Transistor saturation

[sy]

Hello, when a transistor is in saturation it typically has a low VCE (let's say <100mV).
I wanted to ask how this would be reflected on the characteristic curve?
In this diagram, would it be correct to say the transistor reaches saturation at IB6, and has VCE=0.7V when the transistor is in saturation?
Or is the curve for saturation meant to have a steeper slope (reflecting the lower output impedance)?
Thank you in advance

[Leuams]

It's in between points A and B. On the x axis it is labeled as saturation region.

[AnalogTodd]

Transistor saturation begins as you drop below a collector to emitter voltage of 0.7V as based on this curve. What you need to remember is that bipolar transistors like this are used as current gain devices, and you aren't really looking at the lower output impedance, but instead the lower beta (current gain) of the device. When you use a transistor as a gain device, you want the change in Ic to be as linear as possible for the change in Ib. In the saturation region, you lose that linear relationship.

In the active region, you get a collector current equal to Ic/Ib which stays fairly constant except for Early voltage effects and you get that linear gain. In the saturation region, the Ic drops significantly for the same Ib. If you are trying to use the transistor as a gain device, you lose a lot of that gain. There are a lot of other effects that also come into play as well that we won't delve into at this time.

[MarkT]

When the base-collector junction is reverse biased you get normal transistor action, the active region, where charge carriers injected into the base almost all get pulled to the collector by the electric field in the base-collector junction.  This gives the full gain.  When that base-collector junction is no longer reverse biased there is no electric field to pull carriers to the collector and they only get there through thermal diffusion - the gain drops rapidly to 5 or 10 kind of range

One reason why in switching transistors the doping ratios between emitter : base : collector are something like 10000 : 100 : 1 is that the high density of charge carriers in the emitter and low density in the collector drives the diffusion from e to c via entropy, causing better saturation performance.

For the best saturation careful design of the transistor doping profile is needed, allowing the best BJTs to be able to have a Vce(sat) of a few tens of mV with reasonable gains (more than 10).