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BJTs being phased out?
eti:
I'm watching an electronics fundamentals lecture series, and the tutor seemed to be implying that BJTs are being phased out; he said "BJTs are slowwwwwly being replaced by MOSFETS" ... or maybe I've put words into his mouth through misinterpreting what he said...
Is this the case? I find it extremely hard to believe.
T3sl4co1l:
I wonder if the statement might be a misquote of a more quantitative one: MOSFETs surpass BJTs, in terms of number of transistors produced. A change which occurred, what, back in the 70s, early 80s?
Back in the 70s, 7400 and LS TTL were still king for medium-high speed, board-level logic, and computing (and also ECL for the highest speeds). Meanwhile, MOS ranging from metal-gate CD4000 family, to NMOS and CMOS VLSI, took off massively, and never turned back. (VLSI = Very Large Scale Integration, thousands of transistors per die.) A single 8080 probably contains as many MOSFETs as a large card of 7400 (SSI scale) gates?
BJTs were never able to integrate quite as well, whether due to heat dissipation (that card of TTL would dissipate a couple ten watts, easily) or, Idunno, signal quality issues and design complexity? (There was an Integrated Injection Logic, which uses open-collector outputs fanning in to current-sourced inputs; it's completely upside-down to design with, because there's only fan-in, no fan-out to speak of. Signal levels are around a Vbe, so, would be quite sensitive to noise by itself.)
BJTs have been displaced in analog and mixed-signal spaces, too. I'm... not sure offhand what would be an example "breakthrough" shift to MOS, but there are lots of op-amps and such, mostly in lower voltage families, where mature CMOS (feature size ~200nm) dominates in affordable performance: higher bandwidth per supply current, greater functionality and integration (not just rail-to-rail inputs and outputs, but feed-forward techniques, precision auto-zero and chopper types), etc. (The feature size is relevant for two reasons: one, it's cheap of course; but two, it also performs better than bleeding-edge processes do. That's right, the transistors in a modern CPU actually perform worse, pound for pound so to speak; they're just so much smaller that they can get away with much higher frequencies (but not proportionally higher!) and much higher integration density.)
Commodity RF applications have been monolithic for some time. These are mixed-signal designs, including DSPs and high speed ADCs/DACs, as well as analog processing (filtering, mixing, PLLs, amps, etc.). They even use integrated inductors and transformers, in the higher frequency parts (>1GHz) -- crummy ones, but good enough to be useful! And these are all MOS, as far as I know.
Even so, there are some newer applications of BJTs, or similar devices anyway. Many fabs use a SiGe process for advanced op-amps. These devices have even better transconductance and capacitance, allowing even higher bandwidth per supply current, without sacrificing the input noise that CMOS tends to have, nor being restricted to low voltages.
Huh, I seemed to recall SiGe ICs were kind of an oddball or new thing, about a decade ago. Checking press releases, it looks like at least TI, LT and ADI (this was before the latter two merged) introduced parts with this process in 2006-2010. So it looks to be just another quiet innovation these days; you might never realize your op-amp contains germanium!
Tim
magic:
^ A common occurrence in academia, and it affects all industries :P
tooki:
--- Quote from: MasterTech on October 13, 2020, 06:34:44 am ---
--- Quote from: eti on October 13, 2020, 02:50:03 am ---I'm watching an electronics fundamentals lecture series, and the tutor seemed to be implying that BJTs are being phased out; he said "BJTs are slowwwwwly being replaced by MOSFETS" .
--- End quote ---
Your tutor clearly does not design much real stuff
--- End quote ---
Except that the instructor is basically correct: as others explained above, the MOSFET has been displacing the BJT in many applications, and if we include the transistors embedded in ICs, then the MOSFET won ages ago, thanks to each of us owning many billions of them in our processors. Percentage wise, the MOSFET undoubtedly makes up the vast majority of transistors manufactured today.
I only take issue with the wording “phase out”, which implies a premeditated, scheduled abandonment; that certainly isn’t the case. It’s just natural evolution of designs as the components themselves evolve. For example, one reason we can make modern, efficient switch mode power supplies is because of modern, efficient, low RDSon MOSFETs. So that contributed to the abandonment of linear power supplies (whose pass transistors, whether discrete or embedded in a regulator IC, are generally BJTs). Audio amplifiers (save for those in discrete hi-fi amplifiers) have largely switched to switching topologies as well, powered by... MOSFETs.
richard.cs:
--- Quote from: tooki on October 13, 2020, 08:36:23 am ---I only take issue with the wording “phase out”
--- End quote ---
Agreed, phase out to me also implies an eventual complete removal. This is essentially not going to happen with BJTs. As I see it there are a number of applications that pretty much only work with BJTs, a number that pretty much only work with fets, and then a huge swathe in the middle where either could be used and within that area there's been a long-term drift from bipolar to fets.
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