the energy gap between Homo and Lumo determine if the material is conductor insulator or semiconductor. if the energy gap small like in metals the electrons can jump easily from the HOMO to LuMO orbital thus the material conductor if the energy gap is very high the material  insulator and if it is between conductor and insulator the material semiconductor.

Thanks for answer dear Shoshana, please I need any artical is provied this or range to values of energy gap ...

Dr.Rafid

The article "Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors", by P.I. Djurovich et al. / Organic Electronics 10 (2009) 515–520 should answer your question.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.466.5966&rep=rep1&type=pdf

Thank you Dear Dr.Olaf