We have considered two simulations of the electron energy as a function of the quasi-momentum: the parabolic one, typical of isotropic semiconductors, and the Fivaz one, which was introduced in [1, 2] for layered semiconductors with specific chemical bonding in one of the crystallographic directions. Using parameters known for GaSe we find the thermodynamic functions for an electron gas and compare them for both high (quasi-classical) and low (Fermi gas) temperatures. This enables us to identify the cases where a parabolic dependence of the energy on the quasi-momentum (which is appreciable for many particle interaction problems due to its simplicity) can also be used for layered semiconductors.