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BJTs: Hfe vs speed
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exe:
Wow, that was very detailed, thank you! Now I'm starting to understand.

I have another question: which transistor is faster: BD135 (45V Vce maximum) or BD139 (Vce 80V max)? From what I understand, BD139 should have thicker depletion region to withstand more voltage. So, BD139 should be faster, but less Hfe, right?
David Hess:
Datasheets do not normally show a variation.  The datasheets for parts intended for high hfe at low current like the 2N5088/2N5089 show a variation in hfe with collector voltage though.  Datasheets for gold doped fast saturated switches seem to show a pattern of speed versus collector voltage.

I doubt the variation is significant for more general purpose parts but if you want to find out, buy a big batch of the same transistor and measure the hfe, collector voltage, and speed for all of them.
Benta:

--- Quote from: exe on June 13, 2018, 06:20:02 pm ---I have another question: which transistor is faster: BD135 (45V Vce maximum) or BD139 (Vce 80V max)? From what I understand, BD139 should have thicker depletion region to withstand more voltage. So, BD139 should be faster, but less Hfe, right?

--- End quote ---

That's wishful thinking. Process parameters are so wide that there's no way to determine that. Basically BD135, BD137 and BD139 are all the same and grown on the same wafer and just assembled and stamped with whatever part number the market desires right now. I'd expect them all to be BD139s.
In earlier days, they would be tested for maximum Vce, but I doubt that it's done nowadays (Vce testing means increasing Vce until a certain noise level is reached, meaning you're close to avalanche).
For a power transistor costing a few cents, it's cheaper just to produce them to the max. spec.

For expensive high-end transistors it's a different story.

T3sl4co1l:
Higher voltages tend to be slower, but within a family, that may not be a strong correlation.  Both hFE and Vceo vary independently across a family (though they may not be sold that way, i.e. in all 9+ combinations thereof).

Processes are also more consistent today than yesterday, so as Benta says, they may sell the same die in all parts.  Testing is a bit part of production cost and the less they have to test, the better (low leakage is an especially bothersome one that drives up cost).

1N4001-7 are definitely made this way; 1-4 are the same, 5-7 are the same, and the specs are divided in two groups.  You're not guaranteed to get a 400V-capable 1N4001, mind -- but it does seem likely based on reading between the lines.

Tim
rfeecs:

--- Quote from: T3sl4co1l on June 13, 2018, 05:40:34 am ---hFE goes as base concentration * emitter concentration / collector concentration, roughly speaking

--- End quote ---

It should be more like hFE is proportional to emitter concentration / base concentration.

You can refer to equation (8.4.5) here:
https://people.eecs.berkeley.edu/~hu/Chenming-Hu_ch8.pdf


--- Quote ---A typical good βF is 100. D and W in Eq. (8.4.5) cannot be changed very much. The
most obvious way to achieve a high βF, according to Eq. (8.4.5), is to use a large NE
and a small NB. A small NB, however, would introduce too large a base resistance,
which degrades the BJT’s ability to operate at high current and high frequencies.
Typically, NB is around 1018 cm–3.
--- End quote ---

That chapter also describes other ways to improve hFE, like using graded base doping or a heterojunction or poly-Silicon emitter.  This can allow increasing the base doping and still getting a high hFE.

So in general hFE and "speed" are not directly related.
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