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The fabrication and performance of ultra-high-speed 0.3-μm gate-length enhancement-mode high-electron-mobility transistors (E-HEMT's) are reported. By using a buried platinum-gate technology and incorporating an etch-stop layer in the heterostructure design, submicron E-HEMT devices exhibiting both high-threshold voltages and excellent threshold voltage uniformity have been achieved. The devices demonstrate a threshold voltage of +171 mV with a standard deviation of only 9 mV. In addition, a maximum DC extrinsic transconductance of 697 mS/mm is measured at room temperature. The output conductance is 22 mS/mm, which results in a maximum voltage gain (g/sub m//g0) of 32. The devices show excellent RF performance, with a unity current-gain cutoff frequency (fT) of 116 GHz and a maximum frequency of oscillation (fmax) of 229 GHz. To the best of the authors' knowledge, these are the highest reported frequencies for lattice-matched E-HEMT's on InP.