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weird fet

Discussion in 'Electronic Design' started by John Larkin, Jan 28, 2004.

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  1. John Larkin

    John Larkin Guest

    We were experimenting with a Fujitsu PHEMT, a pseudomorphic
    heterojunction gaas-type fet. They are poorly characterized for DC
    specs (typical microwave part!) so we were making pulsed measurements
    to get the DC transfer curves.

    They are normally considered to be depletion parts, but it turns out
    they enhance nicely, to about 1.4 * Idss with maybe 0.4 volt positive
    gate voltage. But the weird part is this: at around +0.5 on the gate,
    the drain current goes *way* up, apparently exponentially on gate

    So, can a PHEMT go into bipolar transistor mode when you forward bias
    the gate? Anybody know anything about this?

  2. Jeff Stout

    Jeff Stout Guest

    Can you be a little more specific about the part number?

    Jeff Stout
  3. gwhite

    gwhite Guest

    Interesting. But square law is still exponential, so I don't know that much
    changed aside from bleeding a bit of gate current off. Why wouldn't it still be
    a channel device with square law response? (I mean at very small increases from
    the .4 V you call out.)

    I've had overdriven MESFET power circuits that had ended up with what I called
    gate-leak bias since it was similar to the tube era grid-leak bias. The DC
    power would go down with increased drive. Of course that was due to the whole
    circuit since the gate bias circuit (in that case) could not source current to
    cancel the leak bias.

    I don't know. Pengelly includes gate voltages up to +0.6 in his MESFET
    ruminations. I haven't looked close, and am in over my head on this one, but on
    a quick persuse it seems like he thinks that a slight positive is within the
    reasonable swing for large signal operation.
  4. John Larkin

    John Larkin Guest

    It's a Fujitsu FSU02LG, really a nice part. The pulsed transfer curve
    is almost a straight line above 20 mA, and the curves are amazingly
    consistant from part to part. I can post it if anybody cares.

    If I get time, I'm going to deliberately bang the gate into forward
    conduction with a really fast pulse and see what the drain does.
    Funny, nearly all the texts and databooks about PHEMTS stop at Vb = 0,
    and ignore this very interesting positive-gate region.

    Most of these kind of parts are specified for small-signal RF use,
    where you sort of pour some RF into the gate and expect more to come
    out of the drain. You're lucky if the datasheets tell you how to bias
    them to do even that.

  5. Ken Smith

    Ken Smith Guest

    This also happens with other depletion mode parts. A common JFET will
    show a higher drain current if the gain is raise by a few tenths of a
    Something very like this also happens on a JFET. If you forward biad the
    gate junction, the drain current increases and the gate shows a neg.
    resistance character. It works like a unijunction.
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