  Current-voltage (I-V) characteristics of quench condensed, superconducting,
ultrathin Bi films in a magnetic field are reported. These show hysteresis for
all films, grown both with and without thin Ge underlayers. Films on Ge
underlayers, close to superconductor-insulator transition, show a peak in the
critical current, indicating a structural transformation of the vortex solid.
These underlayers, used to make the films more homogeneous, are found to be
more effective in pinning the vortices. The upper critical fields ($B_{c2}$) of
these films are determined from the resistive transitions in perpendicular
magnetic field. The temperature dependence of the upper critical field is found
to differ significantly from Ginzburg-Landau theory, after modifications for
disorder.
