Qualitative explaination of Output Characteristic of Transistor (BJT) using Depletion layer between Collector and Base
Due to high doping ratios from base to emitter/collector, there exist 2 exponential curves at the same temp whose ratio gives the linear hFE ratio due to Conduction band bending.
Yet hFE increases &gt;50% from very low currents then decreases typ. past the mid-range current.
Then into saturation, hFE decreases to a useable hFE=10% of max hFE when CB changes from a low leakage current sink to a conductive diode at Vce=Vce(sat) often rated at 10,20 and/or 50 if hFE&gt;500.
The C-E conduction leakage may in a range of 100k to &gt;1M for common types then into saturation the size of the chip rated in Watts is inversely related to the diode Rce quasi linear bulk resistance Rce&lt;=1/Pmax rated down to 50% or so.
Some as low as 10mOhm while PN2222 is 4 Ohms at Vce(sat) for 1/4W rating.
Some physics details.