We report for the first time that replacement metal gate (RMG) work function metal (WFM) modulates the interface defects in Silicon and SiGe MOSFETs. Changing the effective work function (eWF) towards nFET band edge provides lower interface defects and higher mobility than eWF near the pFET band edge for both Si and SiGe substrates. Reducing the electric field across the dielectric (via eWF) improves bias temperature instability (BTI) for both n pMOSFETs beyond expectation. Breakdown voltage increases and gate leakage decreases with increasing eWF for both n pMOSFETs. Therefore, multi-Vt MOSFETs by RMG metal gate exhibit differentiated reliability as well as differentiated performance for both Si and SiGe channel materials.