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2 types of Vth for MOSFET?

  1. May 11, 2012 #1
    So apparently there are two types of Vth for a MOSFET

    How can I tell the saturation apart from linear?

    Right now I am simulating a voltage sweep (on the Gate) of a transistor and the sim results give a saturation and a linear Vth. Now from what I understand saturation depends on whether the drain is higher than the gate by the threshold voltage. So how am I suppose to know if my drain voltage of -0.1V is higher than the Gate voltage by the threshold if my gate voltage is changing?

    And my most general question...does Vth change with Gate voltage?

    edit: Source is at 0V, and this is a pchannel mosfet
  2. jcsd
  3. May 11, 2012 #2
    I want to re-frame your question correctly,
    How can I tell the saturation apart from linear, without prior knowledge of the Vth?

    Well in simulation, Id-Vg in linear region will always be linear and in saturation it will be squared. So that's easy to tell.

    In experiment, you may not know what's linear and what's saturation as you wont get perfect linear curve in linear region avoiding measurement errors. Here you have a guess of Vth and determine linear and saturation. Again take the result Vth as new guess and do the experiment. After two iterations, I think you will know the regions correctly.

    Simple answer is no. Vth is defined as a constant parameter in simple models. However, in higher models in may be dependent of Vg. Also note that the part below Vth in Id-Vg graph will be exponential.
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