Important progress has been made in the passivation of Ge/gate dielectric interfaces. One important approach is by thermally oxidized GeO2 interface and ALD high-k layers, with an interface state density D-it similar to 2 x 10(11) cm(2) eV(1). Another approach is with an epi-Si/SiO2 interface, resulting in similar Dit. Hysteresis and V-th shift, however, are still not optimal. Extensive material characterization and theoretical insights help us understanding the root cause of these remaining issues and show the way to improved interface control. (c) 2008 Elsevier B.V. All rights reserved.