BJT Saturation Characteristic

Question

From what I understand a saturation for an BJT is when the B-E voltage doesnt change the collector current anymore. In other words, Saturation simply means that an increase in base current results in no (or very little) increase in collector current. However, the section for Saturation in the graph is when the current and voltage rise almost linearly together for a fixed base current. Isnt this contradicting? Shouldn't the slope be somewhat similar to the 'active region' and be flat?

I meant if we change B-E voltage, the collector still should be the same. Am I missing something here? Thanks

Answer 1

I understand a saturation for an BJT is when the B-E voltage doesnt change the collector current anymore.

In the graph you presented, the x-axis is \$V_{ce}\$, not \$V_{be}\$. Changes in \$V_{be}\$ are represented by jumping from one parameterized curve to another.

You can see in the saturation reason, the parametric curves overlap, meaning changing \$V_{be}\$ doesn't result in a change in collector current, just as stated in your proposed definition.

Answer 2

Saturation is well defined. It is when both base-emitter junction and base-collector junction are forward biased. The way you said it is approximately correct also. In this graph, the base-emitter junction is always forward biased. So Vbe is around 0.6V or so. Varying slightly with actual base current.

Over on the left side of the graph is where Vce is <= Vbe, so that is the saturation region.

Think of it this way. As you move to the left on any one of the curves, Vce goes down, down, down, but Ic is relatively constant (constant beta). Only when Vce gets lower than Vbe does Ic start to collapse. This is the beginning of the saturation region.

So there is no contradiction. Everything is copacetic.