We developed an original physics-based unified analytical model describing the transport from diffusive to ballistic regimes in double-gate MOSFETs. This model includes a new analytical model of the backscattering coefficient based on an accurate empirical approach. In addition, short-channel effects, carrier quantum-mechanical confinement, and degeneracy are considered. For the first time, we used the model to analyze and quantify the real impact of ballistic/quasi-ballistic transport on circuit performances. We show how the switching times of CMOS inverters and the oscillation frequencies of ring oscillators are improved when considering ballistic instead of diffusive transport. Finally, our model is fully validated at both device and circuit levels using numerical simulation and experimental data.