  The disorder-induced Superconductor-to-Insulator Transition in amorphous
Nb$_{x}$Si$_{1-x}$ two-dimensional thin films is studied for different niobium
compositions $x$ through a variation of the sample thickness $d$. We show that
the critical thickness $d_c$, separating a superconducting regime from an
insulating one, increases strongly with diminishing $x$, thus attaining values
of over 100 {\AA}. The corresponding phase diagram in the $(d, x)$ plane is
inferred and related to the three-dimensional situation. The two-dimensional
Superconductor-to-Insulator Transition well connects with the three-dimensional
Superconductor-to-Metal Transition.
