Saturation region is one in which the output current is independent of the input and remains almost constant. Hence, MOSFETs in saturation are modeled as current sources( whose current is independent of voltage across it)

When a diode is operated as reverse bias the current flow is almost completely blocked. A small amount of current is still able to travel in reverse through the diode and this is referred to as the reverse current saturation.

reverse saturation current is produced by the thermal activity of the diode materials. This current: 1- Is temprature dependant; that is, it increases as temprature. 2- Accounts fot the major portion of diode reverse current

surface leakage current is produced by contamination on the surface of the device, allowing current flow to bypass the junction

its simple.reverse saturation current is because of the flow of minority carriers across the junction when the bias is changed suddenly from forward to reverse.this is why it doesnt depend on forward bias