High-frequency SiGe-n-MODFET for microwave applications

n-type SiGe modulation-doped hetero field-effect transistors (MODFET's) with a 0.25-μm Schottky-gate on a Si/sub 0.55/Ge/sub 0.45/ buffer are presented. The layer structure was designed to enable elevated sheet carrier densities of n/sub s/=7.0×10/sup 12/ cm/sup -2/ at moderate electron mobilities of 1050 cm
²
/Vs. Reducing the thickness of the cap layers enhances the control of the gate on the 2DEG and leads to a high transconductance of 320 mS/mm. Targeting analog applications, we focused on large current densities around 400 mA/mm. Due to advanced RF-characteristics the 100-GHz hurdle of f
_max
was passed for the first time with f
_max
(U)=120 GHz and f
_T
was determined at 42 GHz.