Not being able to get a N MOSFETs to work on a curve tracer - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 29, 2020, 09:11:39 am
content: I got a B&K curve tracer that apparently works fine. According to SM all signals are perfectly fine.I am able to test NPN, PNP, ZD, D, R, JFET... etc... never had a problem.But everytime I try to test a N MOSFET as Negative ( got a bunch of new ones) I get just a straight line. And when I then invert to test as Positive then I get a weird vertical curve. See video bellow.Also by looking at the symbol of the MOSFET there is a diode between S and D. It that diode behaviour the reason for what I am experiencing?As I mentioned I never had a problem testing a JFET. Also A JFET do not have this diode behaviour between S and D.

author: exe
author_rank: Unknown
timestamp: April 29, 2020, 09:56:01 am
content: Quote from: gkmaia on April 29, 2020, 09:11:39 amWell, yes, if by behavior you mean switching it into PNP testing where it is forward biased.I assume that the vertical scale is for current, and the wiring is correct. In this case I'd say the curve tracer doesn't push enough voltage into gate. It should be at least 2.6V (according to your transistor tester), practically even higher than that (I'd say at least 5 volts). I can't see the name of your tester and labels, so I have to ask: what is the maximum gate voltage it can provide? And what's the name of the puppy, I want the same.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 02, 2020, 09:24:26 pm
content: In the video. The first trace you see, that is just an horizontal line is the one that I cannot understand. The tracer is set to NEGATIVE there.I should be seeing something like the N channel plot bellow. Instead I just see a single horizontal line. That grows as voltage is applied.And as no current flows through the signal does not grow vertically and all steps are comprised in one single row as you can see.For example when I tested a N-FET I got the perfect plot. Voltage applied and current flowing as steps progress.But when it comes to any N E MOS I cannot get the tracer to separate the current steps.The second graph you see shows is on POSITIVE mode (I am reverse biasing the MOS) and separated steps of voltage with negative current. I also cannot understand why it is displaying it like that.So... when testing NPN and PNP I am increasing the at each step and on my oscilloscope I see X as voltage and Y as current.Then when testing a N E MOS as I am increasing voltage as each step, should I invert my channels? Then get x as current and Y as voltage?The tester is this one. There are other cheaper ones but get no were near this one. Then only problem is that is cannot apply more than 5volts to components. That is quite limiting if you want to stress diodes for example.https://www.aliexpress.com/item/4000116837945.html?spm=a2g0o.productlist.0.0.2a903c99wEPDj5&algo_pvid=6fc5a626-55dc-4f5e-b0b1-12c074ccfac8&algo_expid=6fc5a626-55dc-4f5e-b0b1-12c074ccfac8-28&btsid=0ab6f82315884546389875713e47da&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

author: exe
author_rank: Unknown
timestamp: May 03, 2020, 07:50:15 am
content: What voltage does curve tracer apply to the gate?

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 03, 2020, 09:40:17 am
content: sweep voltage on 100 I get:+65v on NPN mode and -65v on PNP on C/D.At B/G:I get -7.1v on PNP and 10v on NPN from 1ua per step up to 2ma per step. This positions are for testing BJTs.I get 0v at IDSS/ICES.At 0.05volts per step I get 125mv. Then at .1v/step I get 250mv. Then at 2v/sI get 500ma, then at .5v/s I get 1.2v. Then at 1v/s I get 2.5v. If switched from N to P the polarity is reversed. these positions are for testing field effect transistors.B/G is pretty much ground reference.By looking at VT on my tester I get 2.6v. And as I measured my tracer applies a max of 2.5v @ 1v/step. That is what you mean right. If I by a NEMOS with a VT of less than 2.5v it will work on my tracer? And that is what the plot bellow would represent. A voltage above 2.5v current starts to flow through the drain. SO it is similar to a diode and instead of a .6v VT as a traditional S-diode this NEMOS needs more than 2.6v at the gate to allow the drain to flow. As the drain does not flow the tracer only displays a horizontal progress, no vertical progress as the current at the D is 0.Is that the logic?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 03, 2020, 12:04:01 pm
content: Can you re do that video in a viewable aspect ratio?

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 03, 2020, 05:06:12 pm
content: I would first validate the simple tester by using a DVM to test the diode, making sure about the drain and source terminals. Then I would test it with bench power supplies, one driving the drain and source with a resistive load and another supply driving the gate. That would eliminate any possible mysteries.What is the transistor number?

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 04, 2020, 01:09:19 am
content: Thanks for all the feedbackdatasheet is herehttps://datasheetspdf.com/pdf/608847/ToshibaSemiconductor/K3934/1First video is pretty much the issue I have when forward biasing the transistor where no current steps are shown.The maximum voltage the tracer provides to the gate is 2.4volts at 1 v/step mode.Second video shows using a power supply to provide gate with voltage and then I get current flowing on my curve tracer.