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I've noticed that in almost every JFET datasheet I've looked at, the values for Idss and Vto (also known as Vgs-off vary hugely.

A J111, for example, could have a Vto of anywhere from −3 to −10 volts at Vds = 5 V and Id = 1 A, while the J112's Vto ranges from −1 V to −5 V and the J113 from −0.5 V to −3 V. Some JFETs also have a 2:1 variation in possible Idss values.

Why is this? Why exactly are the tolerances for Vto and Idss so poor?

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  • \$\begingroup\$ Perhaps unwillingness to pay the asked price for laser trimmed discretes may be a factor. On IC fabrication of high-valued JFET input devices (some from Burr Brown come to mind), laser trimming may be worth the trouble. And you are right. The specs for discretes, those few that are still in the market at all, are order-of-magnitude variation and horrible. \$\endgroup\$
    – jonk
    Dec 14, 2022 at 22:41
  • \$\begingroup\$ I've watched what seems like a self-perpetuated spiral from a wider supply towards almost no JFETs anymore. Same has been true for BJTs, as well. For example, small-geometry NPN RF transistors with linear \$\beta\$ and a high \$f_T\$ are pretty much gone. The once BFT25A had \$f_T\approx 5 \:\text{GHz}\$ at \$I_C=1\:\text{mA}\$ and a very linear \$\beta=100\$, which it kept at \$\beta\ge50\$ all the way down into low \$\text{nA}\$. \$C_\text{cb}\approx 300 \:\text{fF}\$, \$V_A\approx 50\$ and cost 30 cents. No longer. \$\endgroup\$
    – jonk
    Dec 14, 2022 at 23:29
  • \$\begingroup\$ @jonk: even 25 years ago I was working with some highly qualified analog circuit designers that were flat out allergic to discrete transistors of any stripe other than power MOSFETs. I don't think the effect is entirely on the supply side of the equation. \$\endgroup\$
    – TimWescott
    Dec 15, 2022 at 0:22
  • \$\begingroup\$ @TimWescott Discretes had their moment. Tektronix built its entire STS line on that moment. Times change. I've witnessed change. That's all. No harm in making an observation. \$\endgroup\$
    – jonk
    Dec 15, 2022 at 2:07

1 Answer 1

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A couple things:

  1. Traditional JFETs are made by diffusion. This is a thermally driven process, and the amount of dopant (concentration and spread) varies strongly with time and temperature.
  2. The channel is very thin (some µm), so it's very sensitive to the amount of diffusion. Cook it just a little too long and you have a pair of diodes instead of a JFET; not long enough and you have a resistor (or maybe a UJT). So, even an extremely finely tuned process can end up with its variations greatly magnified for a transistor like this.
  3. There are other processes possible, like, I see a few hits for epitaxial JFETs. It appears they can make them with a mixture of processes (epitaxy, diffusion, ion implantation), allowing more precise formation of the gate, and the channel within.
  4. If you confine your search to bad* old parts, of course you're going to see bad variances! Take the famous(ly old?) 2N3819 for example: -0.5 to -7.5V pinchoff, IDSS 2 to 20mA! Most families at least had the courtesy to divide them into three or so offerings. Contrast with, say, the classic (now obsolete) BF862, -0.3 to -1.2V pinchoff and 10-25mA IDSS. CPH3910 is more or less the modern replacement, offering -0.6 to -1.8V pinchoff and 20-40mA IDSS.
  5. Which compares alright with MOSFETs: a typical logic-level switching transistor might have 0.8 to 2V VGS(th) for example. Which is partly limited by diffusion (carrier density in the channel), and by gate oxide thickness (which also varies yfs for obvious(?) reasons).

*Just in the sense of this question, I mean. Also for sake of note, Jxxx series parts are all, I think, pretty old, contemporary with 2Nxxxx parts?

Disclaimer: I'm no transistor/IC expert; take this as a jumping-off point for further research. I would expect contemporary (1960s, 70s?) books on semiconductor design and manufacture, to hold many answers? These aren't particularly easy to search for online, but perhaps a physical library can help out.

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