  A generalization of the Thermo Field Dynamics (TFD) for fermionic degrees of
freedom is proposed. Such a generalization follows a previous one where the
SU(1,1) thermal group was used to obtain the closed bosonic string at finite
temperature. The SU(2) thermal group is introduced to construct a general
thermal Bogoliubov transformation to get the type IIB superstring at finite
temperature.
