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My lack of knowledge makes me see this as contradictory, but:

I've read that mosfets are better (especially for logic gates) over BJT, because they don't consume power, don't require so much voltage (or current?), react quicker and such.

However, I also read that if the voltage on the gate is below 10V, they will act like resistors and generate lots of heat. So it makes me feel that both gate and drain-source voltage must almost be equal, for a 12V battery ?

Finally, I also read that mosfet can have noise at the gate, so if it was for a CPU, wouldn't that be a huge problem ?

Thanks!

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    \$\begingroup\$ Probably what you have read about MOSFETs are comparison between a specific MOSFET and a specific junction transistor. There are thousands types of BJT and MOSFETs. Each are superior in term of spesific parameter compare to another. Generally the gate capacitance makes MOSFETs slower than BJT but modern MOSFETs are made with very small gate capacitance makes then super fast. Computer CPUs are now running at around 1V power supply. The fets in the cpu don't need 10V to fully turn on. \$\endgroup\$ – soosai steven Mar 3 '16 at 20:45
  • \$\begingroup\$ If you want to know when BJTs are better, ask BJT manufacturer Zetex: AN21: Bipolar transistor considerations \$\endgroup\$ – CL. Mar 4 '16 at 11:14
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Actually the main reason why almost all logic is made in CMOS (NMOSFETs and PMOSFETs) these days is that CMOS logic in chips (and almost all logic is in chips) scales down much (can be made smaller in size) much better than BJTs.

Have you heard of Moore's law ? Well, this "law" today almost exclusively applies to CMOS logic using MOSFETs. In the 1970s BJT logic could still scale down in size so then it did follow Moore's law. But it was soon caught up by CMOS which results in much smaller power consumption (especially when the logic is static, meaning not doing anything) and when CMOS became more advanced, higher speed also.

Today's smartphones would be impossible to make using BJT logic as the phone would be as large as a refrigerator and it would need a battery of equal size.

"below 10V, they will act like resistors and generate lots of heat"

That is complete nonsense. Forget that, you must have misunderstood that because it is untrue. Today's Intel CPUs use CMOS logic running on 1 Volt or less. These transistors are damaged by only 2 Volts at their gate.

Also behaving as a resistor does not imply that heat is generated. Only if current flows heat is generated. In CMOS logic the main source of power dissipation is caused by the charging and discharging of the gate capacitances.

"mosfet can have noise at the gate"

I also think you misunderstood this statement.

All devices generate noise including BJTs. For logic circuits, noise (I'm talking about flicker noise and thermal noise) is generally not a problem since the signals (even when using a 1 V power supply) are significantly larger than the noise. Noise is only an issue in analog designs, not in digital. Noise or better: disturbances on the power supply is an issue also in digital and that is why a lot of supply decoupling (using capacitors) is needed.

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  • \$\begingroup\$ To be fair the claim that MOSFETs act as resistors when Vgs is below 10 V is not complete nonsense. When comparing power transistors, the gate voltage required for fully turning on a MOSFET is a significant factor in component selection, although low Vgs "logic level" power MOSFETs have become common in recent years (you can buy ones with a treshold voltage lower than 1 V). OP just assumed that the same must apply to CMOS as well after reading some random stuff on the internet, when in reality it doesn't due to the different fabrication processes invoved. \$\endgroup\$ – jms Mar 3 '16 at 21:37
  • \$\begingroup\$ @jms It's not only the Vgs that determines if a MOSFET behaves as a resistor (triode mode). It's Vdssat which is Vgs - Vt. Meaning Vgs must be large, Vds must be small. As Vds > Vdssat the mosfet behaves as a current source (saturation mode). Also you will find in the datasheet of most MOSFETs for switching purposes that Vgs needs to be above (not below) a certain value for a low Rds on, meaning the MOSFET is in triode mode and behaving as a resistor. \$\endgroup\$ – Bimpelrekkie Mar 3 '16 at 21:53
  • \$\begingroup\$ I just wanted to explain to OP how he made the wrong assumption about the treshold voltage since you didn't. I agree with your answer and I do realize that I should have written "...although low Vgs(th) "logic level" power..." instead of "...although low Vgs "logic level" power...", but I don't understand why you started explaining stuff since I never implied otherwise. \$\endgroup\$ – jms Mar 3 '16 at 22:11
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MOSFETs consume no static power to keep them on or off. A BJT would need a base current to remain turned on.

In a digital circuit there is no static current present (except leakage current), therefore the transistors only need power for switching. The power depends on the capacitance of the gate the voltage (squared) and the switching frequency. They are very effective. Due to the large number of transistor in a CPU power consumption can be very high.

In digital design noise of transistors is of no concern, it is negligible compared to the signal amplitude.

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