When did DS-2 go to all silicon diodes?

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laurie
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Re: When did DS-2 go to all silicon diodes?

Post by laurie » Sun Dec 04, 2022 1:17 am

Old_Iron wrote:
Sun Dec 04, 2022 12:52 am
bipolar transistors are two back-to-back diodes, and a PNP is a bipolar junction transistor...
With sincere apologies for not letting go of this subject... this is not the right description of a transistor.

A diode is like a non-return valve. Current will flow "unimpeded" in one direction, and not at all in the other.

A BJT is like a faucet - the amount of current flowing between Collector and Emitter is controlled by the current injected (or removed) at the Base. Two diodes back-to-back simply would not behave like that.

The "large signal" Ebers–Moll model looks like two diodes back-to-back, but that isn't what it is implying.

This diagram gives a good visual:
https://en.wikipedia.org/wiki/Bipolar_j ... %20regions.

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Re: When did DS-2 go to all silicon diodes?

Post by Old_Iron » Sun Dec 04, 2022 1:58 am

laurie wrote:
Sun Dec 04, 2022 1:17 am
With sincere apologies for not letting go of this subject... this is not the right description of a transistor.

A diode is like a non-return valve. Current will flow "unimpeded" in one direction, and not at all in the other.

A BJT is like a faucet - the amount of current flowing between Collector and Emitter is controlled by the current injected (or removed) at the Base. Two diodes back-to-back simply would not behave like that.

The "large signal" Ebers–Moll model looks like two diodes back-to-back, but that isn't what it is implying.

This diagram gives a good visual:
https://en.wikipedia.org/wiki/Bipolar_j ... %20regions.
No problem to not let it go, I appreciate your will to share your knowledge.

Again, I should had being more specific, sorry, but I wasn't describing the transistor's function/works as the same as of 2 diodes.

I was talking about physical construction.

If you ask google : "is a bipolar transistor 2 diodes?", the answer will be:

"Since a bipolar transistor is essentially two diodes, it can function as such. However, since bipolar transistors are not designed to be used as diodes, using them as diodes might cause a problem in terms of current and other ratings."

Another answer, this time for a BJT:

"BJTs can be thought of as two diodes (P–N junctions) sharing a common region that minority carriers can move through. A PNP BJT will function like two diodes that share an N-type cathode region, and the NPN like two diodes sharing a P-type anode region."


As I said, physical construction, and in the context of being affected by temperature, not that they works the same way.

P.S.: I edited the original post to make it more clear and to mean what I really wanted to say, sorry for the misunderstanding.

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Re: When did DS-2 go to all silicon diodes?

Post by laurie » Sun Dec 04, 2022 3:00 am

Ah yes, google. If I didn't have an advanced electrical engineering degree this type of sloppy description from the internet likely wouldn't annoy me anywhere near as much :?

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Re: When did DS-2 go to all silicon diodes?

Post by Old_Iron » Sun Dec 04, 2022 3:46 am

laurie wrote:
Sun Dec 04, 2022 3:00 am
Ah yes, google. If I didn't have an advanced electrical engineering degree this type of sloppy description from the internet likely wouldn't annoy me anywhere near as much :?
The "sloppy description from the internet" presented by google came from the Wikipédia page that you linked.

https://en.wikipedia.org/wiki/Bipolar_j ... transistor


The complete paragraph:

"Theory and modeling
BJTs can be thought of as two diodes (P–N junctions) sharing a common region that minority carriers can move through. A PNP BJT will function like two diodes that share an N-type cathode region, and the NPN like two diodes sharing a P-type anode region. Connecting two diodes with wires will not make a BJT, since minority carriers will not be able to get from one P–N junction to the other through the wire."

More from the same page:

"The thin shared base and asymmetric collector–emitter doping are what differentiates a bipolar transistor from two separate diodes connected in series."



And it seems that people from Toshiba doesn't have the same "advanced electrical engineering degree" that you have. :?

https://toshiba.semicon-storage.com/ap- ... diode.html

"Since a bipolar transistor is essentially two diodes, it can function as such. However, since bipolar transistors are not designed to be used as diodes, using them as diodes might cause a problem in terms of current and other ratings."

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Re: When did DS-2 go to all silicon diodes?

Post by laurie » Sun Dec 04, 2022 6:22 am

These being the relevant statements: "Connecting two diodes with wires will not make a BJT" and "The thin shared base and asymmetric collector–emitter doping are what differentiates a bipolar transistor from two separate diodes connected in series"

The Toshiba page even says "Use dedicated diodes for typical diode applications instead of bipolar transistors."

I suppose at the very highest level a non-technical, non-mathematical statement could be "BJTs can be thought of as two diodes". Arrrrggghhh... A transistor can be coaxed into operating as two diodes in very specific applications. If the statement is that "a BJT can behave (poorly) like two diodes in specific circumstances", then yes I agree - it can. But that is not what it actually is.

There are many models that describe what is actually going on inside a BJT. One common simple model is the hybrid-pi:
.
Hybrid_Pi.jpg
Hybrid_Pi.jpg (8.15 KiB) Viewed 246 times
.
SPICE models are a favorite of electrical engineers and are much more mathematically involved:
https://www.youspice.com/spice-modeling ... datasheet/

None of the engineering mathematical models for a BJT model them as "two diodes". Because they just aint ;)

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Re: When did DS-2 go to all silicon diodes?

Post by laurie » Fri Dec 23, 2022 5:55 pm

Hey Old_Iron .... apologies if I became annoying by being pedantic about the topic above.

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