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I've been reading various answers about the switching speed and frequency of BJTs and MOSFETs, yet I keep finding conflicting answers. Can anyone explain the discrepancy and the real answer? Sources I'm referencing:

  • "MOSFETs have faster switching speeds and lower switching losses than BJTs" Circuit Digest Article
  • "BJT can switch faster than MOSFET due to the less capacitance at the control pin" OscarLiang Website
  • "MOSFETs are slower than BJTs because of enormous capacitances at its junctions" StackExchange Question
  • "The switching speed of the BJT is slower" El-Pro-Cus article
  • "BJT is much slower device than a MOS. ... In BJTs we get a small bandwidth with better gain but in FETs we get a larger bandwidth when compared to BJT but gain is not better when compared to BJT." Quora Question
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    \$\begingroup\$ Yep it's conflicted because few talk about the actual type of switching that is going on. My understanding is that the BJT is faster if you DON'T switch the transistor hard enough to saturate, whereas the MOSFET is faster if you DO switch the transistor hard enough to saturate. Since, technically, "saturation" means modes for BJT and MOSFET, I will clarify here that when I say that mean when increasing Vce or Vds provides no further increase in current. \$\endgroup\$
    – DKNguyen
    Aug 25, 2021 at 19:44
  • \$\begingroup\$ And your last bullet is different from all the rest because now it's talking about analog bandwidth where the transistor is NOT switching but instead acting as an amplifier. Keep the two distinct in your mind. \$\endgroup\$
    – DKNguyen
    Aug 25, 2021 at 19:47
  • \$\begingroup\$ 1st link: > MOSFETs have easier to design gate driver circuits than the BJT’s base > driver circuit. I don't think so. Search for gate driver boards and you'll find that they are way more expensive than the transistor itself. \$\endgroup\$ Aug 25, 2021 at 20:13
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    \$\begingroup\$ are cars faster than trucks? depends on each one. \$\endgroup\$
    – dandavis
    Aug 25, 2021 at 21:37
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    \$\begingroup\$ To me, BJT is faster. I remember a paper focusing on a similar question. IIRC, the author compared the same-size BJTs and MOSFETs in terms of carrier speed, mobility, etc. taking the geometric and material properties into account. And he concluded that the MOSFET is slower. I'll put a link to the paper if I find it again. \$\endgroup\$ Aug 26, 2021 at 5:24

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When you compare the two you have to be fair about it: both devices need to be of similar size. When you do that you will find that the intrinsic speed difference, if any, is relatively small; while the system switching speed is dominated by other factors including wire capacitance.

That said, MOSFET logic speed eclipsed bipolar speed in the 1980s. That's more a reflection of the investment made in MOS compared to bipolar, not of the inherent speed of each.

Related: Why were bipolar junction transistors (BJTs) better than metal oxide semiconductor field effect transistors (MOSFETs) in the past?

For ultrahigh frequencies, HEMT (High Electron Mobility Transistor, a type of FET) beats both MOS and bipolar, as an amplifier.

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    \$\begingroup\$ To my knowledge very high frequency (>100 GHz) amplifiers often use bipolar transistors of BiCMOS technologies. \$\endgroup\$ Aug 26, 2021 at 4:45

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