Low frequency flicker noise in n-type metal-oxide-semiconductor field effect transistors (n-MOSFETs) with 15Å gate oxide is investigated. A noise generation mechanism resulting from valence band tunneling is proposed. In strong inversion condition, valence-band electron tunneling takes place and results in the splitting of electron and hole quasi-Fermi levels in the channel. The excess low frequency noise is attributed to electron and hole recombination at interface traps between the two quasi-Fermi levels. Random telegraph signal in a small area device is characterized to support our model.