The results of investigation of GaAs single crystals heavily doped by
group VI elements are considered. The doping effects are studied by me
thods of free carrier infrared absorption, photoluminescence, ultrason
ic, and calorimetric methods. For the explanation of the observed pecu
liarities in doped gallium arsenide the thermodynamic model of phase t
ransitions is proposed using the properties of complex formation in th
is semiconducting compound.