An analytical formulation and modeling for determining the band-tail p
arameter E-0 taking into account the canrier-impurity and phonon-carri
er interactions and the structural disorder is presented. An analysis
of the different contributions to E-0 at low and room temperature in n
- and p-type GaAs is also done. The theoretical results agree very wel
l with experiment for noncompensated GaAs samples. The agreement was a
chieved in a wide range of carrier concentrations at 5 K and room temp
erature by adding a constant value to E-0, lower than 4.1 meV, related
to the structural disorder.