Author Topic: BJTs being phased out?  (Read 8891 times)

0 Members and 1 Guest are viewing this topic.

Offline etiTopic starter

  • Super Contributor
  • ***
  • !
  • Posts: 1801
  • Country: gb
  • MOD: a.k.a Unlokia, glossywhite, iamwhoiam etc
BJTs being phased out?
« 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" ... 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.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: BJTs being phased out?
« Reply #1 on: October 13, 2020, 04:40:14 am »
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
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
The following users thanked this post: xlnx

Offline magic

  • Super Contributor
  • ***
  • Posts: 7453
  • Country: pl
Re: BJTs being phased out?
« Reply #2 on: October 13, 2020, 07:17:14 am »
^ A common occurrence in academia, and it affects all industries :P
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #3 on: October 13, 2020, 08:36:23 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" .

Your tutor clearly does not design much real stuff
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.
 

Offline richard.cs

  • Super Contributor
  • ***
  • Posts: 1200
  • Country: gb
  • Electronics engineer from Southampton, UK.
    • Random stuff I've built (mostly non-electronic and fairly dated).
Re: BJTs being phased out?
« Reply #4 on: October 13, 2020, 09:39:29 am »
I only take issue with the wording “phase out”
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.
 
The following users thanked this post: tooki

Offline OwO

  • Super Contributor
  • ***
  • Posts: 1250
  • Country: cn
  • RF Engineer.
Re: BJTs being phased out?
« Reply #5 on: October 13, 2020, 11:05:27 am »
No. Show me a FET based MMIC that can give you flat response (let's say within 6dB) to 5GHz and has reasonable noise figure. BGA616 fits the bill and it's BJT based.
Email: OwOwOwOwO123@outlook.com
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #6 on: October 13, 2020, 12:59:03 pm »
No. Show me a FET based MMIC that can give you flat response (let's say within 6dB) to 5GHz and has reasonable noise figure. BGA616 fits the bill and it's BJT based.
Given the nature of the original post, I don't think the OP is knowledgeable enough about microwave electronics to be able to answer your question.

I think the rest of the thread already characterizes the situation well, which richard.cs's comment sums up perfectly:
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.
 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 10031
  • Country: gb
Re: BJTs being phased out?
« Reply #7 on: October 13, 2020, 01:46:20 pm »
I think we pretty much reached an equilibrium some time ago, where anything digital and quite a lot of analogue things use MOSFETs and some analogue things use BJTs. There are some exceptions where BJTs are still king for a switching application. If you want a cheap 1200V transistor as a primary switch in an SMPS that can sit across 3-phase mains, for example, BJTs are king.
 
The following users thanked this post: tooki

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #8 on: October 13, 2020, 02:11:22 pm »
Aren't those all shifting to IGBTs? I mean, an IGBT is essentially a hybrid of the two, so the real answer is even more complex! :D
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #9 on: October 13, 2020, 03:12:35 pm »
Thanks for the added info! In that case, what's the IGBT's advantage that's leading to their increased use in high-power applications?
 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 10031
  • Country: gb
Re: BJTs being phased out?
« Reply #10 on: October 13, 2020, 03:17:50 pm »
Aren't those all shifting to IGBTs? I mean, an IGBT is essentially a hybrid of the two, so the real answer is even more complex! :D
1200V IGBTs get expensive. For higher power levels they may be the better choice. However, low power 1200V BJTs are super cheap, so for low power supplies in a lot of 3 phase control, monitoring and metering applications they are a great choice. That market is big enough that there are SMPS controller chips specifically designed to drive a high voltage BJT. For single phase 240V work you have a lot more choices of good cheap transistors with adequate voltage headroom. This is just an example application. There are others where the price of a suitable MOSFET or IGBT starts to rise rapidly above about 700V, and a simple BJT is an excellent choice.

 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 17427
  • Country: us
  • DavidH
Re: BJTs being phased out?
« Reply #11 on: October 13, 2020, 08:14:52 pm »
Just adding to your list:

- Bipolar transistors have a higher transconductance than FETs.  This also leads to a higher gain-bandwidth product.
- Bipolar transistors have a higher current density than FETs, which leads to less area (cost) for a given current.
- Bipolar transistors have a lower voltage noise than FETs.
 
The following users thanked this post: boB, tooki

Offline Benta

  • Super Contributor
  • ***
  • Posts: 6420
  • Country: de
Re: BJTs being phased out?
« Reply #12 on: October 13, 2020, 08:17:09 pm »
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.

This will happen the same day that the 555 gets phased out. According to most posts on this forum (from my feeling, every third), the 555 will cure Corona, world hunger, overpopulation and global warming as well as solve almost every electronic issue imaginable.   :-DD

Don't worry, the BJT will survive.

I'm much more worried about the JFET, which is a dying species.

 
The following users thanked this post: boB, tooki

Online NiHaoMike

  • Super Contributor
  • ***
  • Posts: 9319
  • Country: us
  • "Don't turn it on - Take it apart!"
    • Facebook Page
Re: BJTs being phased out?
« Reply #13 on: October 13, 2020, 11:12:56 pm »
There was a device called the "ESBT" (Emitter Switched Bipolar Transistor) many years back, it's literally just a high voltage bipolar transistor with a low voltage MOSFET in series with the emitter.
- Bipolar transistors have a higher current density than FETs, which leads to less area (cost) for a given current.
MOSFETs that can switch on the order of 100A are not that uncommon (they're found on every modern motherboard), but a 100A bipolar is pretty exotic.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: BJTs being phased out?
« Reply #14 on: October 14, 2020, 12:55:18 am »
There was a device called the "ESBT" (Emitter Switched Bipolar Transistor) many years back, it's literally just a high voltage bipolar transistor with a low voltage MOSFET in series with the emitter.
- Bipolar transistors have a higher current density than FETs, which leads to less area (cost) for a given current.
MOSFETs that can switch on the order of 100A are not that uncommon (they're found on every modern motherboard), but a 100A bipolar is pretty exotic.

Not exotic, just old.  They reigned in motor drives and such, alongside SCRs, until IGBTs displaced them (in all but the biggest applications, where SCRs and GCTs still reign supreme).  BJTs of 300A (or more?) and 1200V, often with triple-Darlington configuration, are common salvage items.  I wouldn't call it "exotic", but "obsolete" is appropriate. :)

Incidentally, ESBTs are still available (for exactly what applications, I do not know?), and they made a bit of a resurgence when SiC broke onto the market with JFETs and BJTs -- what's old is briefly new again, then old again as SiC MOSFETs and IGBTs were introduced. :)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Benta

  • Super Contributor
  • ***
  • Posts: 6420
  • Country: de
Re: BJTs being phased out?
« Reply #15 on: October 14, 2020, 10:26:54 pm »
@blueskull:

I'm with you all the way in your description of JFET applications. They are wonderful devices.

That doesn't change the fact that only very few are available today. Almost no major semiconductor manufacturers have them in their portfolio. And the ones that still have them are "pruning" their availability list.

Today, only specialty manufacturers such as linearsystems.com supply a comprehensive line of JFETs, like the famous 2SJ74/2SK170.

Like I said, it's a dying species.

« Last Edit: October 14, 2020, 10:46:06 pm by Benta »
 

Offline LeoGray

  • Newbie
  • Posts: 1
  • Country: cn
Re: BJTs being phased out?
« Reply #16 on: October 14, 2020, 11:56:45 pm »
JFETs are extremely widely used in (lower power high voltage) power ICs due to the difficulty obtaining planar MOSFETs at higher Vds (remember, in a planar IC process, D and S are symmetric, so Vdg can also breakdown the fragile gate oxide). It's common practice to series connect a JFET with its gate tied to ground, with a low voltage MOSFET from the JFET's source and ground, to form a cascaded high voltage switch.

Also JFETs can be series connected without individual external gate driving circuitry, they just self balance, making them suitable for very high voltage switching that traditionally only IGBTs can do, but at a much higher speed.

I never know JFET could be used for high voltage. I always want to make a electroadhesion thing that uses around 10KV. But never found a good cheap solution for discharging switch.

Do you mind expending a little bit on the high voltage switching part? What's the buzz word I should search to find good articals and examples?

Thanks in advance.   
 

Online EEVblog

  • Administrator
  • *****
  • Posts: 39025
  • Country: au
    • EEVblog
Re: BJTs being phased out?
« Reply #17 on: October 15, 2020, 10:01:41 am »
I'm much more worried about the JFET, which is a dying species.

I'm feeling a Save the JFET T-Shirt coming on...
 

Offline bsfeechannel

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: 00
Re: BJTs being phased out?
« Reply #18 on: October 15, 2020, 04:46:58 pm »
Today, only specialty manufacturers such as linearsystems.com supply a comprehensive line of JFETs, like the famous 2SJ74/2SK170.

Like I said, it's a dying species.

There's a discrete JFET inside every single electret microphone capsule. Maybe it is becoming a product for niche applications.

I'm feeling a Save the JFET T-Shirt coming on...

Not a bad idea. I've already rescued a bunch of stray JFETs off the streets and now they are happily living in my loving parts bin.
 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 10031
  • Country: gb
Re: BJTs being phased out?
« Reply #19 on: October 15, 2020, 04:56:23 pm »
Today, only specialty manufacturers such as linearsystems.com supply a comprehensive line of JFETs, like the famous 2SJ74/2SK170.
Like I said, it's a dying species.
There's a discrete JFET inside every single electret microphone capsule. Maybe it is becoming a product for niche applications.
True, but MEMS microphones are killing the market for electret microphones.
 

Offline tszaboo

  • Super Contributor
  • ***
  • Posts: 8217
  • Country: nl
  • Current job: ATEX product design
Re: BJTs being phased out?
« Reply #20 on: October 15, 2020, 05:01:38 pm »
I'm much more worried about the JFET, which is a dying species.

I'm feeling a Save the JFET T-Shirt coming on...
We really should! Otherwise how do we make current mirrors to reflect on society.

OP: BJT is cheaper than MOSFET so its not going anywhere.
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #21 on: October 15, 2020, 06:20:47 pm »
Today, only specialty manufacturers such as linearsystems.com supply a comprehensive line of JFETs, like the famous 2SJ74/2SK170.
Like I said, it's a dying species.
There's a discrete JFET inside every single electret microphone capsule. Maybe it is becoming a product for niche applications.
True, but MEMS microphones are killing the market for electret microphones.
Is that true outside of severely space-constrained applications like cellphones and the mikes built into phone earpieces (be it inline in the cord or integrated like in AirPods)? I haven't gotten the impression that any other applications have embraced MEMS mikes.
 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 10031
  • Country: gb
Re: BJTs being phased out?
« Reply #22 on: October 15, 2020, 06:31:46 pm »
Today, only specialty manufacturers such as linearsystems.com supply a comprehensive line of JFETs, like the famous 2SJ74/2SK170.
Like I said, it's a dying species.
There's a discrete JFET inside every single electret microphone capsule. Maybe it is becoming a product for niche applications.
True, but MEMS microphones are killing the market for electret microphones.
Is that true outside of severely space-constrained applications like cellphones and the mikes built into phone earpieces (be it inline in the cord or integrated like in AirPods)? I haven't gotten the impression that any other applications have embraced MEMS mikes.
That's 99% of all mics, isn't it? The electret mic market was mostly phones and cell phones. Phones have almost gone. Cellphones have moved to MEMS mics. Most other mics are condenser or dynamic. There were once some very high quality electret mics, but they seem to have gone, and now we only have condenser, ribbon and dynamic mics for high quality applications. Condenser mics also use a JFET, but their volumes are millions, not the hundreds of millions we see with cellphones.
« Last Edit: October 15, 2020, 06:36:50 pm by coppice »
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #23 on: October 15, 2020, 06:34:29 pm »
True, but MEMS microphones are killing the market for electret microphones.
Is that true outside of severely space-constrained applications like cellphones and the mikes built into phone earpieces (be it inline in the cord or integrated like in AirPods)? I haven't gotten the impression that any other applications have embraced MEMS mikes.
That's 99% of all mics, isn't it?
In terms of sheer unit sales, perhaps. But other applications (like recording, stage and PA mikes, the ones in voice recorders, webcams, etc.) are hugely important and aren't going anywhere.
« Last Edit: October 15, 2020, 06:36:37 pm by tooki »
 

Offline tooki

  • Super Contributor
  • ***
  • Posts: 13156
  • Country: ch
Re: BJTs being phased out?
« Reply #24 on: October 15, 2020, 06:59:44 pm »
Second reply after you edited to add everything that follows the single sentence quoted above.


The electret mic market was mostly phones and cell phones.
Uhhhh… electret mikes were in practically every tape recorder. They're still widely used in webcams, laptops, and all manner of audio devices. They're still in every landline or VOIP phone and headset.

Phones have almost gone.
At home, yes, they're going that way. In businesses, not really. They may not be true circuit-switched landline phones any more, they're VOIP, but they still use handsets with electrets. Call center style headsets also use electrets.

Cellphones have moved to MEMS mics.
Yes, I think we established that! ;)

Most other mics are condenser or dynamic. There were once some very high quality electret mics, but they seem to have gone, and now we only have condenser, ribbon and dynamic mics for high quality applications. Condenser mics also use a JFET, but their volumes are millions, not the hundreds of millions we see with cellphones.
That is categorically untrue. Electrets are still omnipresent in pro audio such as broadcast TV. Because of the poor performance and longevity of early electrets, pro audio distanced itself from the word "electret" and just calls them condenser mikes, because electrets are a subtype of condenser mikes. (It simply means a condenser mike that is pre-polarized, as opposed to ones that need external polarization power.) As such, I think it's eminently possible that a significant percentage (perhaps even a majority?) of "condenser" mikes are actually electret condensers. (In pro audio, you'll see reference to "true condenser" to try and signify an externally polarized condenser mike, even though that phrase is technically imprecise since it attempts to mislead one into thinking that electrets aren't truly condenser mikes.)

For example, the Sennheiser MKE 2 is a $350 lavalier microphone. In a spec sheet, Sennheiser describes it as a "pre-polarized condensor [sic] microphone". And "pre-polarized" is literally the definition of an electret, so it means it's an electret mike in actuality.


Again, I don't dispute that the billions of cheap mikes in cellphones and cellphone accessories have largely moved to MEMS, but to claim that the electret is dead is, well, dead wrong.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf