We report positive bias temperature instability data in replacement metal gate Ge n-channel metal–oxide–semiconductor field-effect transistors with an in-situ gate stack employing an ultrathin (5 A), stable Al2O3 interlayer (IL) using ultrafast (~microseconds) characterization techniques that ensure recovery artifact-free measurements. Comparison with state-of-the-art GeO x IL is also reported besides establishing correlations between band-edge interface trap density ( ${D}_{\text {it}}$ ), mobility ( $\mu$ ), and threshold voltage ( ${V}_{T}$ ) instability. Ultrafast measure-stress-measure (UF-MSM), ultrafast measure-stress-detrap-measure (UF-MSDM), stress-induced-leakage-current (SILC), direct-current current–voltage (DCIV), split capacitance–voltage ( ${C}$ – ${V}$ ), and low-temperature full conductance techniques along with a compact model demonstrate that: 1) trap generation occurs at IL/high- ${k}$ interface during stress; 2) an increase in $\mu$ with reduction in ${D}_{\text {it}}$ does not guarantee better reliability, i.e., ${V}_{T}$ shift and $\mu$ are uncorrelated due to their dependence on separate regions of the gate stack; 3) contributions to total ${V}_{\text {T}}$ degradation from trapping and generated traps are mutually exclusive; 4) UF-MSDM is a powerful tool to estimate trap generation; and 5) ${V}_{T}$ degradation is directly proportional to high- ${k}$ thickness, varies inversely with IL thickness, and reduces with annealing.