Effects of gate-recess structure on high-frequency characteristics of 0.1 μm metamorphic HEMTs

We investigate effects of the gate-recess structure on the high-frequency characteristics of the 0.1 μm depletion-mode metamorphic high electron mobility transistors (HEMTs). We characterize the dc and radio frequency performances of two different gate-recess structures and perform a comparative study using the hydrodynamic device model simulation and small-signal parameter analysis. The narrow gate-recess structure shows significantly higher dc performances than the wide gate-recess structure, and this phenomenon is due to the presence of the negatively charged surface states on the InAlAs Schottky barrier layer surface in the wide gate-recess structure. Despite the superior dc characteristics, the narrow gate-recess structure shows more degraded maximum frequency of oscillation (fmax) of ∼296 GHz than that (∼340 GHz) of the wide gate-recess structure and almost the same cutoff frequency (fT) of ∼130 GHz. The degraded fmax of the narrow gate-recess structure is attributed to a ∼66% higher gate-to-drain capacitance (Cgd). The significant increase of Cgd is caused by the reduction of effective gate-to-drain spacing due to the nonlinear electric potential distribution in the gate-to-drain region.