Greetings,

I have a simple circuit shown in this video:
https://screencast-o-matic.com/watch/cYiO0UEaUj

Both junctions are forward biased which means that the transistor should be in saturation mode and it looks like it is. What confuses me is the fact that the current flows from the emitter to the collector, essentially cancelling the base emitter current. My basic understanding tells me that since there is no base to emitter current flowing therefore the transistor should be OFF. Obviously, that's not the case, so what am I missing? Why exactly does this work?

 

You've allowed the collector to be lower than the base. That transistor is one big NPN, so you're biased it in the "opposite" direction and hence current flows "opposite".

 

OK, so if I understand it correctly, the transistor in this case is biased via base collector junction and the emitter act as a collector would under normal biasing conditions.

 

To correctly turn on a transistor you want to have:

1. Forward bias on the Base-Emitter junction
2. Reverse bias on the Base-Collector junction

If you look at the semiconductor physics you are lowering the potential barrier for electrons to cross the junction. I have heard of operating a transistor backwards, but I'm pretty sure it doesn't work as well as if you do it correctly. Might be interesting to try on a simulator, just to see if the models anticipate this sort of thing.

 

Right, it isn't normally used that way, but it does work. It's still a P-N junction. You could characterize it if you wanted something to do, seeing the E-C current is with varying B-C current, but I don't know why you would want it to work other than in normal biasing.

 

Right, it isn't normally used that way, but it does work. It's still a P-N junction. You could characterize it if you wanted something to do, seeing the E-C current is with varying B-C current, but I don't know why you would want it to work other than in normal biasing.

I certainly wouldn't

 

Greetings,

I have a simple circuit shown in this video:
https://screencast-o-matic.com/watch/cYiO0UEaUj

Both junctions are forward biased which means that the transistor should be in saturation mode and it looks like it is. What confuses me is the fact that the current flows from the emitter to the collector, essentially cancelling the base emitter current. My basic understanding tells me that since there is no base to emitter current flowing therefore the transistor should be OFF. Obviously, that's not the case, so what am I missing? Why exactly does this work?

The transistor is being operated in the reverse mode which is simply when the role of the emitter and collector are swapped. It is not cancelling the base-emitter current, but rather you now have the base-collector current playing that role and the current out of the collector is now the sum of the current into the base and the current into the emitter. As you can see from the simulation, you have 425 mA plus 274 mA yielding 699 mA. This transistor is heavily into saturation (the base-collector voltage is 2 V and the emitter-collector voltage is 1 V) and so the reverse beta is similarly in the tank at only 0.64.

It is possible (and sometimes done on an IC) to make BJT transistors symmetric so that the collector and emitter are names arbitrarily assigned to the two outer terminals. This is very commonly done with IC-based MOSFET transistors with respect to the source/drain terminals, which is why some MOSFET symbols make no distinction between them.

But experience has shown that you can get better performance by making transistors, especially BJT transistors, highly asymmetrical. You can get better current gain, breakdown voltages, speed, and other parameters in one direction, usually at the sacrifice of those parameters in the other direction. Some transistor models reflect this asymmetric behavior in the reverse direction and others do not. Even the ones that do usually don't put as much effort into modeling it nearly as well as they model the forward characteristics.

There ARE circuits the intentionally operate transistors in the reverse mode, but they are few and far between.

 

I remember reading a thread (somewhere?) informed by someone who knew, saying that the areas aren't doped symmetrically in a BJT.

So while you may be able to operate it in reverse...Why would you want to? I think that's what causes tears in the space-time continuum.

 

Maybe the best model for this is a common cathode dual diode, since you need to have the collector of the emitter reverse-biased before it starts acting like a transistor.

 

I have seen small-signal BJTs used in the reverse configuration at low switching currents, since that gives (I know not why) a lower saturation voltage.

 

QUOTE="upand_at_them, post: 1535654, member: 85302"]
I remember reading a thread (somewhere?) informed by someone who knew, saying that the areas aren't doped symmetrically in a BJT.

So while you may be able to operate it in reverse...Why would you want to? I think that's what causes tears in the space-time continuum.
[/QUOTE]

Not only are they typically doped differently, the layouts are generally quite different.

As for why someone might want to operate them in reverse, that's simple. Why do we have so many different BJT transistors in the first place? Why not just a one-size-fits-all NPN and a similar PNP? Because different applications need different transistor characteristics, so you look for a transistor that has characteristics that you need for your application. Well, there are some applications that need characteristics typically found in transistors operated in reverse mode and there aren't transistors that have those characteristics in the forward mode because the need is small enough that it isn't economically viable to manufacture them.