JFET mysteries - Page 1
Forum section: electronics / beginners

author: hirada
author_rank: Regular Contributor
timestamp: December 28, 2018, 01:02:32 pm
content: Hello all,So I've thought to finally having found the holy grail for high side switching in symetrical JFETs, simply connect the source to V+ and when the Gate is high, VGs=0 and it conducts, when the gate is low, Vg<Vs, so it blocks, and therefore I went shopping and done some testing during the last days.Of course, it was an utter failure. Now I am not sure wether I have misunderstood the interchangeability of Drain and Source mentioned in the specs or wether I just may have killed both JFETs while soldering them to the SOT23->through hole adapter. I am not the worlds most skillfull SMD solderer.Anyway, the big disappointment was, that they completely behaved identical, no matter wether I connected Source or Drain to V+.So no matter which way I've connected them, they always behaved, as if the reference voltage for the gate was the voltage on the terminal closer to ground and not fixed to the voltage on the source terminal, as suggested by the datasheets.Meaning what is specified as Vgspractically has kind of become Vdsand therefore defying any meaning of connecting them upside down. For me interchangeability meant, that current may flow in both directions, but not, that the reference voltage for the gate switches as well.But maybe I have done something fundamentally wrong. Hence my cry for enlightenment.The other mystery is, when the Gate is high (that is V+) and I am disconnecting V+ from the source (or drain, if connected instead), the gate current rose dramatically from a couple of μV to up to half a milliamp and more. In fact the gate current equalled the drain current (or source current, if connected the other way round). Enough to dim the LED and so we are not talking about the initial gate current needed to charge the gate capacitor.What is this? Is this the mysterious "Forward Gate Current" found in the datasheets and that I have yet to find an explanation for?And why would anybody want to do this that makes it worth being specified in the datasheet? It was a mistake on my side, that made me discover this behaviour, but why would anyone run a JFET without one terminal being connected?Thanks for any insightEdit: The ADG419BN in the picture is really just a placeholder for a manual switch to change the gate voltage between GND and V+.

author: oPossum
author_rank: Super Contributor
timestamp: December 28, 2018, 01:17:49 pm
content: What is the part number of the JFET you are using?

author: hirada
author_rank: Regular Contributor
timestamp: December 28, 2018, 01:23:07 pm
content: Sorry. I knew I've forgotten something. Here we go:PMBFJ109https://www.nxp.com/docs/en/data-sheet/PMBFJ108_109_110.pdf?PMBFJ309https://www.nxp.com/docs/en/data-sheet/PMBFJ308_309_310.pdf?

author: madires
author_rank: Super Contributor
timestamp: December 28, 2018, 02:28:26 pm
content: The FET Constant-Current Source/Limiter:https://www.vishay.com/docs/70596/70596.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 02:53:57 pm
content: Most of the JFETs are really that symmetric - so there is no difference between the source and drain terminals. For a N channel JFET the relevant voltage is between the gate and the more negative D/S pin. So for switching one may need to provide an auxiliary signal at the level to switch for the on phase.There are a few non symmetric JFETs, usually for RF applications. Here the gate to source and gate to drain capacitance can be different and also the trans-conductance could be different when D and S are interchanges. However if used for simple slow switching one could still interchange drain and source.It is not normal to use a JFET with 1 terminal open. But there are uses with 2 pins directly connected. Either as a current limited (G connected to source) and as use as a low leakage diode (D and S together).

author: Nerull
author_rank: Frequent Contributor
timestamp: December 28, 2018, 09:01:15 pm
content: If you put a positive bias on the P-type gate relative to either end of the N-type channel than a JFET acts as a forward biased diode, because that's what it is.There is no daemon inside a JFET which switches connections around to make it work, it's just semiconductor junctions. In a simple JFET model there is no difference between the drain and source terminals, though real components may differ.

author: Wolfgang
author_rank: Super Contributor
timestamp: December 28, 2018, 09:47:31 pm
content: Quote from: Kleinstein on December 28, 2018, 02:53:57 pmHi,even for symmetric JFETs there is a thermal asymmetry caused by the fact that the chip is bonded to a metal carrier on the back side. Heat is generated in the drain portion, so if the JFET is run hot the mounting asymmetry could cause different drift behaviour. Another issue is capacitances due to lead and bond wire arrangement. These too need not be symmetrical.

author: hirada
author_rank: Regular Contributor
timestamp: December 28, 2018, 10:39:07 pm
content: Thanks for all your replies.QuoteThanks. There goes another option for my switch. I was afraid, that would happen.QuoteI still have not really gotten a grasp for all that junction stuff. I've seen animations, read about holes and electrons, doping, but unfortunately still no real feel for it.My hope was moreover, that maybe the manufacturing itself makes some bind between gate and source. Since those graphics are basically more general explanations.But thanks very much for your effort.QuoteI'll see, what I can learn from it. First impressions seem rather advanced.

author: spec
author_rank: Frequent Contributor
timestamp: December 28, 2018, 10:47:48 pm
content: Hi hiradaNot too sure of your objective, whether it is to understand JFETs or high side switches. So here is a bit about JFETs, low side switches and high side switches.JFETSJFETS are specialist components now, and not really suitable for switching loads on and off. But for certain applications JFETs are a godsend. The other thing about JFETs is that they are depletion mode and are normally on and can only be turned off by making their gate more negative than their drain (for NJFETs that is). The other thing is that the voltage necessary to turn a JFET off varies wildly, so they are difficult to use. And, even when they are turned on, they have comparatively high drain to source resistances. Finally, JFETs are quite delicate and easily damaged.LOW/HIGH SIDE SWITCHMOSFETs are the devices of choice for low/high side switching, as illustrated in the attached image below. MOSFETs are normally enhancement mode, which means (for an NMOSFET) that when their gate and drain are at the same voltage no current flows from their drain to their source. Note that you can also get a few depletion mode NMOSFETs. To make NMOSFETs conduct current from their drain to their source, you need to make the voltage on their gates more positive than their sources. For PMOSFETS, simply reverse the polarity of all voltages. Note that, unlike JFETs, current never flows into or out of the gate of a MOSFET (the situation is more complex at high frequencies though, but that will not concern you for this application).The beauty of MOSFETs is that they are capable of controlling high currents, 100A in some cases.For your application chose an N or P MOSFET with the following characteristics:VGD: 20V minVGS: +-8V min magnitudeID: 1.8A minVthreshold: 2V maxPower 1.8W minRDS 100mR max (at VGS=5V)There are literally hundreds of N and P MOSFETS to choose from.A MOSFET with the above capability will be way more than is needed to drive a few miliamps through your LEDs but it would give you more scope to drive other, more demanding loads, in the future.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 11:03:38 pm
content: JFETs can still be attractive for switching high impedance analog signals, as they can have relatively low leakage. Especially of one has a suitable voltage available to control the gate: signal level for on (this keeps the gate current low) and a rather negative voltage for off (N channel).In many cases CMOS switches like DG419 or similar are simpler to use and have replaces JFETs in quite some switching applications.For switching things like an LED MOSFETs are usually better suited.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2018, 01:45:37 am
content: Yes, precisely: the reference terminal is the lower of the two channel terminals.Same happens for invertible BJTs, you can swap emitter and collector because it's a symmetrical N-P-N structure (not usually actually symmetrical, due to doping differences -- it's notcommon, but symmetrical parts arepossible), and the emitter is again the lower of the two main terminal voltages.The only devices this isn't true for, are: thermionic tubes (no reverse conduction), the SCR (no reverse conduction), and the TRIAC (always common to MT1 -- the gate equivalent circuit is a back-to-back diode to MT1, so its voltage is never very far, and current flow in either direction will activate MT1-MT2 latching, to varying degrees of success depending).Tim

author: hirada
author_rank: Regular Contributor
timestamp: December 29, 2018, 11:39:42 am
content: QuoteBoth. I came along JFETs by searching for a low latency high side switch solution for small signals, eventually driven by a push pull output to be used as a (not really precise) buffer or impedance changer. There will be a load in parallel to the R/LED, that is why I require high side switching.Preferably a high side switch, that works, as one would expect: A "high" on the base/gate turns on, a low off and the voltage drop being more or less constant with variying load (within in a certain range, 1-30mA in this very case). Kind of like a relay really, but for energy reasons, I would like to stay clear of a dedicated IC switch for this presumably simple task, I've once thought, this would be.But with this final bastion of hope has fallen now, I've learned, this is not possible with silicium. Unless you build more complex units like a npn driving a pnp or p-mos or bootstrap/pump an n-mos - as your graphics show as well: The p-mos is plain in the wrong state with regards to the gate. Back in the days I was hoping to fix this with symmetrical JFETs.Or of course by accepting a higher voltage drop I could use a npn alone. All quite unelegant solutions to the simple task of switching, but thats our choice. So my next thread will be about mosfets. And push-pull outputs.As you've written, mosfets are more easily found for larger currents.*But then they may need a driver and have way slower turn on/switching times due to larger gate capacities (though I do not really have a need for that extra high speed, except enjoying a clean square on the scope and maybe the learning effect by taking propagation into account from the beginning). On the first glance, JFETs looked quite simple and less demanding than mosfets. I may have squinted here quite a bit.However, I have a couple of p-mos on my list that could fit my bill to test next.QuoteYes, I've noted that too. None of the JFETs I've tested would really turn off at 0V. The 2N3819s were the best/most consistent, but still did not turn off completely. But that was not as mysterical to me as the forward gate current, but more a matter of wrong reading of specs.QuoteMy next step. Though quite surprising, as integrated switches and lots of other chips are made of JFETs. And a lot of those seem to be able to work fine without a negative voltage. Where I was not even able to fully turn of my few devices with 0V. But then again, I am way less complex.QuoteThank for confirming this again. Though bad news.*: And it is not that easy to find those that actually do work properly with logic levels, especially for p-mos. But that did not really work out well for my JFETs either.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 29, 2018, 02:29:37 pm
content: The silicon JFETs are always depletion mode. So they are on at zero G-S voltage. As an odd point, there are SiC and GaN JFETs, that are actually enhancement mode and thus of at zero voltage. But these are expensive high voltage parts.The silicon JFETs are usually low current (< 50 mA). The small discrete MOSFETs are similar resistance and also similar in gate capacitance, but there are larger ones available. Really small ones are rare. So the driving is not that much easier with JFETs, it's different voltage levels however.The integrated switches usually use MOSFETs where substrate and source are separate. Such MOSFETs are also available as discrete parts, but rare. There are a few older Chips with JFETs too, but these are more like the exception.For a normal high side power switch the obvious choice is a P-MOSFET. Driving is not that much different from an N-JFET - mainly opposite logic.

author: Nerull
author_rank: Frequent Contributor
timestamp: December 30, 2018, 12:32:34 am
content: n-channel jfets work by choking off conduction through the n-channel by creating a depletion region, this requires reverse-biasing the PN junction, just as it does in a diode. You aren't going to find one that turns off at 0V because 0V is their fully on state, they turn off with a more negative VGS and should never be driven with a positive VGS, since this drives the PN junction into forward bias and the gate begins to conduct.

author: hirada
author_rank: Regular Contributor
timestamp: December 30, 2018, 11:50:20 am
content: QuoteRight. I was just thinking about the TL072CN and the AD SW06, both of which I am playing with right now. But those are a minority. Thanks for the heads up.QuoteI've had them on the high side. So the source was not at 0 Volt, but around 2-3V. Anyway, still not enough. Which is fine, for one model I remember the sheets specified a minimum of -1V, but up to -6V.QuoteI believe, finally I've gotten it now. I suppose, this is the gate forward current mentioned in the datasheet. Makes sense now, since, after disconnecting V+ from drain, Vgs rose from 0.1V to 0.7V - and the gate current rose to the draincurrent.Thanks very much to all for helping me out!

author: nick_d
author_rank: Regular Contributor
timestamp: December 31, 2018, 11:11:13 am
content: Regarding the complaint that power MOSFETs are hard to drive (high gate capacitance, etc), if you only want to switch light loads, the 2N7000 is an N-channel MOSFET that can drive 300mA and has a tiny gate capacitance (maybe 10s or 100s of pF). Apparently a BS250P is a reasonable P-channel equivalent of a 2N7000. If you want high=on then the simple solution is to drive it through an inverter such as a 74HC04. Of course this setup requires everything to be at 5V, otherwise the high side driving is rather tricky.If the voltage being switched is up to about 15V then an appropriate MOSFET can be found such that a gate voltage of 0V (or essentially -15V relative to your supply that's being switched) won't kill it. If it is more, you will have to use either bootstrapping or isolated drive (transformer or opto coupled). If you are forced to do it this way, it is often just as easy to have the gate drive sitting above the rail, for instance suppose your supply is 30V, then you could use the 30V as the ground of an isolated gate driver chip and provide another voltage say 10V higher as the supply of the gate driver chip. If you do that you can use N-channel MOSFET as the high side switch and save money and stocking costs. And interestingly you have high=on as requested, provided the isolated gate driver chip doesn't invert the sense.As to JFETs I do not know, but I learned a lot from this thread. Thanks!cheers, Nick

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 11:28:59 am
content: They used to make higher voltage and higher current "power" JFETs but they were never competitive with bipolar power transistors or power MOSFETs because of high channel resistance.4 pin MOSFETs with a separate substrate connection can be used as bidirectional switches and choppers but they were never very common. The same parts made good RF amplifiers.Linear SystemsandCalogicmake some small signal ones. Micrel (Now Microchip) has a couple of 4 pin p-channel power MOSFETs intended for high side power applications; it is too bad they are not higher voltage and available as n-channel parts also. I have no idea how to find their selection guide on Microchip's atrocious web site but the highest voltage part number isMIC94030.The cheapest solution is to use a pair of back-to-back power MOSFETs but if you want good small signal performance, then JFETs and the above 4 pin MOSFETs from Linear Systems and Calogic are the way to go.Quote from: nick_d on December 31, 2018, 11:11:13 amThe 2N7000 is 60pF input, 25pF output, and 5pF reverse maximum capacitance which is actually very high for a small signal application. A 3N part or JFET will have 1/5th to 1/10th that much capacitance and can be 10 times faster.For switching LEDs this does not matter but even the smallest vertical power MOSFET is useless for something like a sample and hold application.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2018, 11:55:46 am
content: Those 4 PIN MOSFETs with separate substrate are difficult to get now. But the same element is commonly used in switch chips. For some experiment is might be even worth getting the old CD/HEF 4007 chip (old CMOS Logic) - it includes such FETs with no too many connections.There are also depletion mode MOSFETs, that in many respects behave similar to a JFETs (turn on at 0 gate voltage, and turn off with a negative gate voltage). However they still have the reverse diode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2018, 12:29:40 pm
content: Quote from: David Hess on December 31, 2018, 11:28:59 amSmall, usually-intended-for-RF MOSFETs are both damned hard to find, and also hard to use with respect to ESD -- even if you're quite good about ESD normally!They're also in a bit of a death-valley market segment: too small to handle any real power (like a jellybean 2N7000), too large and too slow to do any serious RF (where PHEMTs, GaNFETs and such dominate). Everything else is ASICs, even in modest quantities (100k's?) so there's no one buying discretes in quantity.The smallest I know of is the RUM001L02 and brethren. It's about 1/4th the size of a 2N7000, electrically speaking (for that matter, mechanically too -- it's a tiny bugger, hope your soldering iron tip is sharp). This comes at the price of Vds(max) and Pd. It's definitely a newer process, with Rds(on) being quite comparable, while Qg being so much smaller.Any smaller (electrically speaking), and you should be looking at arrays (e.g., CD4007?), analog switches, oh or those "precision zero offset" MOSFETs by ALD ($$$), or doing it in an IC (which may involve a boring ADC-DSP-DAC scheme,that still wins on power consumption-- it's dumb, it works, get over it?..).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2018, 03:42:21 pm
content: For RF use there are dual gate MOSFETs (e.g. BF 998). In principle they should also work for logic switching if needed. One does not really need to actively use the 2 nd gate - it's quite common to have it at a fixed potential and than behave quite normal, like a small low voltage MOSFET with low input capacitance.A slight problem is that they are so fast that they tend to oscillate quite easy at frequencies the entry level scope may no even show.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 03:54:05 pm
content: Quote from: T3sl4co1l on December 31, 2018, 12:29:40 pmPricing and availability are definitely a problem but I linked two manufaturers to show that they (not the dual gate parts) are still available. They are useful for choppers and electrometer applications where a JFET is not quite good enough.I never really noticed the connection between the RF and chopper/electrometer 4-pin MOSFETs until going through my old Signetics book where they specified them both ways.PNP RF transistors are beyond the valley of death.QuoteIt depends on the application; not all are suited to an integrated part. I usually see these part in old precision mixed-signal designs like electrometers and high resolution multimeters. They show up sometimes in high performance active mixers in place of JFETs.Quote from: Kleinstein on December 31, 2018, 11:55:46 amThis becomes more apparent looking through the Signetics datasheets. Not only are they the same element, they are exactly the same part which gives a rare look into the performance of individual transistors found in old CMOS processes.QuoteThere were also depletion mode 4-pin MOSFETs but I do not know the part numbers or if anybody makes them now.Quote from: Kleinstein on December 31, 2018, 03:42:21 pmDual gate MOSFETs are not a substitute for 4-pin source-gate-drain-substrate MOSFETs. I have never seen them used in a chopping application but I am sure someone tried it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2018, 04:45:30 pm
content: Quote from: David Hess on December 31, 2018, 03:54:05 pmYeah, I shouldn't say unavailable; and yeah, definitely not mainstream (they're not carried by Digikey; though Calogic is carried by Future which isn't too bad).QuoteGet your BFT92s now. Last call!Tim