What is reverse saturation current?
Under reverse polarization, the externally applied voltage V pulls the holes in the the p side and the electrons in the n side away from the junction. The width of the depletion Layer and the height of the barrier increase accordingly.

The increase in the barrier energy is measured in eV. This rise of the barrier height reduces the current to a negligible value, since very few majority carriers can now cross the junction by surmounting the enhanced barrier.

On the other side, the number of minority carriers moving down the potential barrier is left unaffected since this number depends (mostly) on the temperature. As a result of this mechanism, a small current called the "reverse saturation current" flows. The reverse saturation current is almost independent of the applied reverse bias voltage, but increases with the rise of temperature of the junction diode. This is because the minority carrier density contributing proportion increases with the rise of temperature.