Design and fabrication of Si/SiGe n-type MODFETs

The impact of several buffer concepts on the resulting MODFET behaviour has been investigated. Best DC and RF results are found for thick linear graded Si60Ge40 buffers from which enhancement and depletion mode MODFETs were simultaneously fabricated in one chip using a differently deep-gate recess. The layer sequence grown on a 2 mu m thick linear graded buffer consists of a strained Si two-dimensional electron gas channel and for the first time of highly Ge containing layers (Si0.55Ge0.45) High Hall mobilities (mu = 1190 cm(2)/V s) and elevated sheet carrier densities (n(s) = 4.4 x 10(12) cm(-2)) are achieved at room temperature. Applying a 0.15 mu m e-beam T-gate (gate-source spacing 0.2 mu m) yields reduced parasitic resistances (R-G = 250 Omega mm(-1), R-S = 0.18 Omega mm) and high saturation currents up to 320 mA/mm. Depletion HFETs show room temperature and 77 K transconductances of g(me)(RT) = 286 mS/mm and g(me)(77 K)= 411 mS/mm, respectively, cut-off frequencies f(t) up to 43 GHz and f(max) up to 92 GHz - the highest f(max) reported so far for SiGe MODFETs. Enhancement HFETs with g(me)(RT) = 476 mS/mm or g(me)(77 K) = 784 mS/mm and f(t) up to 30 GHz and f(max) up to 52 GHz have been realized. (C) 1998 Elsevier Science B.V. All rights reserved.