Reverse saturation current of BJT?
The reverse saturation current in the collector-base junction is origined by the diffusion of minority carriers from the neutral regions to the depletion region.

It is very dependent from specific parameters of the junction itself, such as the donor and acceptor concentrations, the diffusion coefficients of holes and electrons, the cross-sectional area.

In general it must be kept small, since you don't want a transistor conducting when the base-emitter junction is not directly polarized.

It is exponentially dependent form temperature (as a general rule it doubels every 10°C), and independent from Vcb