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In MOSFET datasheets they often lists parameters like VGS (ON) , Id (ON). Is VGS (ON) is the testing gate- source voltage that is being used (of course above threshold voltage) or it's a particular constant gate-source voltage which turns the MOSFET FULLY TURN ON? similarly what is this on state drain current?

As far as I know the degree of "on-ness" depends on the gate voltage, so what does it mean to have a "on" state drain current Id on? It is meaningless because the degree of "on-ness" depends on the user hence, different degree means different drain current. So, is on state drain current is the saturation drain current for a testing gate voltage?

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    \$\begingroup\$ \$RD_{on}\$ is an attempt to simplify MOSfets for logic-like on/off application. You're wanting to look more closely at on-ness, so go to the MOSfet's detailed data sheet. \$\endgroup\$
    – glen_geek
    Commented Mar 13, 2021 at 16:14
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    \$\begingroup\$ I've never seen the term V_GS(ON) used in any MOSFET data sheet. Ditto V_DS(ON) and I_d(ON) so, what you should perhaps do is link a data sheet that does list these things even if it's just for fixing my incompetence in these matters. \$\endgroup\$
    – Andy aka
    Commented Mar 13, 2021 at 16:14
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    \$\begingroup\$ You've also stated \$V_{GS}(ON)\$ twice so I assumed you meant one of those to be \$V_{DS}(ON)\$ which would be preferably written as \$V_{DS(ON)}\$ if it has any actual meaning (sorry for not knowing it). \$\endgroup\$
    – Andy aka
    Commented Mar 13, 2021 at 16:17
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    \$\begingroup\$ @Andy aka I might be wrong with the term V_GS(ON) pardon for that. Could you please elaborate other two parameters? \$\endgroup\$
    – Sayan
    Commented Mar 13, 2021 at 16:18
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    \$\begingroup\$ particular constant gate-source voltage which turns the MOSFET FULLY TURN ON? No, in theory, as you increase \$V_{GS}\$, the resistance (\$R_{DS,on}\$) will decrease. The limit is the maximum \$V_{GS}\$, if you go above that, the gate oxide is damaged. So in practice you need to apply as much \$V_{GS}\$ as is feasible (maximum your circuit can deliver but not exceeding \$V_{GS,max}\$) and that gives you a certain \$R_{DS,on}\$. \$\endgroup\$ Commented Mar 13, 2021 at 16:19

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@Andy aka see here: mhtml.alldatasheet.com/html-pdf2/50067/FAIRCHILD/2N7000/405/1/… , see on-state drain current and on state drain-source voltage

The only occurrence of the term \$I_{D(ON)}\$ in the data sheet for the Fairchild 2N7000 is here: -

enter image description here

And the reason why most MOSFET data sheets don't list this value is because it can be easily extracted from the MOSFET forward characteristic seen here (with added red and blue lines): -

enter image description here

For \$V_{DS(ON)}\$ the only occurrence is here: -

enter image description here

And, if you look at the same MOSFET forward characteristic (as above) you'll see that ties in with the purple markings I've added to it below: -

enter image description here

As far as I know the degree of "on-ness" depends on the gate voltage, so what does it mean to have a "on" state drain current Id on?

Correct and that is why most data sheets place emphasis on the forward characteristic curve because those (extremely) rarely mentioned parameters can be found in that graph. The forward characteristic graph tells you many many things and, you can easily interpolate as I have done.

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  • \$\begingroup\$ to be specific just can you explain this alone : what is Id (ON)) ? As mentioned in the data sheet , here is my assumption : they are choosing a particular TEST GS voltage and a particular TEST DS voltage, and now measuring the drain current under this condition and lists this as Id (ON) , is this true? \$\endgroup\$
    – Sayan
    Commented Mar 14, 2021 at 11:39
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    \$\begingroup\$ \$I_{D(ON}\$ is measured (in the data sheet) at a gate source voltage of 4.5 volts. 4.5 volts is chosen because it represents a decent lower limit value of a standard 5 volt digital signal. So, with 4.5 volts applied and sufficient drain source voltage, you can typically expect the drain current to be 600 mA @Sayan. So, yes, it is true but, remember that the graph and value quoted are typical values. In the table, the drain current might be as low as 75 mA in extremes. \$\endgroup\$
    – Andy aka
    Commented Mar 14, 2021 at 11:46
  • \$\begingroup\$ what do you mean by "sufficient"? Do you mean the voltage which can take the mosfet in saturation region corresponding to GS=4. 5 V? \$\endgroup\$
    – Sayan
    Commented Mar 14, 2021 at 11:54
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    \$\begingroup\$ @Sayan yes, sufficient drain source voltage needs to be applied or you will be in the linear region where the current isn't constant. So, \$V_{DS}\$ at 10 volts ensures the MOSFET is saturated. \$\endgroup\$
    – Andy aka
    Commented Mar 14, 2021 at 11:56
  • \$\begingroup\$ sorry for my late reply, I think now it's clear. Answer is accepted. \$\endgroup\$
    – Sayan
    Commented Mar 15, 2021 at 15:23
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The on-ness of a MOSFET depends on the Gate-Source voltage, the threshold voltage and the Drain-Source voltage. It is meaningful because if you have resistors connected to the circuit, there will be a voltage drop on each resistor which will depend on ID and the Source voltage may change according to that changing the on-ness.

Because in the comments there is a lot of confusion, let's go through the basics: A MOSFET can work as a voltage-controlled resistor when Vgs-Vth>Vds and we don't have a source resistor. If we have a source resistor, the resistance still changes with Vgs but not as much. When Vds>Vgs-Vth a MOSFET is a current source which depends on Vgs.

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