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Effective mobility in MOSFET

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bonzer:

--- Quote from: Wimberleytech on July 21, 2019, 10:05:43 pm ---
This plot does not apply to the operation of an MOS transistor--don't use it for that.  This plot can be used to determine the conductivity of doped silicon (e.g., a p+ doped resistor).  Mobility of the channel of an MOS transistor is the mobility of the "inverted" silicon.  It is much lower.  Typical mobilities for Nch and Pch long-channel transistors modeled with the Level-1 model are 600 and 300 respectively (cms/V-s).

--- End quote ---
Thanks for your response. You got me, my doubt is right here. Yes, I understand you at this point, I've already seen that at the end it's lower.
I've never heard about this concept of inverted silicon. But what I know is that I've seen somewhere that they used as approximation in this case for the mosfet mobility, the mobility from the graph in case it's nMOS: "mu_n" this way: mu_mos = 0.5*mu_n(NA). So in an nMOS for istance it can be approximated to half of the mobility from the graph of the electrons inside phosphorus doped silicon at the concentration NA of the boron that we have in the bulk. (assuming that there's only boron doping in the silicon and no n-pre-doping).

But what's the origin of this stuff? It kinda surprised me. Have you ever heard about this approximation? If yes, is it always valid?

Wimberleytech:

--- Quote ---I've never heard about this concept of inverted silicon.

--- End quote ---
Sorry, sloppy language on my part.  I should have said "inversion layer" which is the conductive region below the gate that forms once the the gate-source voltage is greater than the threshold voltage (even this is an approximation).

Wimberleytech:

--- Quote ---
But what's the origin of this stuff? It kinda surprised me. Have you ever heard about this approximation? If yes, is it always valid?

--- End quote ---
No, I have never heard of this approximation and would be interested to see your reference where it is cited.

The reason mobility is different in the inversion layer versus bulk silicon is similar to the difference in your mobility driving the autobahn vs. a narrow dirt road.  The inversion layer is very thin whereas bulk silicon is the wide open spaces!!

bonzer:
Oh thanks for the explanation, now I got it! Unfortunately I have no reference because this was one of some exercises that a (italian) university professor solved.

Wimberleytech:

--- Quote from: bonzer on July 24, 2019, 05:37:33 pm ---Oh thanks for the explanation, now I got it! Unfortunately I have no reference because this was one of some exercises that a (italian) university professor solved.

--- End quote ---
You are welcome...happy to answer questions about this topic!!

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