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My Gigabyte GA-MA790XT-UD4P motherboard is marketed with a Lower RDS(on) MOSFET. I'm curious what this actually means.

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up vote 5 down vote accepted

Rds (the ds should be subscript) means "Resistance(drain-to-source)", the drain being where the current goes to and the source being where it is coming from.

MOSFET are generally used as a better alternative to power transistors and are used for high current switching applications.

Having a lower Rds basically means that less power is going to be lost across the MOSFET as per ohms law and by saying that their MOSFET are low-Rds they are basically saying that their boards are more power efficient and will thus produce slightly less heat as a by-product of the MOSFET.

Rds(on) is basically just saying that the Rds is low when the MOSFET is in the "on" state. In the Off state the MOSFET will not be conducting so you don't care about the resistance.

A little detail...

In its simplest use a MOSFET (or Metal Oxide Semiconductor Field Effect Transistor) is used as a direct replacement for power transistors and relay switches. The symbol for the MOSFET is somewhat similar to the transistor but has a gap to illustrate the fact that there is no direct connection between the gate and the other parts of the transistor, hence field effect transistor.

Mosfet: MOSFET Transistor: Transistor

Due to the fact that the gate is effectively isolated from the current path (source to drain) of the device this makes it much more useful for higher currents as there is much less leakage through to the gate, thus increasing the power efficiency of the device considerably.

Per wikipedia:

A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs. Bipolar transistor-based logic (such as TTL) does not have such a high fanout capacity.

Due to the construction of the MOSFET there is still a resistance between the source and drain, even when fully activated (saturated) and this resistance causes some of the current being passed through the device to be wasted and thus produce heat. Reducing this resistance reduces the amount of wasted power and so also reduces the amount of heat generated by the MOSFET.

Compared to a transistor a MOSFET is highly efficient, a low-Rds MOSFET is enhancing that power efficiency.

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Do note that the correct term for the non-field-effect transistors mentioned in this answer is bipolar junction transistor. Both are transistors (as mentioned in the Wikipedia quote). – AndrejaKo Dec 11 '11 at 17:00

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