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{\large
\centerline{\bf Excitons in Semiconductors:}
\centerline{\bf A T-Matrix Approach}}

\ 

\centerline{\underline{T. Schmielau}, K. Henneberger}

\centerline{\em Universit\"at Rostock, Fachbereich Physik,
                18051 Rostock, Germany}

\ 

\noindent

We discuss the influence of electron-hole pair correlations on the
optical and thermodynamic properties of semiconductors.

Within the Green's function approach, pair correlations are represented
by the $T$-matrix. We obtain the $T$-matrix by numerical solution of it's
Bethe-Salpeter equation.
Special attention is paid to the degenerate case, which is not described
correctly by common approximations as the quasiparticle approximation,
but is a key feature in semiconductors as the reason for optical gain and
possibly for Bose-Einstein condensation (BEC).
The effect of screening of the Coulomb interaction is demonstrated,
however, it is of minor importance at parameters where BEC may occur.

Using these results, we discuss the concept of a dissociation equilibrium
of excitons and unbound carriers, the Mott transition, and the consequences
of the fermionic character of the exciton constituents for BEC of excitons.
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