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I read that MOSFET are more power efficient than BJT and that they are capable to handle higher current. In this case, why using a BJT ?

I see most circuit use BJT to turn on / off some LED with a MCU GPIO for example. But why not simply use MOSFET instead ?

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  • \$\begingroup\$ Have a look at the safe operating area of both. Likewise look at the conduction losses (tip but wins) \$\endgroup\$ – JonRB Sep 4 '16 at 17:41
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    \$\begingroup\$ Don't let a 50 watt FET fool you into thinking you can dissipate 50 watts in it like you can a 50 watt BJT. You can if it's saturated, and blasts from on to off in microseconds, you can't if you try to use it in a linear mode, like the pass element for a linear regulator, controlled load, or audio amplifier output device. The Safe Operating Area has timing constraints, the array of cells in the FET unbalances thermally if dissipating for too long. \$\endgroup\$ – Neil_UK Sep 4 '16 at 19:17
  • \$\begingroup\$ Possible duplicate of When is a MOSFET more appropriate as a switch than a BJT? \$\endgroup\$ – Cole Johnson Oct 27 at 1:29
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I would not make the generalization that MOSFETs can handle higher current and are more power efficient. It very much depends on the application.

BJTs can be cheaper than FETs. This is especially true for high voltage switching where the much larger die area of FETs make them much more expensive. That's why you see IGBTs used for higher voltage motor controls. IGBTs still can't compete with FETs for switching speed (the minority carrier issue) but often 20kHz operation is fast enough.

BJTs also have some differences from FETs than can make them more convenient in some applications. BJTs start to turn on around 0.7V, where FETs mostly have much higher threshold voltages. For low supply voltages that can be useful.

For high frequency power conversion with small magnetics the BJT can't touch the FET and FETs are now used almost exclusively in these applications.

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There are about 8k active stocked unique part numbers in each category of BJTs & FETs. BJTs won't be going obsolete.

BJT with similar characteristics for switching current are 1/3 the price. This is important for mass production.

But FETs with similar price have better characteristics if you only consider power gain for switched output vs input.

So it depends on design requirements for cost and performance.

When you get into HV and high currents, you may consider the best characteristics of both devices with FET input and Bipolar output or IGBTs.

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