  Understanding of the puzzling phenomenon of high temperature
superconductivity requires reliable spectroscopic information about temperature
dependence of the bulk electronic density of states. Here I present a
comprehensive analysis of $T-$evolution of bulk electronic spectra in Bi-2212
obtained by Intrinsic Tunneling Spectroscopy on small mesa structures.
Unambiguous spectroscopic information is obtained by obviation of self-heating
problem and by improving the spectroscopic resolution. The obtained data
indicate that the superconducting transition maintains the mean-field character
down to moderate underdoping, and is associated with an abrupt opening of the
superconducting gap, which is well described by the conventional BCS
$T-$dependence. The mean-field critical temperature reaches maximum at the
optimal doping and decreases with underdoping. Such behavior is inconsistent
with theories assuming intimate connection between superconducting and
antiferromagnetic spin gaps, and support proposals associating high temperature
superconductivity with the presence of competing ground states and a quantum
critical point near optimal doping.
