[SOLVED] BJT in the reverse active region easy but unfamilair question

hi

i have been asked a question by my professor and he challenged me to find an answer for this question and I have been unsuccessful.

If we have a bipolar junction transistor in the reverse active region (also know as inverse). If someone did not where the reverse bias is operated it is when we
(for the NPN)
-apply reverse bias to the emitter base junction
-apply forward bias to the collector junction

ok?

This case is not used lately and is as some people might call it as useless!!!

The point behind the question is that is there any application where the BJT is used in the reverse active region?
(i mean by application is that the saturation region of the mosfet is used as an application for an amplifier or for an inverter)

I really want to be up to the challenge

PS: I do not plagiarize I would state the sources :idea:

thank you :grin:

Does original TTL logic operate at least one transistor in reverse?

Keith

Well I have asked him about 1 application an he told me that the TTL is opperated in reverse bias (that was given by my prof).
Then I posted on intel discussion about this and told them that the TTL is used but I got an answer that it was not used and the community was not able to help me.
I did not add TTL to remove confusion just in case.

Is there any other application?

Thank you for your fast and helpful reply

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Keith

thanks does The Gummel-Poon BJT Model or Integrated injection logic use this region?

Ha. I was just searching and came across I2L logic. Maybe I was mixing that up with TTL.

Keith

Yes, there are applications.

A BJT in common emitter configuration can be used as analog switch (C-E is switched), e.g. for choppers or synchronous demodulators. Unfortunately, it has a DC offset voltage of 2 to 20 mV in on-state (Vce,sat @ Ic = 0). In inverse mode, the offset voltage (Vec,sat @ Ie = 0) is only 0.05 to 0.5 mV.

keith1200rs does the Gummel-Poon BJT Model use the BJT in reverse active region ?
C. The Gummel-Poon BJT Model
and does the
IIT operate in reverse active?
Integrated injection logic: Definition from Answers.com


FvM please may you explain a bit more?May you give me a website to check it my self?or a pdf file?

thank you:grin::grin::grin::grin::grin:

I have used inverse BJT analog switches in some designs.

I only know a reference from a German text book (U. Tietze/Ch. Schenk, Halbleiter-Schaltungstechnik, 11. Edition, Ch.2 p. 69). It lists the saturation voltage ranges I mentioned above, according to empirical results.

There's also a derivation based on the current gain in normal and inversed mode:

Vce,sat(Ic=0) = Vt ln(1+1/Bi)
Vec,sat(Ie=0) = Vt ln(1+1/Bn)
Vt=26mV, Bn = 50...500, Bi = 1...10

Click to expand...

The english version of the text book: Electronic Circuits

The differences in regular and inversed mode saturation voltage should be treated systematically correct by a SPICE simulation, because it's based on the Gummel-Poon model. The model covers of course the complete transistor behaviour, including inversed mode. That's pretty obvious, if look you at the model equations in your link. Also Ebers-Moll basically treats the inverse active mode.

I did not remember any integrated circuit which I had built which use the inverse properties on a vertical bipolar.

I also think that accurate modelling of saturation in forward mode where primary the parasitic vertical substrate PNP is activated is different to the inverse operation.

From the physical view the inverse operation is getting worse if you try to improve the forward operation. There are two basic effects resulting from the different doping levels close to the base at the emitter and the collector. The doping level defines the space-charge zone. A high doping gives a low connecting resistance to the inner transistor, a low breakdown voltage of the diode and a high majority capacitance.

For the circuit operation you want to have a high breakdown voltage of the collector to base diode, a low parasitic emitter resistance and a low collector to base capacitance. That is exactly what is achieved by the asymmetric bipolar.

I anticipate that physical optimization and did not concider the inverse mode.

I think there are very rare applications where both, the forward and the inverse mode is used to an advantage at the same time.

I only remember some symmetric application of a lateral field PNP in a BiCMOS process where two P+ emitter regions are used for a nonlinear current distribution but the difference to a two diode connection is low because the lateral current gain is less than about 10 and need a high doped N+ buried layer to reflect the minorities by the doping gradient field. So the symmetry was used but not the inverse operation. By the way we found the parallel connection of two ring P+ collector lateral field PNPs more efficient than a single two stripe symmetric field PNP.

The parallel connection of an asymmetric bipolar could be used to make the operation symmetric at the price of less performance.

so do you have any application like the one kieth1200rs gave me??

thanks