Power Semiconductors: What is On-Resistance?


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Contributing author: Stephen Russell

A common question our Power Semiconductor experts encounter is: What is on-resistance or RDS(ON)? Stated simply, on-resistance is the total resistance between the drain and source terminals of a FET.

We also frequently see the follow-up questions:

  • Why is RDS(ON) so important in a power semiconductor device context?
  • How can we use RDS(ON) as a metric to compare different products?

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Stephen Russell, Subject Matter Expert - Power Devices, explains RDS(ON) in this 8-minute YouTube video.

He discusses the constant tug-of-war that takes place between three technical factors of power devices:

  • The On-state
  • Switching performance
  • Robustness and reliability

Improving one of the above factors in a power device tends to compromise at least one of the other two, so how do power semiconductor designers navigate this tug-of-war effectively? Not to mention considerations of cost...

RDS(ON) is not unique to power transistors, however it is extremely pertinent to them. Power devices are analogue not digital, they deal with the flow of electrical energy itself rather than signals. So, due to the extreme levels of power involved even fractions of a percentage loss in efficiency can be significant.

As an example of how different device structures can have a radical impact on this figure, Stephen compares some analysis we performed on a couple of GeneSiC silicon carbide MOSFETs - their 3rd generation 750 V device and also their recently released 3.3 kV device. The drift region differs by greater than a factor of 5 which changes the overall value of RDS(ON) and the region of the device with the most significant contribution (shifting from the MOSFET channel in the 750 V case to the drift region in the 3.3 kV case.)

Finally, Stephen discusses a standardized metric by which different power devices from different vendors can be compared the specific on-resistance or RDS(ON)*A.


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