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I am using a MMBFJ270 P-Channel JFET and if you look at the characteristic graphs it is saying that there are two values for VGS(off) and in one graph it quotes three different values... Why does it do this and what determines the value for the VGS(off)?

A couple of the graphs are below so you don't have to trawl through the datasheet.

enter image description here
enter image description here

In the datasheet it says in the characteristics table as well that the VGS(off) is 2.0V... I can only assume it depends on the IDS or VDS but I really have no idea...

Can anyone give me some pointers as to what value VGS(off) this FET will actually be or how I can work it out?

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  • \$\begingroup\$ Kinda weird... maybe in the past they made a number of Jfet's with different thresholds and the data sheet is a hold over from then. \$\endgroup\$
    – George Herold
    Commented Mar 30, 2015 at 17:01

1 Answer 1

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That data sheet is a wrong IMHO but first, a bit of background. The device you are wanting to know about is a P channel JFET and these types of transistor conduct drain current when there is zero gate-source voltage. To turn the device off you raise the gate-source voltage positively. For an N channel JFET you would raise the gate-source voltage negatively.

So let's look at the first two graphs in the OP's question: -

enter image description here

They shows typical characteristics for one device that has a VGS(off) voltage of 4.5 volts and the other characteristic is for a device with a VGS(off) voltage of 2 volts. It makes a mistake (IMHO) in that the 4.5 volt device appears to be listed as having a VGS(off) voltage of -4.5 volts - if it were it would be an N channel JFET and not a P channel JFET. So, that is there first mistake I believe.

It also looks like this device has exactly the same characteristic as the J177 P channel mosfet - the graphs are identical as far as I can tell and I suspect that Fairchild may have dropped a major clanger: -

enter image description here

So back to the front page and the specification in the table is incomplete in my opinion. Using the J177 as an example, it lists a range of JFETs that are basically the same device but, due to manufacturing process variations have different VGS(off) voltages: -

  • The J175 has a range of 3V to 6V for VGS(off)
  • The J176 has a range of 1V to 4V for VGS(off)
  • The J177 has a range of 0.8V to 2.5V for VGS(off)

In all other specifications they are the same device.

So here's the rub - the MMBFJ271 exists and has a VGS(off) value ranging from 1.5 volts to 4.5 volts.

Shame on Fairchild!

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  • \$\begingroup\$ Good answer! Especially "Fairchild may have dropped a major clanger" :) \$\endgroup\$
    – bitsmack
    Commented Mar 30, 2015 at 18:15
  • \$\begingroup\$ So (excuse the noob like question) is VGS(off) a fixed value? Like it won't change depending on other variables like currents or VDS? \$\endgroup\$
    – MrPhooky
    Commented Mar 30, 2015 at 21:09
  • \$\begingroup\$ @MrPhooky yes, VGS(off) is fixed for any device and in the two graphs in my answer they demonstrate what a 4.5 volt device does and a 2.0 volt device does when the gate-source voltage is altered. The graph on the left is for a negative drain current and the graph on the right for a positive drain current. \$\endgroup\$
    – Andy aka
    Commented Mar 30, 2015 at 21:13
  • \$\begingroup\$ It's possible both devices are made using the same process therefore the same graph could be relevant to both, with the J175 to 177 parts being binned according to the actual Vgs(off) at test, and the MMBFJ271 being binned differently for whatever reason. \$\endgroup\$
    – user16324
    Commented Mar 30, 2015 at 21:46
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    \$\begingroup\$ It probably is 2 volt max so ignore the 4.5 volt curves OR contact a supplier ecause somewhere down the line there are data sheet errors. In all my many years I've never seen a data sheet from Fairchild that was this contrary lol. \$\endgroup\$
    – Andy aka
    Commented Mar 31, 2015 at 8:20

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