forward leakage of a diode ... - Page 1
Forum section: electronics / projects

author: free_electron
author_rank: Super Contributor
timestamp: August 24, 2017, 05:11:22 pm
content: I'm having trouble finding the forward leakage of a diode. nobody seems to specify that ...for example : if on a 1n4148 ( standard silicon diode ) i put 0.1 volts : how much will it leak ( theoretically nothing since it is below the 0.7 volts , but in practice it will leak due to impurities )here's my problem : i need to protect a high impedance 1 volt input. my idea was to put two or three silicon diodes in series ( 1.2 volts .. 2.1 volts ) ( the chip can handle 2.7 volts , but the signal to be measured is guaranteed never to be above 1 volt , besides the a/d reference is 1.024 volts ...) so i thought to clamp the signal , past the input series resistor ( there is a 1 meg resistor from the input to the sensing amplifier. sticking the two diodes there would solve the problem.problem is the diodes have an impact on the signal to be measured ! they leak...finding TVS diodes that go that low is not doable... zeners that low are simply stacked silicon diodes.and no manufacturer seems to specify the leakage current in the 'non conductive' zone of a diode. trial and error... i don't feel like measuring this on a bunch of diodes and find what works best.i may have to do the trick keithey uses in their electrometers : a pair of b-e junctions in antiparallel ( but stacked )

author: duak
author_rank: Super Contributor
timestamp: August 24, 2017, 05:47:42 pm
content: I recall that either Bob Pease or Jim Williams had some info on the forward characteristics of various types of diodes.The 1N4148/1N914 diodes are poor choices for this application because they are gold-doped to increase switching speed which has the side effect of increasing the forward current at low voltages. Being in glass cases also makes them photosensitive. It's better to use a bipolar transistor instead but the higher junction capacitance might be a problem. There are low leakage non-gold doped diodes available too but no numbers come to mind.Cheers,

author: edavid
author_rank: Super Contributor
timestamp: August 24, 2017, 05:58:52 pm
content: Quote from: free_electron on August 24, 2017, 05:11:22 pmNo, it is not theoretically zero. The junction equation is continuous from 0V up.For some measurements, see The Art of Electronics 3rd ed figure 5.2The winners are the same as for reverse leakage: 1N3595, 2N3904 BE junction, diode connected PN4117A

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2017, 06:50:47 pm
content: Quote from: edavid on August 24, 2017, 05:58:52 pmIndeed, continuous from some -V (not quite towards breakdown*) to +V. The Shockley diode equation is:\[i_f = i_s \left( e^{\dfrac{v_f}{\eta v_{th}}} - 1 \right) \]Under forward bias, the -1 is negligible, but near zero, it is critical. Simply put: smaller \$ \eta \$ and smaller \$ i_s \$ is all you need.*IIRC, SPICE matches the avalanche curve at some point (piecewise continuous, or continuous derivative), then sets the exponent to cross through (BV, IBV). Or you could use a TANH function to interpolate between avalanche and normal conduction (thus getting all continuous derivatives). I don't know what's more physically realistic, but it probably doesn't matter enough to tell.Tim

author: fcb
author_rank: Super Contributor
timestamp: August 24, 2017, 07:11:55 pm
content: I use a pair of 1N3595 in one of my designs. I seem to remember them being mentioned in the Keithley low level handbook, 1pA rings a bell.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 24, 2017, 07:17:41 pm
content: Quote from: free_electron on August 24, 2017, 05:11:22 pmAre you taking the piss or just gently trolling?I was just about to buy an electric car...Leop.s. I got 33nA fromhttps://www.diodes.com/assets/Datasheets/ds12019.pdf

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 24, 2017, 07:41:00 pm
content: What is the bandwidth of the signal?

author: nctnico
author_rank: Super Contributor
timestamp: August 24, 2017, 07:54:37 pm
content: Quote from: free_electron on August 24, 2017, 05:11:22 pmMost diodes' datasheet has a forward current versus voltage graph so there is your answer.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 24, 2017, 07:57:24 pm
content: The low forward voltage behavior can be modeled by the shockley diode equation, with the caveat that the ideality parameter eta is basically never specified in datasheets, although you may be able to estimate it from the provided parameters, or just guess that it is about 2.Note that you can also calculate the small signal impedance of a diode near DC by simply taking the derivative of the diode equation. You get: dvf / dif = \eta v_{th} / i_s. You may be shocked to find values < 1 megaohm even at zero DC voltage for common signal diodes. That is at zero DC voltage!If you need high impedance: use an ultra-low leakage diode such as the FJH1100, or use a transistor. Alternately, you can use an LED -- they have orders of magnitude lower leakage than the best silicon diodes but have a correspondingly higher forward conducting voltage. Make sure you completely black it out to avoid photocurrent! If you only care about low frequency you should probably bootstrap the diode by providing a buffered copy of the input to the other terminal of the diode. That will keep the DC voltage across the diode very close to zero as long as the signal is in range.

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2017, 07:58:09 pm
content: I see diode-connected transistors used when you need low leakage.Temperature is very important, if your product will be in a hot environment. I believe leakage is log temperature.

author: nctnico
author_rank: Super Contributor
timestamp: August 24, 2017, 08:58:25 pm
content: For the problem at hand I'm wondering what the chip's ESD diodes can handle and what the required bandwidth is. The solution may be as simple as sticking a series resistor at the input which limits the current into the ESD diodes which do exactly what you want: clamping without distorting the signals. As an extra measure you could put a zener diode or shunt regulator across the power rails to sink the current.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2017, 09:18:40 pm
content: In general, if you are trying to protect an input driven from a substantial source impedance (where the diode "leakage" current is important), you should establish a voltage with a suitable diode network to ground, biased from a resistor to the power supply (not much bias current required) and then connect a low-leakage diode from the input node to that bias voltage. Depending on the voltage, this can be series-connected PN or Schottky diodes, or a suitable Zener.I have used the base-collector diode of low-noise audio transistors (2N2484 NPN or 2N4250 PNP) and the gate-channel (source and drain connected together) of low-current JFETs to get low leakage currentin the reverse-bias directionbefore the safety circuit conducts (with the low-leakage diodein the forward-bias direction). When the input is very positive, the current will flow through the forward-biased diode to the diode network, limited by the source resistance to keep the diodes from smoking.

author: free_electron
author_rank: Super Contributor
timestamp: August 24, 2017, 10:47:26 pm
content: Quote from: nctnico on August 24, 2017, 07:54:37 pmexcept that curve is unreadable. i am interested in the 1millivolt,2 millivolt up to 0.4 volt range ... ( well below the knee of the diode )What i'm trying to do is the following : i am sensing a small current through a sense resistor. i want to clamp the voltage across the sense resistor to maximum 1.2 tot 2.1 volts. i want the current to flow through the sense resistor, not the diodesThe sense resistor is in the order of 1 megohm. ( the source impedance is high and i can't change it .)i want to protect the input from ESD events and nitwits connecting it directly to a voltage source. so i put a 100k series resistor and clamp it using a few diodes. ( it's a current source so series resistance doesn't matter )i made a typo in the drawing. current is max 1 micro amp. so 0 1 microamp gives me nicely 0 to 1 volt which i can pick off using a nice unitiy gain instrumentation amplifier with a few pA input bias . (i could use gain in the amplifier but prefer not to ... )signals are DC ...i don't want any current going through the protection diodes (until the protection needs ot activate of course ) . any current loss there gives a measurement error. ideal diodes (0.6 volt forward voltage ) will work since they won;t trip until 3x0.6 is reached. clipping my input at 1.8 volts. but in the real world diodes even draw current at 0.1 volt ...So i am trying to find a really good diode with very low leakage until it is supposed to conduct. And datasheets are very lacking in the 'non-conducting' area of the curves ...Or maybe i'm going about this the wrong way.I was thinking about this after watching the teardown of th electrometer . they use b-e junctions to do the protection. so i thought : that would solve it. problems is nobody specs that stuff...

author: free_electron
author_rank: Super Contributor
timestamp: August 24, 2017, 11:03:43 pm
content: Quote from: Leo Bodnar on August 24, 2017, 07:17:41 pmhow did you get to that number ? i don't see that anywhere in the datasheet.

author: edavid
author_rank: Super Contributor
timestamp: August 24, 2017, 11:05:44 pm
content: If you use an op amp current to voltage converter, the voltage at the sensing node is zero, so there is less of a leakage issue.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 24, 2017, 11:17:43 pm
content: Quote from: free_electron on August 24, 2017, 10:47:26 pmDiode is not a relay or a fuse.Datasheets are written with expectations that you roughly know what is happening outside the measured graphs. They usually only mention things that you do not expect the part to do.It sounds harsh but the best you can do is take a break from your design, honestly admit to yourself that you don't understand how semiconductor diode works and read a good electronics engineering college coursebook or TAOE. They have much better talent at teaching than a random internet forum.But if you still demand the secret curves - here they are.Pease says ~60nA for 1N4148 at 0.1V

author: free_electron
author_rank: Super Contributor
timestamp: August 25, 2017, 12:36:26 am
content: Quote from: Leo Bodnar on August 24, 2017, 11:17:43 pmi'm not even going to go there ... i don't need a theory lecture. i wanted to see the Vf curves at low currents and find a diode i can buy at digikey that has the lowest If for the highest VfQuoteThat's what i wanted to see. i didn't want to do that experiment. It figured that Pease of all people would have done that.So a yellow led will do the trick. up to 1 volt it draws less than 1 nanoamp.Now to find a nice smd yellow LED that i can put in a dark enclosure

author: agaelema
author_rank: Contributor
timestamp: August 25, 2017, 01:30:55 am
content: Very interesting topic.I'm reading some pages and is common the use of 2N3904 as low leakage diode using BE junction (if the voltage is between the limits) or BC junction, but generally reverse biased, not forward biased as the diodes in your circuit.-https://www.electronicspoint.com/threads/diode-with-very-low-reverse-leakage-current.26296/-https://electronics.stackexchange.com/questions/16966/transistor-as-low-leakage-diodeMaybe can you use the BE/BC junction conected between the input and an auxiliar supply (another regulador or zener) of 1.2V. In a normal condition the diode will reverse biased (like in the picture).

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 02:03:33 am
content: LEDs painted black also make good low leakage diodes but of course they have a low peak reverse voltage so not really any better than a transistor base-emitter junction.If the signal is low frequency, then the diode clamps can be bootstrapped to follow the input voltage reducing leakage further and then a second second of diode clamps used to clamp the bootstrap output. I have seen this done in several HP designs and it is also used now on very low input current CMOS operational amplifiers.Bob Pease discusses this subject in his book.

author: free_electron
author_rank: Super Contributor
timestamp: August 25, 2017, 02:23:04 am
content: Unfortunately those things (biased or bootstrapped clamps don't work in my case. i don't have aux supplies. My system runs off a coin cell (cr2032) and has to run for 2 years.All i want is clamp diode that prevents the opamp input to get more than roughly 1.2 volts across it. ( the a/d power rail is 1.8 volts. and i want to protect the input from large incoming voltages such as esd spikes , metallic noise on the cable etc.The main CPU shuts down the analog front end when not sampling to preserve power so the clamp needs to still work .... the opamp is the first active element. i need to protect that as it sees the two wires coming from the sensor (the big bad outside world). my idea is : stick a series resistor in the loop ( it is current anyway , so series resistance doesn't matter ) . then clamp it. but : the clamp is not allowed to drain anything until its trip point. neither from the loop (as that falsifies my reading ) , nor from the system power (i don't have the power budget )Classic zeners don't work. A 1.8 volt zener leaks 1uA at 1 volt ... i only have 1 uA coming in ....TVS diodes are even worse. a 1.8 volt TVS diode( trip voltage : 1.5 volts ) sucks 10uA at 1 volt ... well below her trip point.Specialised clamp diodes begin at 2.5 volt but they pull 50nA at 2 volt ( before they conduct ). their behavior is not specified (no curves from 0 volt to their snap point ) , and even if such curve existed it will be non linear. i have a 10 bit A/D.So i can digitize 1 nA. that diode would already throw my system off by 50nA ... not good.The system is basically a coin cell operated LoRaWan node that samples the current coming from an electrochemical cell. That cell is a current output (0..1uA into 1Mohm giving 0..1volts) that is being digitized. I want to protect that 1Mohm input by placing a protection diode across it. Since we are dealing with a current mode output : anything i place in parallel with the 1Megohm falsifies my reading. Especially if its behavior is non-linear ... classic diodes leak too much in forward mode. ( iwas thinking of stacking two in series ... 2x0.7 volts .. but diodes also conduct at 0.1 volt and 0.2 volt ... i can't have that ( at least not if they conduct microamps ... )I didn't feel like doing the legwork of spending a whole day playing with various diodes to find out which one leaks the smallest current.Fortunately Bob Pease has done this already ... yellow LED. 1nA at 1.2 volts. perfect. well within the noise of my A/D.Now i have a starting point to see if it will work ( i'll build it on bench and measure it. i have keithley electrometers en picoampere meters ) so i can sense the exact voltage across my 1meg/led parallel circuit and measure the exact current through the combination.

author: agaelema
author_rank: Contributor
timestamp: August 25, 2017, 02:49:27 am
content: Quote from: free_electron on August 25, 2017, 02:23:04 amWhy not?If a new source is a problem, connect the BE junction directly to positive pole of battery (or the 1.8V of A/D rail that you cited). The voltage at the input can exceed the rail by Vbe and the OpAmp output will saturate. To protect the A/D you can put a serie resistor to limit the current and another diode connected between the input of A/D and the 1.8V rail.A 2n3904 BE junction (second the one of the links) has 1pA of reverse leakage, much less then 1nA of yellow led.

author: free_electron
author_rank: Super Contributor
timestamp: August 25, 2017, 03:15:07 am
content: Quote from: agaelema on August 25, 2017, 02:49:27 amproblems : The power rails are off when not sampling ... so protection would not work (the bias voltage goes to zero so the diode would turn on... which would pulls current from the sensor, a current which is larger than the 1uA. which is a nono... the more current i pull the faster the sensor ages. it's complicated. this thing is a based on an electrochemical reaction. draw more current and the reaction speeds up : eating away the electrodes in the sensor...Unfortunately i'm not a liberty to give more info on the sensor. Suffice to say it is made for a max current of 1uA into a 1M sense resistor. if you lower the resistor value then the current delivered by the sensor will increase and that ages the sensor. It is also not allowed to send current into the sensor as that destroys one of the electrodes due to an electromigration process.This bloody thing is made for a 1meg load and gives 1 volt at full reading ( actually 0.98 volt). you can lower the resistor but that comes at the price of a shorter lifespan. There is some passive circuitry inside this sensor in the form of a resistor bridge. i don't have acces to this as the whole thing is potted... otherwise i would stick my signal conditioning in there ... and clamp in a low impedance domain but that doesnt work.normally the sense amplifier is mounted directly on the sensor. in my case there is a few meters of wire ... so i want a esd clamp there ...i already verified that cutting the power rail of the opamp does not change the input current draw of the opamp. it is safe to shut down the entire input path without causing additional load to the sensor.anyway. this is dancing around the subject. all i want to find is a diode that does not leak in forward mode.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 04:09:58 am
content: Quote from: free_electron on August 25, 2017, 02:23:04 amBootstrapping the input protection diodes does not require a separate or higher voltage supply but the buffers do draw more power.QuoteWhat kind of operational amplifier are we talking about?Obviously it has low input bias current but bipolar input parts (1) include back-to-back diodes across their inputs to prevent base-emitter breakdown from high differential voltages if that is a problem.With a 3 volt supply voltage, protecting each input with a pair of low leakage diodes like the mentioned 2N4117 JFET or a 2N3904 base-collector junction which go to the supply voltages is usually sufficient if the operational amplifier's internal protection diodes are not. Add resistors at the inputs and also between the protection diodes and the operational amplifier inputs. The later resistors also isolate the capacitance of the diodes from the inverting input which would otherwise reduce phase margin. Do not make the resistors any larger than necessary.Quote from: free_electron on August 25, 2017, 03:15:07 amThat sounds a lot like a standard cell. They have similar requirements. I am not recommending you do this but the Linear Technology LT1012, LT1008, and LT1097 datasheets show an example where a depletion mode MOSFET with its gate tied to the positive supply is used to disconnect a standard cell from the operational amplifier input so removing power does not damage the cell. This would be difficult to do in a 3 volt circuit.When shutting off the power to the operational amplifier, why not leave the input protection diodes connected to power? They will be reverse biased anyway.QuoteThat is a little odd. Most CMOS operational amplifiers have protection diodes between the inputs and the supply pins so if the supply voltage drops, the inputs end up shorted to the supply pins. The high series input resistance solves this problem but it also solves the problem for the external clamp diodes anyway.(1) Super beta input bipolar operational amplifiers are suitable for megohm input resistance circuits and have the advantage over many low input current JFET and MOSFET parts of not having their input bias current double every 10C which is important if high temperature operation is a consideration. If you are running on a 3 volt coin cell however, they are probably not suitable do to higher supply current unless the amplifier is being shutoff. They do have the advantage of higher precision and lower noise however.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 25, 2017, 06:01:23 am
content: Quote from: free_electron on August 24, 2017, 10:47:26 pmDoesn't matter. There is actually no such thing as a "diode knee". The current is exponential following the shockley diode law that multiple people have posted about, there is no threshold. 0.6 V is just a convenient reference point. The IV curve flattens out at high current due to self heating and the bulk resistance in series with the junction, but you can extrapolate from the data sheet values down to much lower currents. For instance, the 4148:https://www.diodes.com/assets/Datasheets/ds12019.pdfyou can see that at low currents, the IV curve is a straight line on the log log plot.Another example, the ultra low leakage FJH1100:https://www.fairchildsemi.com/datasheets/FJ/FJH1100.pdfYou can read off from the table that the voltage drop is about 80 mV / decade at low currents(corresponds to an ideality parameter around 1.3). So you will get 1 nanoamp at ~300 mV forward bias, and 1 picoamp at around ~50 mV forward voltage.So you could put 4 of these in series if you wanted to get about 1 nanoamp @ 1200 mV. But the LED solution is much better (and several orders of magnitude cheaper!)The curves/ratings are usually at 25 C. The scale current is exponential in temperature, so if your device has to operate at 35 C, the forward current will be much higher at the same voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2017, 06:30:54 am
content: Quote from: free_electron on August 25, 2017, 12:36:26 amThen give up now. They don't exist. I don't know of any manufacturer that makes promises about current flow at low voltages.Youmustsolve this problem either by:1. Give up. Assume that most diodes don't leak much down there, and just go with it. Accept the consequences.2. Measure all incoming diodes (as a spot or sample check, or up to 100% test, it's up to you) to confirm their properties in this application. Accept the monstrous increase in unit cost.Tim

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 25, 2017, 07:44:20 am
content: Vincent, these curves are 25 years old in case you are using them as selection tables or design references and they were taken from just one random product sample. Super-bright LEDs were not even invented back then. I think Bob's intention was not to compile a selection table but to illustrate his thinking process - I was hoping you'd get the book and read it.This is amazing thread. As a free bonus, there is some discussion about diodes here.LeoP.S. Could we please stop using "leakage" in relation to normal forward conduction? It confuses the hell out of me and probably a lot of others.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2017, 07:52:55 am
content: In the very low current range one might have to check / measure the parts used. Except for expensive special low leakage parts, they diodes are not tested for leakage down to the low pA range, as this is expensive (because it takes time).LEDs might be a good option - though you need to check. The typical current will be low, but one can not be sure. If the high forward voltage is not a problem, especially blue ones would be worth a try.Extrapolating the diode equation / curve down to very low voltages can be tricky. Quite often there are other contributions than the idea diode current you see at higher currents. Quite often there is at least a contribution with a higher (e.g. 2 ) ideality factor at low currents, that is just not relevant at the more normal currents.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 08:01:11 am
content: Here are the graphs from my measurements for the 1N4148 and BAV199 forward (50mV -500mV) and reverse (50mV -100V) currents at about 26C ambient.CheersAlex* I've measured a number of samples of each diode, as well as BAV199 diodes from five different manufacturers. The comparative curves are average for the 1N4148 and for the worst make of the BAV199. The best makes of the BAV199 have almost ten times lower forward and reverse currents.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 25, 2017, 08:10:58 am
content: This is an interesting thread ...IMO using one or more diodes in forward direction to achieve the OPAMP protection isn't the right way.Most opamp inputs are well protected if one limits the maximum input current to some value in the 5mA ballpark. If you limit the voltage across your sensor input to say +/- 7V using back to back connected transistor BE junctions as zeners, one can achieve OPAMP protection using a simple 4k7 resistor. You might need a zener across the OPAMPs supply pins to catch that injected current, depending on your power supply switching scheme.I don't know the exact figures of the BE zener leakage, it is believed to be very low. So I'd just consider a architecture change of the protection network.

author: tszaboo
author_rank: Super Contributor
timestamp: August 25, 2017, 10:25:29 am
content: First of all, the correct term, you are looking for is "subthreshold" leakage. It help a lot with the google-fu.And yes, diodes are not very well defined in this region. But transistors are. You can use 2 transistors of your choice, by connecting the base to the collector, and look up the Collector Cutoff Current. That will be almost your value. It is 0.2nA-15nA for a BC546. Temperature dependent a lot of course.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 25, 2017, 10:38:45 am
content: Quote from: NANDBlog on August 25, 2017, 10:25:29 amScore!http://www.google.co.uk/search?q=%22diode+subthreshold+leakage%22

author: oldway
author_rank: Super Contributor
timestamp: August 25, 2017, 11:07:25 am
content: BAV25 is the diode you are looking for.....it has a leakage of 0.1pA at 100 mV forward voltage.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 11:23:08 am
content: Quote from: oldway on August 25, 2017, 11:07:25 amCould you provide a link to a data sheet on this diode (Google search draws blank)?CheersAlex

author: AndyC_772
author_rank: Super Contributor
timestamp: August 25, 2017, 11:37:07 am
content: I'm struggling to see the physical mechanism by which a component might be damaged, with a 100K resistor in series with it.IC damage is caused by local heating, when a non-trivial current flows through somewhere it shouldn't. How much current are you expecting to be able to draw through that 100K resistor under fault conditions?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 11:53:40 am
content: Quote from: free_electron on August 25, 2017, 03:15:07 amBAV199CheersAlex

author: oldway
author_rank: Super Contributor
timestamp: August 25, 2017, 12:51:34 pm
content: Quote from: Alex Nikitin on August 25, 2017, 11:23:08 am

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 01:12:59 pm
content: Quote from: oldway on August 25, 2017, 12:51:34 pmSo it is BAV45 (obsolete from Philips and the replacement from Central Semi has nothing in the data sheet on the forward current values at these levels). About the same performance as the BAV199, which is easily available for a fraction of the price. There are more low leakage diodes available, including FJH1100 from Fairchild (very expensive, photo-sensitive and not as good as the BAV199), PAD series from Vishay (also expensive and not easily available, no specs on the forward current v voltage).CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 01:31:23 pm
content: Here is a possible solution for the topic starter. If it is possible to sacrifice a couple of uA to forward bias two diodes, the summary leakage through D1 and D2 diodes in this circuit at 0 to 1V on R2 would be under 0.1pA at 25C (typical figure).CheersAlex

author: oldway
author_rank: Super Contributor
timestamp: August 25, 2017, 01:45:13 pm
content: Also the diode FDH300 (very low forward leakage)http://www.onsemi.com/PowerSolutions/product.do?id=FDH300In SOT-23, MMBD1501/A

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2017, 02:14:32 pm
content: Quote from: AndyC_772 on August 25, 2017, 11:37:07 amThe high currents involved can also cause electromigration which is both current and temperature dependent. CMOS gate structures may suffer from punchthrough of their gate oxide. The parasitic thyristor in CMOS junction isolated processes may be activated crowbaring the supply if powered. Base-emitter breakdown damages bipolar transistors when the hot carriers cause dislocations lowering hfe by increasing recombination; this is why bipolar precision operational amplifiers include back to back input diodes to protect against high differential input voltages if their base-emitter junctions are exposed. (1)Just a series resistor may not be sufficient if its breakdown voltage is exceeded. For the same reason, creepage and clearance matter. In the past they used carbon composition resistors for their good short term overload capability but today one of those inexpensive fusible high voltage film resistors would be suitable if you are paranoid enough and cannot rely on additional shunt protection. The rise time of an ESD pulse is fast so bypass capacitance matters as well.Things are worse when you consider that it is the machine ESD model which likely applies instead of the human ESD model.(1) Many PNP input operational amplifiers avoid this problem but I do not know that they are any more ESD resistant. NPN input cascode operational amplifiers also avoid it but again, I do not know that they are any more ESD resistant.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 25, 2017, 02:15:08 pm
content: Quote from: oldway on August 25, 2017, 01:45:13 pmNot really in the same class, less than 400mV for 1uA forward current typical. The BAV199 has around 500-550mV at 1uA .CheersAlex

author: free_electron
author_rank: Super Contributor
timestamp: August 25, 2017, 04:07:15 pm
content: so sorry. sensor output is 1mA .... not 1uA ...Me : then where does this 1m megaohm requirement come from ?they: that is input resistance of multimeter so you get no erro ... (the spelling of erro gives it away right ? )screw it i'm putting a simple TVs diode across there. and i'm gonna use a 100 ohm sense resistor with an x10 amp.the only requirement is that the burden voltage of the system should not go above 1.2 volt. as that causes unwanted electromigration in the electrodes.During normal operation the output is in the order of 50 to 60 microamps. only at full reaction does the sensor deliver 1 mA. but that is not realistic.something along these lines (image below) will clip enough incoming crap (esd, people with screwdrivers ... ) that it has no impact and protects the input opamp.Anyway. Interesting puzzle. How to protect a line that has a high impedance ? Most diodes don't work as they leak to much in the 'subthreshold' zone, and manufacturers don't specify the data for that area. so good luck finding something that works. the transistor as a diode trick would work. That's why keithley uses a matched pair antiparallel as input clamp for their electrometers. ( dave's recent video reminded me of that and i was working on this 1uA pretection thing so one thing lead to another )And yes , diodes follow that equation but even that is useless information to solve this problem. you don't know half of the parameters used , manufacturers don;t list em. so all you are stuck with is trial and error. which i wanted to avoid in the first place.One thing learned : when looking for some information that is hard to find : check Bob Pease books FIRST. most likely he's already done it.

author: dcarolan
author_rank: Newbie
timestamp: July 22, 2024, 06:19:37 pm
content: Here is something I did a while ago and am glad to share.If youre looking to clamp a hi Z stage. Be very cautious of the diode connected JFet trick. It only works for a couple of milliamps then it looks like a resistor.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 22, 2024, 06:44:28 pm
content: Quote from: dcarolan on July 22, 2024, 06:19:37 pmYour data on the BAV199 is somewhat suspect (perhaps it was a counterfeit device). Even the worst makes I've measured are much better than the BAV99 standard diode. Here are my measured results for a number of BAV199 from five different manufacturers. Even the worst one is considerably better than your graph shows , and the better makes are similar to what you have measured for the BAS716 (hardly surprising as it is the same device as the BAV199, but a single in a different package).CheersAlexP.S. - on a JFET connected as a diode. It will look as a diode+resistor for larger currents, however the value of that resistor would depend on the channel resistance. The MMBT4117 is a very small JFET with a very high channel resistance (over 10K). If you use a larger JFET with the channel resistance in tens of Ohms it will happily work as a diode up to tens and possibly even hundreds of mA, though the leakage current will be higher too.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 22, 2024, 08:57:36 pm
content: Considering the P N Junction of a FET....Would a FET with a very low Reverse Leakage also have a low forward sub threshold leakage?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2024, 10:04:54 pm
content: Impressed that it's that low: e.g. for 10 to 30mA, Vf goes from ~1.1 to ~1.4V, or ~15Ω.Yeah not sure what's up with the BAVs, those have some serious uA-nA leakage at low voltage. Surface contamination? Radiation exposure? Schottky? Well it ain't schottky, the drop isn't nearly low enough in the mA.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: July 22, 2024, 11:46:20 pm
content: how about a BAS low leakage diode

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2024, 12:42:04 am
content: Quote from: Wallace Gasiewicz on July 22, 2024, 08:57:36 pmI suspect that is the case, but I do not think there is any relationship between reverse leakage and forward conductance. If there was, then a diode connected JFET would be recommended for log/antilog circuits, and I have never seen that.Leaky gold doped switching diodes are also leaky when forward biased.A base-emitter junction is about the best silicon diode; it has low forward and reverse leakage, and high conductance.Visible light LED junctions with their high bandgap have low reverse and forward leakage, but in the dark.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 23, 2024, 07:21:38 am
content: Very interesting thread, I have thought about lots of these considerations for sensor projects too.One important thing that was missing yet: If I understood it right, you measure a DC signal. That means you can easily get rid of e.g. ESD spikes by- catching them in a higher voltage suppressor diode- smoothing the output with an appropriate RC filterIf you do the maths or if you simulate you´ll see:- e.g. a 2500V ESD spike will be reduced to around 15V by a 6.8V suppressor- the remaining 15V will be reduced to around 0.2V by e.g. a 100kOhm/2.2nF RC filter(values are just exemplary for one system I experimented with long ago - they vary greatly with the immanent stray inductance. But the principle stays the same.)A 100kOhm filter resistor before your 1MOhm shunt will increase the meters impedance by 10% - but maybe that´s a fair price for the pulse protection.In the RC filter only the cap could create a leakage current, but taking a decent foil cap you can get into the picoampere range.For some high voltage sensors that have been prone to arcing we even cascaded the protection: Sensor --> 50kOhm HV resistor --> Spark gap --> 50kOhm resistor --> Suppressor diode --> RC 100kOhm 2.2nF --> LM 662 input with several hundreds of MOhm feedback. Of course all higher frequency components of the signal are blocked that way, but we even wanted some analogue averaging there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2024, 09:36:58 am
content: Large-value resistors are problematic for analog applications; I would avoid maybe even 10k, almost certainly 100k, with regular diodes (BAV99 etc.), certainly 10k with schottky (BAT54S etc.); but lower is okay.This will be for middle-of-the-road say 12 ENOB applications, not quite generic and loose stuff, somewhat precise, but nothing wild.For use with LMC662 (there's no LM662?), a bootstrapped diode would be most suitable: a back-to-back diode from input to ESD clamp node (which gets the supply clamp diodes), then a low resistor from that to OUT, with the op-amp wired as unity-gain follower. Little voltage drop means little current flow, even with leaky diodes. Not so effective if the same amp needs voltage gain though.Tim

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 23, 2024, 09:55:40 am
content: Yes, you´re right, LMC662 - I never correctly remember any namesWhat do you mean with no more than 10k? The resistor in a LC-filter? Or the feedback resistor of the opamp-circuit? Is thermal resistor noise your concern?We used it with a lot of averaging to get around 0.5pA stability. The ionization sensor was never absolutely calibrated so this was not important. But the cascade of spark-gap + TVS + RC was finally stable so that no LMC662 was killed even with frightening HV arcing.

author: dcarolan
author_rank: Newbie
timestamp: July 23, 2024, 04:04:28 pm
content: They could easily have been counterfeit. They came off of EBay.The question of the JFET diode connected and better behavior is an interesting one.I never have the time to look into the impact of the different figures of merit but I'd be curious to know.Don

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2024, 09:29:43 pm
content: Quote from: Phil1977 on July 23, 2024, 09:55:40 amIn terms of Thevenin input resistance -- signal source itself, dividers, RC filters, what have you. A typical acquisition input circuit being a divider, buffer/filter (optional), and ADC. Particularly for muxed ADCs, a lowish source impedance is required, how low depending on sample rate generally. Diode leakage introduces an unknown (and thermally dependent) offset for larger-value resistors; and of course schottky are leakier. They also have some nonlinearity near the rails (it's not a constant current). Slow signals and generic MCU ADCs, you can usually get away with just the divider, but faster/more accurate systems, or higher impedance sources, benefit from buffering.Thermal noise wouldn't be much a concern at 12 bits and 100kΩ, but say 16b+, voltage noise starts to dominate, and you also get into the precision range where special resistors, very good refs, etc. become required -- give or take of course how much gain accuracy you really need (since it could also be something like audio where samples are entirely relative, no one really cares about gain or range, just noise).QuoteAn extreme case, and quite a robust one indeed.Tim

author: exmadscientist
author_rank: Frequent Contributor
timestamp: July 24, 2024, 02:40:58 am
content: A couple of parts relevant to this discussion are the Vishay FRED hyperfast power diodes. That's right, big ass power diodes and small signal analog reallycanmix. They have relatively high forward voltage and excellent reverse leakage. I don't recall testing forward subthreshold behavior specifically (I usually use the B-E junction) but they have all the ingredients to be good at this, with no putting RTV over LEDs required.There are a lot of parts in this series and they're all a little different, but the best reverse leakage performer on my list is the VS-3EJH02-HM3 (fun names, aren't they?) which is 1nA @ 50V. That's actually BAV99 territory (at least on the datasheets), but can the BAV99 handle 2A... continuously? And 65A surges? Yeah, not so much. (Some few unlucky circuits need to be protected against greater terrors than other, luckier circuits. And sometimes the big power diode that was already on your BOM just turns out to have other sides to its personality.)For sure, not the solution for every problem. But also better than covering yellow LEDs in black RTV (a old, traditional light proofing technique). That gets old fast.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2024, 04:58:21 am
content: Considering they're Pt doped, or beta irradiated, low leakage is encouraging. Give or take what they're doing these days -- not sure which ones are using MOS and other structures, but there are some impressively fast types out there.Tim

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 24, 2024, 08:11:23 am
content: These are all good tips with more or less exotic special diodes...But: Does the author of this thread need them at all? As far as I understood he does not want to logarithmize the signal, he just wants overvoltage protection. And as I wrote earlier, for slow signals a larger voltage TVS + simple RC filter dampens the surges more than any diode alone, and it may also help a slow battery powered ADC to get stable readouts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2024, 08:38:25 am
content: OP is a seasoned engineer -- he already knows probably everything else that's been discussed. Also note that he hasn't posted in this thread for about six years(!). All that's left is regulars chatting, or bringing things up for future readers.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 24, 2024, 09:20:37 am
content: Quote from: free_electron on August 24, 2017, 10:47:26 pmThe resistor alone should be enough to protect it from most likely abuse ( for ESD maybe minmal capacitance across the resistor and add a PCB spark gap). Damage is caused by current flow, not voltage alone.ISTR you mentioned it was DC, so a parallel cap will protect from any fast transient that can get past the resistor.QuoteWhat's wrong with relying on the chip's internal diodes, assuming it has them - they are there anyway so will be rated to not cause issues, and once they start conducting you're in a "don't care" situation as regards accuracy etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2024, 09:25:16 am
content: Resistors are nice, as long as they don't arc over (or blow up!). ESD-rated chip resistors aren't very abundant (or effective, or easy to use..!) so it can still be better to use clamp diodes. Or filter caps, if you can afford large enough ones.(I've seen one or two series that *were* rated for ESD; they stipulated their 10kV rating is only achieved when embedded in potting, as you might guess!)Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2024, 11:16:02 am
content: I never found that adding resistors for ESD protection was very effective for the reason T3sl4co1l gives; ESD voltage levels are high enough to jump over the resistor or cause the resistor to breakdown. Adding shunt capacitance helps if your circuit can tolerate it.Diodes and TVSes work, but another option, especially where low leakage is required, aregas discharge tubes. In the past neon bulbs were used and they are still an effective option.

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 24, 2024, 12:05:06 pm
content: That´s also what we had to learn building the ionization sensor: The first series resistor in a highly ESD robust picoampere-meter input needs to be not too small and not too large - it needs to be matched to dissipate the sparks energy as soon as the spark gap or gas discharge tube has ignited. During this time nearly the whole HV drops at this part, so if e.g. 8kV drop over a 50kOhm resistor then it dissipates roughly 1kW - though only for a very short duration, but anyhow far too much for 0805 or even 2W through hole.We used glass coated metal film that were specified for this type of operation. I don't know the supplier anymore.

author: thm_w
author_rank: Super Contributor
timestamp: July 25, 2024, 12:33:53 am
content: BAV199 is commonly used for input protection of thermocouple input amps, should be good in various scenarios.Quote from: dcarolan on July 23, 2024, 04:04:28 pmPlease don't waste your time and others by measuring data from ebay sourced parts, unless labeled as such.You can buy genuine BAV199 for 3 cents each:https://www.lcsc.com/product-detail/Diodes-General-Purpose_Diodes-Incorporated-BAV199-7-F_C155248.html

author: dcarolan
author_rank: Newbie
timestamp: August 07, 2024, 09:01:57 pm
content: A series resistor is to limit the input current so your device doesn't get baked.Yes, it should have a good voltage and power rating. Some of the better ones like Vishay have a pulse withstand rating that is a good thing to keep in mind for overload events.Don

author: peter-h
author_rank: Super Contributor
timestamp: August 07, 2024, 09:31:48 pm
content: I haven't read the whole thread but I get great results for ADC input protection etc with a BAS116. I use two of them, one to GND and one to say +2.7V (that rail is set with a 2.7V zener so it can sink a fair bit of current) and the signal to be clamped goes to their junction. Reverse-biased they leak less than an other diode I tested. As stated early on, 1N4148 leaks badly.Forward biasing a diode is no good; they leak badly even at 100mV Vf. But it depends on the application...

author: TimFox
author_rank: Super Contributor
timestamp: August 07, 2024, 10:47:07 pm
content: Rather than connect a protection diode to ground, where the protection is through a forward-biased junction, I always connected each of two diodes to an appropriate bias voltage to keep them reverse-biased. The best way is to use Zener diodes with a reasonable current through each, sizing the Zeners to pass the required current (above the small quiescent current) when the protection diode goes forward bias. Choice of the protection diodes involves leakage current (with reverse bias) and pulse current rating (with forward bias). With reasonable choice of bias voltages, the reverse voltage rating should not be important.

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 07:03:11 am
content: Yes that is the best way for cheap. Here I am protecting three signals to op-amps which are powered from +3.3V (and have a -0.3V -ve rail)If you need less leakage then you need to use an op-amp voltage follower to drive the other end of the protection diode(s) so the diode has zero volts across it, like one does with the classic guard ring on a PCB, but then you need more components because the op-amp output needs protecting.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2024, 08:12:42 am
content: The low leakage diodes like BAS116 , BAV199 have a rather high forward voltage ( ~ 1 V). Clamping with those diodes along may not be enough for protection, as the OP or µC internal "diodes" will conduct before that. One may want an additional resistor (e.g. 220 Ohm) between the diode and the part to protect to limit the current reaching the chip to a safe level.Clamping to the 2.7 V zener may work, but may still want a diode to the positive supply so that the clamp works better when the power is off.

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 11:45:32 am
content: Yes a very good idea. In fact a depletion JFET between 2.7V and GND, with the gate to -5V or so, would do that bit... Does anybody still make those?2N3819.In the above, the 2k2 resistors are obviously important.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 04:09:25 pm
content: What's a JFET going to do about surge currents..?More to the point, actually, just putting a stonking big bypass cap on the 2.7V will sink ESD just fine. And by "big" I mean a mere 1uF or so. Maybe one per diode pair, if they aren't grouped close together.The difference in clamping voltage with/out bias is so small anyway, I wouldn't even bother, as long as there's a series resistor between diode and IC.There's already C32 there, though it's between lines, and I wonder if the cut-off C31, 33 are comparable values. R23-25 also suggest series resistance, or maybe the signal is coming in another direction, I don't know; perhaps there's another clamping or filtering element ahead of them.Tim

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 04:48:44 pm
content: Yes all those caps are 1uF and yes the inputs are from the left so the 2k2 are the current limiting resistors.The objective was to withstand 24V indefinitely, plus static protection. For 230V AC you have to a bit more workThe JFET idea was to produce a "short" across that 2.7V, to protect the devices in question even when power is OFF (+3.3V rail is not powered).

author: Smokey
author_rank: Super Contributor
timestamp: August 08, 2024, 05:39:21 pm
content: This thread is great. So much info. No matter how much I think I know, you guys never fail to remind me I don't know much. I might need to actually put that SMU to use and check some diode leakages now.

author: free_electron
author_rank: Super Contributor
timestamp: August 08, 2024, 09:39:50 pm
content: wow , this discussion went on for a long time..for completeness : i used a SSTPAD10 diode . They are built for that.

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 09:58:46 pm
content: Impressive spec: 10pA max at 20V Vr. Nice price tooAt pA currents you also need guard rings on the PCB, driven by voltage followers.

author: free_electron
author_rank: Super Contributor
timestamp: August 09, 2024, 01:33:16 am
content: Quote from: peter-h on August 08, 2024, 09:58:46 pmTeflon standoffs. It's was an electrometer style setup. I sort of copy-cat designed a similar input stage like the keithley electrometers. Problem is that the special transistors they use in to-5 ( dual jfets) they used for protection are now complete unobtainium. I have two more of the dual input fets to repair mine if they ever fry. Linear systems is out of stock on themKind of cute , soldering a sot 23 with two legs on the board and pin 3 sticking up in the air. i used special high voltage resistor from susumu. for 2$ i had the protection i needed. There was an even better one with even lower leakage but it was not avaialble and we didn't need it.Companies to remember if you need "exotics"- Linear systems ( NOT Linear technologies, borged by analog devices )- Interfet

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2024, 01:57:41 pm
content: Quote from: free_electron on August 08, 2024, 09:39:50 pmThe PAD series of diodes are JFETs without the third lead connection.If you do not want to test 2N3904s for leakage, then a better low leakage diode is a 2N4117A/2N4118A/2N4119A JFET which is 1 picoamp max, 50 milliamps, and more readily available.

author: peter-h
author_rank: Super Contributor
timestamp: August 09, 2024, 05:51:44 pm
content: Most sites say No longer manufactured. Nice price too - 10-20 quidHere is a summary but no prices:https://www.rhopointcomponents.com/product/components/jfets/2n4117a/I know that company. They specialise in weird analog stuff and super high perf ADCs etc.Interesting parts, where you need that. And the "BC109" TO18 package, which basically needs hand soldering.

author: edavid
author_rank: Super Contributor
timestamp: August 09, 2024, 06:17:50 pm
content: MMBF4117 is still available:https://www.findchips.com/search/mmbf4117Or if you must have through hole, InterFET sells PN4117:https://www.findchips.com/search/pn4117

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2024, 06:38:01 pm
content: The transistors can be low leakage, but rarely very low leakage like the special diodes or small JFETs.The main reason to try the transistor is of one needs a metal case or wants the lower forward voltage and still relatively low leakage.The metal case 2N4117 are getting rare, but SOT23 and maybe also TO92 versions are still easy to get. Depending on the version the test limit for leakage can be different.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 09, 2024, 07:44:06 pm
content: Speaking of InterFET, be mindful that when they say "bulk packaging", they mean "bulk packaging". No one wants a bag of 100 SOT23 parts. Especially not my assembler.(Actually I didn't even try and shove those down their throat... by the time they arrived, the same part was back in stock from onsemi, so I just bought the usual cut tape instead....)

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 09, 2024, 10:23:31 pm
content: Here are some measurements of the forward (measured with Keithley 263) and reverse (measured with Keithley 617) leakages of MMBF4117 compared with BAV199 (Infineon) diode.CheersAlex

author: peter-h
author_rank: Super Contributor
timestamp: August 10, 2024, 06:49:17 am
content: That's amazing - the BAV199 is still below 1pA.But that is at room temp, presumably.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2024, 12:46:06 pm
content: Quote from: peter-h on August 09, 2024, 05:51:44 pmLast time I looked them up they were inexpensive and available. Mouser carries the SOT-23 part:https://www.mouser.com/ProductDetail/InterFET/SMP4117A?qs=1NkvD5QbL4EvVoeJ8vXqaQ%3D%3DAnd Interfet apparently has the SOT-23 or TO-92 for half that if you buy 1000 on a reel.The largest advantage of the 4117A is that the 1 picoamp specification is actually tested. The BAV199 is only tested to 5 nanoamps.

author: peter-h
author_rank: Super Contributor
timestamp: August 10, 2024, 06:38:29 pm
content: How would you wire it up to form a diode with low reverse leakage?I don't think it will match a standard diode e.g. a BAS116

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2024, 07:01:01 pm
content: The arrow indicates the diode. The channel is a common strip of same-doped material: treat it as a resistor between terminals; preferably, short D+S together.It's worth repeating for those readers in the back: when you see a diode in a [traditional, grumble*] semiconductor symbol, it indicates a PN junction.A diagonal line to a substrate also does this; hence C and E in the NPN transistor, where the emitter is a PN with the N going to the terminal (arrow pointing out), and C is implied by symmetry to be the same way.Square lines (like the JFET D/S) indicate ohmic contacts to a substrate.The UJT thus is drawn essentially identically to a JFET, and really isn't much different in construction either: the catch is, in the JFET, the channel is made thin enough to pinch off under reverse bias, while the UJT uses a wide and long channel that can't be pinched off (or at least, not by very much), but where conductivity modulation due to charge injection (forward-biasing the "gate") is exaggerated, hence its negative-resistance behavior. THey chose to draw it with a tilted "gate" to differentiate it from a JFET.In the MOSFET, the channel is indicated with a continuous strip (depletion mode) or three segments (enhancement mode), the substrate with a PN diode pointing at the middle of the channel. Normally, substrate is tied with source, making the familiar three-terminal device.*Grumble grumble: the regrettably-popularmodernsymbol is to draw the MOSFET in analogy to the BJT, where voltages are referenced common to the be-arrowed terminal, andthe arrow points outward for the N-type part. The N-MOS thus becomes a "square legged" NPN. Which as you can see from the above, is... utterly meaningless, you can't have an ohmic AND rectifying contact at the same time, that's a contradiction. What's worse, I've seen dozens of examples where this symbol is confused with the traditional symbol, bringing back the substrate line segment, swapping around which one has the triangle, which direction the triangle is pointing, etc. Major manufacturers like TI can't even get the symbology correct; it's a huge festering mess.Also, note that MOSFETs are often drawn as a MOSFET in parallel with a body diode, but, as you can tell from the above description: the body diode is already there, that's what the triangle is pointing out to you! I suspect, newbies missed this for a long time, so manufacturers started showing the parallel diode to emphasize that's what's in there. But still to this day, you see schematics on the regular where newbies have connected external diodes (even with woefully inadequate ratings compared to actual body diode ratings, i.e. they'll never carry significant load current anyway) across symbols that show two diodes already. The solution was never -- and will never be -- increasingly ornate symbology; newbies just don't know how to parse those symbols at all, they're just " means ", and I mean what else do you have to go on if no one ever discusses the meaning of these things -- and I have never seen an appnote mentioning it, and I doubt many curricula discuss symbology in common use either. So, I have made it a point of adding this lecture whenever semiconductor symbols come up -- because they are, in fact, quite descriptive and physically relevant, and you can make sense of them, given a little knowledge of the bits used to draw them!Tim

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 10, 2024, 07:18:28 pm
content: Quote from: T3sl4co1l on August 10, 2024, 07:01:01 pmStronglyagree with this. I used to struggle with this stuff until I found some version of this lecture online, and now I have no trouble. My excuse was that I wasn't educated as an EE (see username), but that doesn't seem to have had anything to do with it. Nowadays every now and then I end up mentoring an intern or junior engineer, and theyuniversallyalso have the same problem. Then they get the lecture (which is great on a whiteboard -- erase the bits of a depletion MOS channel and it really makes an impression) and they always seem to get it.Why is this always missing from EE education? (OK, I worked for universities for a while... maybethisisn't the place to start.) Whatever it is, if there are any senior engineers out there mentoring juniors: please find a way to make sure they receive this knowledge. It really does help.

author: peter-h
author_rank: Super Contributor
timestamp: August 10, 2024, 08:56:02 pm
content: OK so the SOT-23 4117 diode would be reversed relative to a normal diode.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2024, 11:35:49 pm
content: Quote from: T3sl4co1l on August 10, 2024, 07:01:01 pmBut the single body diode shown depends on the substrate being shorted to the source, which is common but not universal for power devices. There should be *two* body diodes shown from the source and drain to the substrate, with one shorted out by the source to substrate short.The other diode symbol matches with the parasitic bipolar transistor which is present, and nominally has a low resistance between its base and emitter to prevent activation. Activating the parasitic bipolar transistor used to be a big problem with power MOSFETs which could lead to secondary breakdown and destruction.

author: free_electron
author_rank: Super Contributor
timestamp: August 13, 2024, 09:59:09 pm
content: Quote from: T3sl4co1l on August 10, 2024, 07:01:01 pmi like the semiconductor way of drawing a mosfet. p-fet has a dot on its gate (like a not gate) to indicate you pull it low to activate ( drive with zero ) . an N-fet does not. you drive it high.Source, drain... all nonsense in the semiconductor world. the channel doesn't care, the gate doesn't care. it matters where you tie the bulk or doped region. The trillions of mosfet in that GPU or hex-core Ryzen don't have a paralell diode.Quotehold it ... that should only be explicitly drawn in the symbol if there IS an actual ( not the body) diode in the package. Many power mosfets have an actual diode in parallel witht he body diode. the body diode is not made to carry high current and is often "slow" . the arrow to/from channel tells you where the bulk is tied to, and that becomes the "source".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2024, 10:32:54 pm
content: Yep, the symmetrical IC symbol works fine, and just something to show P/inversion suffices. Substrate is implicit in CMOS, and can be drawn in when non-VDD/VSS connections are used. The same thing with an "emitter arrow" though...Quote from: free_electron on August 13, 2024, 09:59:09 pmI'm not aware of any non-body diode MOSFETs, that aren't co-pack (or maybe they're monolithic or even interleaved) schottky?The body diode can definitely handle as much forward current as the channel, it's only limited by power dissipation. Haven't seen a single datasheet that said otherwise. It would be very hard not to, I think, unless the device fails because of thermal runaway (imagine, a monolithic diode that can't diode?); AFAIK, injected minority carriers are so much higher concentration than majority carriers, no matter how tightly you pack the trench channels.My understanding is, the body diode is intrinsically poor, a side effect of the doping needed for voltage rating and MOS function, and so they kind of just do as well as they can. Usually with two options, fast and slow (and neither one all that good compared to lifetime-killed PN diodes), and this is also inversely correlated to avalanche robustness.Conversely, it's better at low voltage, hence the acceptable recovery times there. Though the faster switching (maybe) and less tolerance of voltage drop (i.e. mostly doing synchronous rectification at lower voltages) make this not very useful or important in practice.Or there's those weird-ass Microsemi? modules with the series diode then antiparallel FRED just to waste your forward voltage, but if you have a very reactive process with arbitrary switching, it might pay off compared to body diode recovery. Anyway, that's pretty clearly a co-pack... I think??Or parts that technically aren't MOSFETs, like GaN HEMTs, but yeah, can't have a substrate diode if there's no substrate, *touches head*.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2024, 01:16:19 am
content: Quote from: free_electron on August 13, 2024, 09:59:09 pmThe body diode is the collector of the parasitic bipolar transistor, which has the same voltage and current capability as the MOSFET since it encompass the entire area of the MOSFET. It is not the fastest, but is considerably faster than a standard recovery rectifier. Long ago power circuits sometimes used power transistor base-collector junctions as power diodes because they were better than the purpose made diodes at the time.The emitter is shorted by the source to substrate short, which should prevent activation of the bipolar transistor, however the large area adds significant resistance between the short and a lot of the emitter. Thyristors have a similar problem where the bipolar transistor can still be activated unintentionally.The body diode failures I have seen all involved allowing it to be forward biased, and then shorting it out with the MOSFET or allowing reverse conduction at high currents leading to high power dissipation during reverse recovery.Quote from: T3sl4co1l on August 13, 2024, 10:32:54 pmIgnoring small signal devices, discrete 4-pin power parts with a separate substrate connection exist and have their uses when voltage blocking capability is desired in both directions.https://www.microchip.com/en-us/product/mic94030Vishay had one but it is apparently discontinued, and it looks like Microchip is going that way also.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2024, 01:45:34 am
content: Oh yeah, those oddball things. Saw that years ago, didn't write it down, lost track of it. No suppliers care about substrate pins so they're impossible to find.Of course in that case, the substrate is still diodes to D and S; just not from S(+SS) to D as for the 3-terminal caseTim

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2024, 06:44:42 am
content: Quote from: T3sl4co1l on August 14, 2024, 01:45:34 amSeems Microchip has a newer one so they are still available, and at a reasonable price:https://www.digikey.com/en/products/detail/microchip-technology/MIC94050YM4-TR/1031450There are also 3N and SD series devices, but they are small signal like a JFET:https://www.mouser.com/c/semiconductors/discrete-semiconductors/transistors/mosfets/?q=3n&id%20-%20continuous%20drain%20current=150%20uA~~200%20mA&number%20of%20channels=1%20Channel&package%20%2F%20case=SOT-143-4%7C~TO-72-4&technology=Si&instock=y&sort=pricing%7C1&rp=semiconductors%2Fdiscrete-semiconductors%2Ftransistors%2Fmosfets%7C~Id%20-%20Continuous%20Drain%20CurrentQuoteExactly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2024, 06:54:43 am
content: Yeah, the Linear Integrated Systems product lineup has some interesting (archaic, specialized, etc.) parts in it... and pricing to match!Advanced Linear Devices too, e.g.https://www.aldinc.com/pdf/ALD1101.pdfAlso their EEPROM one-time-programmable offset parts, which is...fascinating, but very boutique appeal; not sure what I would even use them for, but it's cool that they exist.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2024, 07:44:17 pm
content: Quote from: T3sl4co1l on August 14, 2024, 06:54:43 amI think I have some samples from ALD. Those were more important back when digital correction with microcontrollers and DACs and stuff was more expensive.There are some other manufacturers. Calogic is suppose to havevarious goodies, including the 3N devices. I thought there was someone else but they are not in my notes. Calogic changed their website since I last checked them so maybe that is it.I usually see the small signal 4 lead MOSFETs in high resolution multimeter front ends or other similar test instruments.

author: free_electron
author_rank: Super Contributor
timestamp: August 17, 2024, 03:09:45 am
content: Calogic is still around.All these guys like Linear Systems, Interfet sit in the same backyard...Calogic : 237 Whitney Pl, Fremont, CA 94539Linear Systems : 4042 Clipper Ct, Fremont, CA 94538Micross : 2520 Junction Ave, San Jose, CA 95134 USAFMD ( Fremont Micro Devices ) : 42982 Osgood Road Fremont, CA 94539

author: mawyatt
author_rank: Super Contributor
timestamp: August 17, 2024, 01:00:24 pm
content: There were some versions of Si on Insulating substrates, like the early 70s SOS (Si on Sapphire). Vaguely recall a version of the RCA 1802 in SOS, and later Peregrine for switches and synthesizers (SOS version of National synthesizer chips).Recall Harris had an Isolated substrate process called Bonded Wafer in their UHF1 and UHF2 processes. The wafers were created by using a pair of wafers and growing an oxide on one and bonding the two wafers together. The top side was then thinned before wafer processing on an effective insulating substrate. We delved into this, but moved to a new Bell Labs Complementary Bipolar process, then later to IBMs SiGe BiCMOS for specific reasons including bipolar device speed and available CMOS.IBM early on had developed SOI (Si on Insulator) and the PowerPC was implemented in this SOI CMOS process (we tried to get others to consider these for Rad Hard applications, sad story we can discuss if interested). IBM also developed some of the early CMOS FinFets in SOI which became the limited factor as the thermal impedance of single SOI FinFets devices was higher to substrate than Junction Isolated FinFets. Since thermal power density was the limiting factor (suspect still is) in digital device density, IBMs early FinFets never became popular as others chose the Junction Isolated FinFet technology path.Also certain CMOS processes have available Triple Well NMOS device, here the NMOS sits in a Junction Isolated "tub" from the chip substrate, with connections to such. They have certain advantages, for example analog/RF/MW switching which we took advantage of a couple decades ago.Best,

author: ArdWar
author_rank: Frequent Contributor
timestamp: September 12, 2024, 08:15:13 am
content: Quote from: T3sl4co1l on August 10, 2024, 07:01:01 pmIf anyone wondering:https://www.nexperia.com/product/GANB4R8-040CBABack to the original topic. How much can I trust manufacturers SPICE parameters to derive these forward current characteristics? Just as a first approximation before I buy a bunch of them to individually characterize.The idea is simple. Since I don't think anyone present IB/VBEdata in easily comparable tabulated format, parsing and comparing SPICE model parameters is much easier and faster. Given Shockley equation, pick one with small IS for lower "knee" current, lower NF for steeper curve, reasonable ISE for decent current floor, etc etc.The problem is that those models sometimes have unexpected values and spread.ISvalues sometimes varying by several orders of magnitude between manufacturers of an equivalent P/N. Some models having negativeXTIvalue, which I'm not sure if that's deliberate or not considering NXP's rather sophisticated models, and so on.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2024, 08:47:52 am
content: Quote from: ArdWar on September 12, 2024, 08:15:13 amStarting point:https://www.eevblog.com/forum/beginners/please-help-understanding-relative-permiivity/msg5586663/#msg5586663

author: Njk
author_rank: Frequent Contributor
timestamp: September 13, 2024, 12:58:54 pm
content: Thanks for the discussion. Interesting fact: for some Zener diode vendors, it was common to specify the min. and max. currents in the absolute maximum section of the data sheet. Because of that, it was mandatory for designers to make sure the actual current is not less than the specified min. value

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 13, 2024, 01:02:34 pm
content: Quote from: Njk on September 13, 2024, 12:58:54 pmHopefully they have been delivered with a battery supply. And how do you solder the diode without interrupting the current???

author: Njk
author_rank: Frequent Contributor
timestamp: September 13, 2024, 02:12:00 pm
content: Just nonsense for an engineer. But if there are a design rules in the organization, e.g. for every component, the electrical load shall not exceed 30% of its rating for each the parameter, and you have to get the design passed the formal review, that nonsense can create a problem

author: arijav
author_rank: Contributor
timestamp: October 04, 2024, 11:16:50 am
content: Hello, I am having a similar problem. In my case I have to perform leakage measurement tests of semiconductors up to 1000V down to 1nA. I have a setup with a logarithmic amplifier but I need to protect its input, which is at stable 0.5V. I have tried putting an InterFET SMPPAD1 Diode between the input and ground in forward direction. The issue is that it seems that at 0.5V I have a leakage of around 8nA (very different as the stated 1pA in reverse). I have tried as well to put 2 Diodes, one from the input to the guard, which has the same voltage as the input (+/-20mV) and the guard to ground, it improves but I still have 6nA leakage.Is there any better suggestion to protect my input (while keeping the 0.5V constraint) and still being able to measure 1nA?Thanks

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2024, 03:25:25 pm
content: Quote from: arijav on October 04, 2024, 11:16:50 amIf I understand you correctly, that is the circuit change I would have recommended. If it did not improve the leakage significantly, then could there be another leakage path?Could your InterFET SMPPAD1 diodes be fakes?

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 04, 2024, 06:10:28 pm
content: Quote from: arijav on October 04, 2024, 11:16:50 amIf you don't need very fast transient response and can live with the input impedance, then I would propose such a setup:The resistors in general prevent super high currents that magnetically couple to somewhere else. The gas discharge tube (GDT) does most of the work by dissipating the energy of ESD or high voltage arcing. The cap smooths the transients to a rise time that can be handled by the following diodes.With this setup I could successfully protect FET-inputs of opamps from 8kV-HV-pulses that regularly arced over in a ionization sensor. Most of the time we just used a neon-indicator bulb as a GDT and it had no measurable leakage current. For the resistor between the GDT and the cap we needed to take a high-power high voltage type because usual small signal resistors also flash over at 3kV.

author: arijav
author_rank: Contributor
timestamp: October 04, 2024, 09:18:56 pm
content: Thanks for the answers. The SMPPAD1 have been bought some days ago over Mouser, so they should be the real deal. I am wondering if the leakage is provoked by the rework soldering to fit the diodes, because I was specially disappointed with the results of the test with the 2 diodes. Since the guards have roughly the same voltage as the input I was expecting an If in the range of picoamps. I will try to rework another channel, this time with blackened white or blue LEDs with high Vf and check if the results improve. I am using as LogAmp the ADL5303 by the way.Regarding the GDTs, it seems a nice solution but my boards need to fit 3 channels in one Eurocard and with them it will be tricky, as I have as well some overcurrent protection circuit which opens Reed-Relays (in the future maybe compact SSR) when the current approaches 10mA.I am wondering if I just add a 80K resistor in series and check if the protection is sufficient without diodes. Any thoughts on this?Any other black magic suggestion to solve this challenge is very welcomed!

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 05, 2024, 11:08:23 am
content: A GDT for this purpose can be just a small neon indicator bulb like in a power switch, not something big as for lighting...A TVS-diode may work too, at voltages <1V they also have very low leakage current (<1nA is typical)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2024, 02:30:27 pm
content: Some resistance that can withstand the high voltage is definitely needed. Bootstrapping the diodes, so that there is not much voltage at the low leakage diode is definitely a good idea. My choice would be something like BAV199 for the low leakage part and a 2nd pair for the bootstrapped voltage.A dirty PCB can also show quite some leakage.The ADL5303 itseld may have some bias current. It may not be the best choice for currents below 1 nA. For the stability the RC combination at the input is likely also important.

author: voltsandjolts
author_rank: Unknown
timestamp: October 05, 2024, 03:06:50 pm
content: Quote from: arijav on October 04, 2024, 11:16:50 amThe stable low input voltage is an advantage. Maybe something likethis.

author: arijav
author_rank: Contributor
timestamp: October 05, 2024, 04:51:13 pm
content: Great inputs from everybody. I will definitely have a look to the small GDTs and low leakage TVSQuote from: Kleinstein on October 05, 2024, 02:30:27 pmI will try other diodes like the BAV199 from the Input to the Guards and later to ground maybe TVS. Regarding the ADL5303, with the PCB as it is I have measured down to 700pA with a one teraohm resistor and a 700V HV source. The board itself seems to work quite good and the ADL should be able to read down to 100pA if well designed. The issue are the dirty modifications for the protection diodes. Nevertheless, would you suggest something different to the ADL5303? I am open to explore other designs. I need to be able to measure 1na till 10mA