http://onlinelibrary.wiley.com/doi/10.1002/zaac.201400042/abstract;jsessionid=267C553C7F5D3F8871400D962D4DE9B9.f04t02

Paper might help

A semiconductor is a material with an energy gap between its conduction and valence band, and the Fermi level located in this gap. The valence band is filled with electrons and the conduction band is empty. Thus there are no "free" charge carriers.

In contrast, in a semi-metal the conduction and valence band have an energy overlap and the Fermi level is located in the overlap region. The top of the valence band has empty states, while the bottom of the conduction  band contains electrons. This makes the material "a little" conductive.

The transition from a semi-metal to a semiconductor behaviour occurs if the valence and conduction bands are "moved" one away from another.

This "movement" is a result of a change in the electronic band structure of the material, induced for instance by a structural phase transition or quantum confinement.

Thank you very dear Sir Johann,

in fact it is a photo catalyst material LiTaO3 with a small transition of the parameter c of the crystalline latice ( analyzed by XRD at high temperature) at a temperature close to 300 degrees C that it could be a semi-metal semiconductor transition that remains difficult for me to prove that. Knowing that this compound known by its famous Ferroelectric transition around 650 degrees C. In other hand, is that I can speak of a quantum confinement for a particle size of the order of some microns?

Many thanks!