In this paper, we investigated the quantization in the MOSFET inversion layer by solving the Schrödinger EQuation (SEQ) in the momentum space according to the 3D Full Band (FB) structure of silicon. For the first time, we explain the dependence of the eigenvalues on the momentum k in the plane of transport and discuss its periodicity. Furthermore, we discuss the solution of the SEQ around the energy minima, compare the Non-Parabolic model with the FB results, and propose a new efficient procedure to achieve an accurate energy dispersion in the subbands.