The aim of this work was to build a thermodynamic energy band diagram for the system of n-type Si(100)/HF that is in dynamic equilibrium at the interface. The construction concept was based on the shift of energy levels such as Fermi energy (EF), conduction band energy (Ec), and valence band energy (Ev) before and after the contact of silicon with HF solutions. Through measurements of the open-circuit potential and flatband voltage (VFB), the energy band diagram for the Si/HF system was established. This diagram was useful in estimating the activation energy for the photoelectrochemical etching system. The kinetic study demonstrated that the etching rate of the silicon increases to a maximum with an increase in HF concentration from 0.5 to 2.0 M, then decreases with further increase of the HF concentration. Based on the energy band diagram established and the electrochemical kinetic data measured, we discuss in detail the etching rate of the n-Si/HF and make clear the concentration effect of HF on the photoelectrochemical reaction.