Fabrication and performance of 0.1-μm gate-length AlGaAs/GaAs HEMTs with unity current gain cutoff frequency in excess of 110 GHz

A multilevel resist process has been developed that is capable of producing 0.1-μm T-cross-section gates using 50-kV electron-beam lithography. A ratio of upper to lower dimensions greater than three provides a low gate resistance of 450 Ω-mm, allowing improved microwave performance over high-resistance trapezoidal gates. Careful characterization and control of gate recessing resulted in less than 300 Å of ungated channel recess for minimal parasitic channel resistance. Several material structures were compared, varying the dopant incorporation from 80-Å spike doping to 40-Å spike doping to atomic planar doping. The growth of the spike-doped structures was optimized to provide a high n_s⩾1.2 10¹² cm^-2. In addition, these spike-doped structures had heavily doped caps of 100 Ω/□ sheet resistivity to provide low parasitic source resistance. Performance was evaluated by on-wafer S-parameter measurements. The peak measured unity current gain cutoff frequency ranged from 90 to 113 GHz, depending on the material structure. This performance is attributed to careful attention to the details of gate formation, layer design, and MBE (molecular-beam epitaxial) growth