This work presents a built-in effective body-bias effect (VBS, eff) in ultra-thin-body and BOX (UTBB) hetero-channel MOSFETs. This effect stems from the discrepancies in the electron affinity, the effective density-of-states, and the band-gap between the high-mobility channel and conventional Si channel. Physical VBS, eff models quantifying this effect are presented for nFET and pFET, respectively. Our study indicates that the DIBL of various hetero-channel devices can be worse than what permittivity predicts because of the built-in forward body-bias effect. Moreover, we have shown that this detrimental effect can be suppressed by the quantum-confinement effect. This effect has to be considered when designing or benchmarking various UTBB hetero-channel MOSFETs.