Typical ON resistance of a FET

Discussion in 'Homework Help' started by Froskoy, Feb 22, 2012.

  1. Froskoy

    Thread Starter New Member

    Jul 22, 2010

    I was just wondering what a typical value for the ON resistance of a FET was? I've calculated 333kΩ, which seemed very high - I wanted to check whether this sounds about right or not?

    With very many thanks,

  2. crutschow


    Mar 14, 2008
    It depends greatly on the model of the FET. High current MOSFETs have an ON resistance much less than a tenth of an ohm. Its value is stated on the data sheet for the MOSFET.

    How did you calculate 333kΩ? :confused:
  3. Vahe


    Mar 3, 2011
    I am guessing that you calculated the on resistance by division (Vds/Id). What is the mode of operation for the MOSFET while doing the resistance calculation (cutoff, ohmic (triode), saturation). In cutoff, Id=0 so the resistance would be infinite. In saturation, the MOSFET acts almost like a current source but will have some resistance in parallel with it -- although it might be high. In the ohmic (triode) region, the resistance should be less than the one computed in the saturation region.

    If this is a power mosfet, the on resistance should be pretty small (as already mentioned by Froskoy). The construction and structure of the power mosfets are fundamentally different than the low current ones used in ICs. The current flow in IC mosfets is lateral and power mosfets is vertical if I remember correctly.