  In the minimal supersymmetric standard model (MSSM) the one-loop
finite-temperature corrections from the squark-Higgs bosons sector are
calculated, the effective two-Higgs-doublet potential is reconstructed and
possibilities of the electroweak phase transition in full MSSM ($m_{H^\pm}$,
${\tt tg}\beta$, $A_{t,b}$, $\mu$, $m_Q$, $m_U$, $m_D$) parameter space are
studied. At large values of $A_{t,b}$ and $\mu$ of around 1 TeV, favored
indirectly by LEP2 and Tevatron data, the threshold finite-temperature
corrections from triangle and box diagrams with intermediate third generation
squarks are very substantial. The four types of bifurcation sets are defined
for the two-Higgs-doublet potential. High sensitivity of the low-temperature
evolution to the effective two-doublet and the MSSM squark sector parameters is
observed, but rather extensive regions of the full MSSM parameter space allow
the first-order electroweak phase transition respecting the phenomenological
constraints at zero temperature. As a rule, these regions of the MSSM parameter
space are in line with the case of a light stop quark.
