ABSTRACT
Metamorphic InGaAs/InAlAs-HEMTs were grown by Molecular Beam Epitaxy on GaAs substrates using linearly graded InxAl1-xAs buffers to accomodate lattice misfit. A compositional variation in x of 0.5 per μm in the buffer layer resulted in practical buffer thicknesses of one μm or less, with moderate cross hatching of 5 nm height for x = 0.32 and 10 nm for x = 0.52. This surface roughness had only little influence on the transport properties of the metamorphic heterostmctures in comparison to lattice matched structures on InP-substrates. DC- and RF- characteristics of 0.13 μm-HEMTs with an In-content in the channel of 0.53 were excellent, with maximum drain currents of 800 mA/mm, extrinsic transconductances of 720 mS/mm and cut-off frequencies of 185 GHz for fT and 215 GHz for fmax, respectively. Metamorphic HEMTs with a channel of 12 nm In0.53Ga0.47As on top of a 20 nm In0.32Ga0.68As subchannel showed dramatically reduced output conductances of 20 mS/mm, thus giving very high fmax-values of 350 GHz, while still retaining convincing transit frequencies of 160 GHz.
