InP-Based enhancement-mode HEMT's 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(S)). With specially designed cap layer structures, nonalloyed ohmic contacts to the device channel were obtained providing contact resistance as low as 0.067 Omega . mm. Furthermore, in device fabrication, a Pt-based buried-gate approach is used in which depletion-mode HEMT's are first intentionally fabricated, and then, the Pt-based gate metal is annealed at 250 degrees 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 HEMT's are changed to enhancement-mode, while the channel region between the source and gate electrodes remain undepleted, and therefore, the small R(S) of 0.2 Omega . mm can be maintained. Excellent maximum transconductance of 1170 mS/mm was obtained for a 0.5-mu m-gate device. A maximum current-gain cutoff frequency f(T) of 41.2 GHz and maximum unilateral power-gain cutoff frequency f(max) of 61 GHz were demonstrated for a 0.6-mu m-gate enhancement-mode HEMT.