For example this MOSFET in the first page mentioned that it is a logic MOSFET.

But is this transistor a logic MOSFET as well? It says VGS(th) is between 2V and 4V. In some forums they claim this is not a logic MOSFET. Im kind of confused what parameter indicates being logic MOSFET.


I simulated the following circuit swept the gate voltage from zero to 10V:

enter image description here

And I got the following plots for Vg(blue plot) and the power(green plot):

enter image description here

And here is Ids versus Vg:enter image description here

Doesn't the power plot indicate as long as the Vgs is over 4V we can use this as a switch?


VGSth is the threshold voltage where the MOSFET only just barely turns on.
The datasheet for that IRF3205 tells you that VGSth is likely to give you a ID drain current through the MOSFET of only 250uA.
If you want to use a MOSFET as a 'logic-level' switching device, you need to know that the voltage level applied to the gate by your logic will be able to turn the MOSFET on properly and not just barely.
Probably the best way to do this is to refer to the MOSFET datasheet and look for the
ID vs VGS graph, like these:
enter image description here enter image description here
The one on the left is the IRF3205, and you can see that it's not particularly good with low VGS values. I wouldn't use it as a switch with anything less than 7V available to drive the gate (probably even more).
The graph on the right is for the IRL3803, and you can see that its VGS requirement is significantly lower.

  • \$\begingroup\$ To some point I understand what you explained very well. But Im lost at this point how you interpret: " I wouldn't use it as a switch with anything less than 7V available to drive the gate" What made you to conclude that? I mean at 7V there is a corresponding ID(drain-to-source current) drain in that graph. How does it tell you whether the MOSFET is ON properly? \$\endgroup\$ – user16307 Jun 13 '17 at 17:02
  • \$\begingroup\$ Those graphs show typical values; you should look for guaranteed RDS(on) values for the desired VGS. \$\endgroup\$ – CL. Jun 13 '17 at 17:02
  • \$\begingroup\$ Please see my edit. \$\endgroup\$ – user16307 Jun 13 '17 at 17:26

As you guessed VGS(th) needs to be low enough to be within the Voh level of whatever Logic family and power supply(Vcc) you are using. 4V would be insufficient for 3.3V devices to drive, TTL @5V you might get away with assuming your load current is small.

  • \$\begingroup\$ So you do not call it logic MOSFET or do you? And 3.3V is between 2V and 4V. You mentioned "4V would be insufficient for 3.3V devices" but what about comparing to the lower limit of the threshold which is 2V? 3.3V is greater than 2V. Should we always exceed or match the upper limit of the VGS(th) to fully turn on the transistor and ignore the lower limit? Hope I made my question/confusion more clear now. \$\endgroup\$ – user16307 Jun 13 '17 at 16:53
  • \$\begingroup\$ @dicksonchargepump in order for it to be guaranteed to work your Voh min must be greater than VGS(th) max. The "Logic" description is actually fairly arbitrary. \$\endgroup\$ – Trevor_G Jun 13 '17 at 17:00
  • \$\begingroup\$ Seriously a down-vote with no comment.. tsk tsk. \$\endgroup\$ – Trevor_G Jun 13 '17 at 17:01
  • \$\begingroup\$ @dicksonchargepump The MOSfets you're looking at are monsters - with small fraction of an ohm "ON" resistance. At what point would you say the MOSfet is ON for your application? Take a look at the gate voltage required to get to this point. Trevor is suggesting that this voltage may be higher than your logic control can provide. You'd call such a MOSfet "not-a-logic" MOSfet. \$\endgroup\$ – glen_geek Jun 13 '17 at 17:22
  • \$\begingroup\$ please see my edit \$\endgroup\$ – user16307 Jun 13 '17 at 17:26

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