  The semi-empirical models of the solar chromosphere are useful in the study
of the solar radio emission at millimeter - infrared wavelengths. However,
current models do not reproduce the observations of the quiet sun. In this work
we present a theoretical study of the radiative transfer equation for four
semi- empirical models at these wavelengths. We found that the Chromospheric
Solar Milimeter-wave Cavity (CSMC), a region where the atmosphere becomes
locally optically thin at millimeter wavelengths, is present in the
semi-empirical models under study. We conclude that the CSMC is a general
property of the solar chromosphere where the semi-empirical models shows
temperature minimum.
