Just a technical point. What you want is exactly NOT to have the mosfet in saturation mode.
When a MOSFETs changed is saturated, it is effectively operating in a constant current mode with a large voltage drop.
You want the mosfet to be operating in its triode region where the channel behaves like a resistor.
Technical use of the word "saturation" aside, it's clear what you want. I'm not saying that what you want doesn't seem clear
To get the lowest resistance across a MOSFET, you need to provide a suitably large voltage between source and gate, and limit the current so that the MOSFET doesn't saturate. If you have access to the datasheet, you want to ensure that the Vds vs. Id curve remains in the steeply increasing portion (where Vgs increases only slightly with increases in Id). In this part of the curve, the slope of the line is an indication or Rds, and you will note that Rds decreases (the curve gets steeper) with increasing Vgs. At some point the curves level off, with Id remaining almost constant for increasing Vds. This is the saturation region.
Some MOSFETs require a gate voltage (Vgs) up to 15V to get the Rds(on) specified in the datasheet.
If you need a high Vgs and you can't provide it, there are several solutions.
- You can increase your supply voltage. This may be inconvenient it impossible in some cases.
- You can boost the voltage you have. This may be complex.
- You can use a gate driver which creates its own boosted gate voltage. This can be expensive, and the parts may be hard to get or come in hard-to-use packages.
- You can choose a MOSFET with the same Rds(on) at lower Vgs. So-called "logic-level MOSFETs" are an example of a marketing label which is indicative of this feature. Beware that having a reduced Rds for a given Vgs may be accompanied with other changes in behaviour (such as reduced Vds(max) and/or Vgs(max), and increased device capacitance.
- Placing multiple MOSFETs in parallel. This works like resistors in parallel. Not that the gate capacitances will accumulate.
You state that you want to get the Rds to the lowest possible value. It is best to work from the other end and determine (from allowable voltage drop and/or dissipation limits) the maximum Rds you can tolerate, and then determine how to achieve it.