Is there a reason you're trying to find the exact voltage that puts it into saturation mode, instead of just driving it at the 10V as specified on the datasheet?
I am not, I am trying to understand why 10V. Though finding the exact point would probably help with my understanding of why.
I think this is more of a question about motors, as I know how MOSFETs work and their regions of operation, etc. Imagine Mike's circuit and replace the resistor with a motor. What happens?
The motor looks like a small resistance if I am correct. Thus, when in cutoff, there is ~0 current in the resistor. The transistor sees a VDS of 30V. Thus you need a fairly sizeable VGS to get it to saturation mode. If more current is required and you want to keep power dissipation the same, you need a higher VGS to get more carriers to the channel so they can conduct.
If the above paragraph is correct then my question is answered.
Also, this should be correct:
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For BJTs:
Amplification: Forward-Active mode desired
Switching (digital, power): bjt Saturation mode desired
For MOSFETs:
Amplification: mosfet Saturation mode desired
Switching (digital, power): mosfet Saturation mode desired, minimizing the time the transistor spends in the ohmic region.
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EDIT: I also meant to put this entire thread in general electronics chat rather than projects. I am not working with that specific mosfet, though it seemed like a good example one.