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I know this might sound a little bit easy for some of you but i do really need a clarification on this.

I have a question about the operation modes of the Bi-Polar Junction Transistor.

In the graph below (bjt characteristics) we all do agree that there are four regions for a transistor to operate :

  1. Forward Active
  2. Saturation
  3. Cut-off
  4. Reverse Active

Most of the refrences I have looked into said that in order for a transistor to operate in the "Reverse Active" The biasing voltage on the Base-Emitter-Junction must be in Reverse, which makes sense to my understanding of the BJT.

Unfortunately, my Professor at the University has another approach and he said that if the Voltage of the Collector-Emitter-Junction is negative (Reverse) the transitor is Reverse in general, therefore we must swap the Collector with the emitter and then we keep solving currents in order to know if the Transistor is in (Rev SAT/ or /Reverse Act)

Can anyone explain that to me?

Also is there is any method to detect when a BJT is operating in the Reverse Active mode?

enter image description here

enter image description here

Any help will be appreciated

Thanks in advance

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    \$\begingroup\$ For NPN transistor BJT is in Forward Active when base is at 0.6V higher than emitter and collector voltage larger than the base voltage. But if you revere the B-E voltage, now the emitter is at higher voltage than the base. But in this case BJT is in Cut-off region (Ic = 0A). NPN transistor is in Reverse Active only when base-collector junction is in Forward (base voltage larger than collector voltage (0.6V)) and emitter voltage is larger than the base voltage (we swap the Collector with the emitter). And now emitter work as a collector and collector work as a emitter. \$\endgroup\$ – G36 Nov 25 '16 at 16:40
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    \$\begingroup\$ Input transistor in TTL gates work in Reverse Active mode when input are 1. This means that your Professor is right. \$\endgroup\$ – G36 Nov 25 '16 at 16:42
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What you should understand is that the transistor is symmetric about the base.

That being said, the level of doping in the three regions, base, emitter and collector are vastly different. If we consider an n-p-n transistor, the collector and emitter are theoretically interchangeable but current amplification factor popularly denoted by beta is high, about 50 - 300 for an average transistor in the active mode and much lesser, about 0.1 - 2 in the reverse active mode. This is decided by the level of doping in the different regions.

In active mode, B>E for forward bias and C>B for reverse bias, hence C>>E. Similarly in reverse active mode, B>C for forward bias and E>B for reverse bias, hence E>>C. So yes, all your sources including your professor and your textbook are correct.

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