This paper discusses the effects of additional oxidation after Ge condensation on electrical characteristics of fully depleted germanium-on-insulator (FDGOI) p-channel MOSFETs. We highlight the passivation of the back interface of GOI layers by the additional oxidation. Moreover, the electrical characteristics of the fabricated GOI pMOSFETs were systematically analyzed with varying the additional oxidation time and resulting compressive strain. It is found that 12-nm-thick GOI pMOSFETs with 0.94 % compressive strain were realized without any additional oxidation by utilizing strained silicon-on-insulator (sSOI), showing the drive current over twice as high as that of unstrained Ge pMOSFETs. However, the devices exhibit large positive threshold voltage (V-th) shifts, large subthreshold swing (SS), and high off-state current (I-off), attributable to the poor MOS interfaces with buried oxides (BOX). This back channel MOS interface can be effectively passivated without significant growth of the BOX thickness by additional oxidation, leading to the restoration of the Vth shifts, improved SS, and dramatic reduction of Ioff. It is also found that the improvement of the back interface (Ge/BOX) and the strain relaxation occur simultaneously by the additional oxidation, which is corroborated by Raman spectroscopy, transmission electron microscopy (TEM), and the electrical characterization of GOI pMOSFETs. As a result, a possible physical model to explain the observed phenomena was proposed. (C) 2015 Elsevier Ltd. All rights reserved.