Phil, What makes a JFET less noisy than a enhancement mode CMOS device?
Would a depletion mode CMOS device be quieter? ...Jim Thompson
Phil, What makes a JFET less noisy than a enhancement mode CMOS device?
Would a depletion mode CMOS device be quieter? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
At some point I remember reading (AoE?) that there was more 1/f noise in MosFet's... more crud in the gate or something like that. It seems like they are getting better and better at that... Nice Cmos opamps these days. But still not as high a voltage as Jfet's... I don't know why that is.
George H.
[snip]
I don't need high voltage, my supplies are +3.3V/-2V ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
The gate of a MOSFET is at the surface (and so is the channel); for a JFET, your gate and channel are under the silicon surface. Dirt on the surface is a major concern (and, of course, not repeatable nor easily specified).
I don't know in any great detail. The physics of the carriers in the channel will be vaguely similar except that there's no chemical potential to drive the formation of a depletion region there is with a JFET or BJT. I suspect that there are a lot more traps and surface states when you have all those layers that the E field has to go through, compared with a nice buried epitaxial junction.
That's backed up by the horrible 1/f noise of MOSFETs, but of course "traps and surface states" is the solid state guy's version of "market sentiment and program trading" on the Friday stock market report. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Jim, you really should get yourself a copy of AoE III and read chapter 8. There we have loads of details about what other IC op-amp designers have been able to accomplish with their fabs. It's 120 pages of good low-noise stuff. You can compare JFET and CMOS, including 1/f noise breakpoint frequencies, and you can quickly compare low- and high-voltage processes.
-- Thanks, - Win
I have a copy... but I've just skimmed it... too busy with real work... finally!!
Super!! I'll read Chapter 8. THANKS! ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
You can skip to Table 8.3, page 522 or thereabouts.
-- Thanks, - Win
Do you address which companies provide foundry services? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
It would be sort of interesting to look at the noise behaviour of lateral MOSFETs with their back gates pinned out separately. I bet the
1/f noise would be a lot less if you ran the back gate at the edge of forward bias. That would force all the channel conduction to happen well away from the Si-SiO2 interface.You can't get those parts any more, of course.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Joerg recently pointed out a device that is still available (SST211).
Interesting, thanks.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.