Current in Saturation and Active regions of BJT

Question

In the image(http://www.allaboutcircuits.com/vol_3/chpt_4/4.html), the result of SPICE simulation for active mode operation of BJT is shown.

The current source is set at a constant value of 20uA and the collector bias voltage is varied from 0 to 2V.

If you see the description of the image, it's mentioned that:

"A Sweeping collector voltage 0 to 2 V with base current constant at 20 µA yields constant 2 mA collector current in the saturation region"

Constant 2 mA collector current in the 'Saturation Region'? As I have understood, the saturation region is the part that lies to the left of active region(where Vce is almost zero) From what is shown in the image is it not that the 2 mA collector current in Active Region?

If the voltage is increased above 2V, there would be no change in the collector current as it's limited by the base current. The maximum current that we have as output is 2mA. How do you say that the transistor is now in Saturation mode?

Please explain me why the saturation region is indicated to the left of active region? From the characteristic curves, it seems that the region that is marked as 'active', is actually the Saturation because there is maximum collector current there.

Answer 1

You have understood this correctly and it appears the author of the document you linked is wrong - for a MOSFET they have perfectly described the saturation region but, this is a BJT circuit and the saturation region is the left hand side. Here's the drawing they talk about: -

Saturation is when the current and voltage rise almost linearly together for a fixed base current.

Answer 2

Perhaps it helps to give some explanation to the term "saturation". Yes - there are two different meanings of the term "saturation" - dependent on the transistor type: BJT or FET. In this context, we should answer the question: Where is this term coming from - that means: Which quantity is "saturated?

1.) BJT (with collector resistor): Increasing Vbe (or Ib) leads to rising collector current Ic, which causes a continuous decrease of the collector-emitter voltage Vce - until a lower limit is reached at app. (0.4...0.6)volts. A further increase in input voltage/current does not cause a further increase of Ic. Thus, Vce has reached it´s lower limit and cannot become smaller. We say: The voltage Vce has reached the state of "saturation"(and - at the same - time the current Ic cannot assume larger values).

2.) In contrast to this effect, for FET´s the term "saturation" means something else. For small drain-source voltages Vds we have approximately a linear relationship between Vce and the current Id (for a fixed control voltage Vgs). This region is called "linear region" or "resistor region". However, for further increasing the voltage Vds beyond a certain threshold (pinch-off) the current Id does not increase anymore (in fact, a slight increase due to channel width modulation can be observed). That means: The current Id has reached the state of "saturation".

Summary: It is an unfortunate situation that two different meanings of the term "saturation" are in use (for BJTs and for FETs), but - due to historical reasons - we have to be aware of these two different definitions.

Answer 3

Here's a typical Ic vs Vce diagram showing the saturation region of a BJT.

In this case if Ib is set at 20uA and Vce varies between 0 and 2V you can clearly see that Ic will also vary from about 12mA (Vce=2V) to about 8mA @ Vce = 0.5V (very non linear) to 0mA @ Vce = 0V. Clearly either the original author is just plain wrong or I need to rethink everything I know about BJTs.