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# How to plot the I-V curve of a tunnel diode?

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Question

Brief description of the problem

How to plot the I-V curve of a tunnel diode?

Background

I am curious to know how the tunnel diode works. So I googled and wikied and after a couple of hours reading, I found some related ideas:

(1) A tunnel diode shows the negative resistance characteristics.

(2) A tunnel diode can be used to build a very high frequency oscillator.

Problems encountered

I have a very rusty EE diploma from Hong Kong Technical College, where I learned basic electronics circuit theory and practice. I once earned my living as a electronics technician. The electronics industry in my place sadly collapsed long long time ago. So electronics to me now is just a hobby, but I am still curious and interested to know more about electronics, and my recent interests are negative resistance and tunnel diode.

I must confess that I do not have the prerequisite knowledge to understand the negative resistance theory, because I don’t have adequate solid background in ideas such as differential voltage, dynamic resistance etc, which I think are essential to fully understand the concept of negative resistance.

Learning by doing

A couple of years ago I was happy to learn the MIT offered free online courses including EE. So I took a foundation course, but I dropped out very soon, because could not keep up with the very fast pace of lectures and tutorials, and most important of all, I could not attend the hardware and software labs and experiments.

But one of the few things I learned from MIT is the motto: “Learning by doing”. My experience as an electronics troubleshooting is that there are theoretically abstract things which you might not fully understand if you only study the equations and graphs, but if you use a multimeter and to measure and display current and voltage values, and use a scope to watch the variables’ time dependence, then you learn faster, deeper, and remember theory longer. Another thing is that if you, say, design a circuit, try it, and if it does not work, but you learn from your failures, and as Thomas Edison says, “I have not failed. I’ve just found 10,000 ways that won’t work.”

Research so far

Now let me explain and summarize what I have been doing so far:

(1) Objective – Plot the I-V characteristic graph of a tunnel diode.

(2) I have already bought a couple of cheapy tunnel diodes (part number 2SB3, US\$3 each)

(3) I have also got a couple of ordinary diodes 1N4148 to compare and contrast, sort of paring/swapping testing/caribration/troubleshooting.

(4) I have the following equipment to do the measurements:

a. A couple of chaapy US\$4 or so, multimeters that can measure down to 200mV and 200 uA.

b. A digitally button adjustable 0V ~ 10V, 3A regulated switching power supply, in increments/decrements of around 0.01V (10mV).

(5) The setup and testing procedure are simple:

a. Connect PSU in series with a protective, current limiting resistor, and the tunnel diode.

b. Increase/decrease voltage level, in steps as fine as mV, and measure corresponding current values, as fine as 0.1mA

c. Plot the I-V graph, and day is done.

Failure analysis

Well, my experiment of course failed flatly, and that is why I am asking here for help. Originally I expected to get a I-V characteristic graph, as we see in all datasheet of rectifying or flyback diode, an exponential curve starting from origin, starting shallow, then goes steeper and steep as voltage increases.

I did expect that for the tunnel diode, there should be something weird in the middle of the curve, so called negative slope which denotes the negative resistance, …

Let me summarize the characteristic of the tunnel diode, before I describein detail why I failed:

Tunnel diode 2SB3 datasheet summary:

a. Ipeak voltage ~= a few mV.

b. Ivalley voltage ~= a ew hndred mV

c. Ipeak ~= tens of mA

d. I valley ~= don’t know cannot measure!

Now the failure details is the following:

a. As I increase voltage from 0 to IpeakV, the current increases as expected,

b. But once approaching the IpeakV, the current suddenly jumped from tens of uA to some 400/500mA. In other words, I just missed the most important measurements, those of the negative resistance region.

Now my questions:

Why I cannot measure the current I as soon as the gradually increasing voltage V enters the negative region? How can I tell the tunnel diode not to “skip” the tunnel?

Discussion

This is the very first question I submitted in CoDidact. Please feel free to comment or suggest to improve my question.

References

Appendices

Appendix A – Negative Resistance Graph

Appendix B – The Test Circuit Design

Actually the first problem that came up to my mind is how to get a stable voltage source to power the tunnel diode. I vaguely know that the test range should be between 0V and perhaps at most 1V, and the negative resistance region should be somewhere between 0.2V to 0.8V. This is just a wild guess, because I don’t have a datasheet.

I don’t have any low voltage PSU in hand, and using a simple voltage divider consisting of one resistor and one stupid analog pot should be face losingly damage my reputation, and all my bad maker friends would LOL.

So the first thing I considered is to use an op amp to step down from say, 0V to 10V to 0V to 10mV.

Ah, bed time, so I just upload my final design picture here (schematic later), and explain how I came to this quick and dirty, sloppy design tomorrow. 🙂

Appendix C – Tunnel Diode 2BS3 Sample 2 Measurements and Curve

Notes

a. Manual measurements not that accurate.

b. The negative resistance region is somewhere between 25 mV ~ 450 mV

Appendix D – @circuit fantasist’s Tunnel Diode Curve Plot

@circuit fantasist’s experiment, explaining the tunnel diode is in bistable state, so the negative resistance region is not stable, resulting the tunnel diode flip from one stable state to another. In other words, the tunnel does following the I-V curve all the time, it is moving too fast, so not observable by my stupid human eyes, and not even by the fast 50MHz Tektronix scope, … 🙂

Appendix E – @tlfong01‘s Tunnel Diode Curve Tracer Circuit Design v0.1

I am thinking of using ICL8038 sig gen for the following reasons:

a. I now know that the tunnel diode 2SB3’s current is of order of a couple of mA, and the total sweep is only 0V to 0.5V (as I said earlier, I guess the negative resistance region is between 24mV and way below 450mV)

b. ICL8038 sig gen can sink 25 mA max, so there is no worry that the signal source would be overloaded and it signal distorted.

/ to continue, …

1 day agotlfong01‭ 14diodetunnelCC BY-SA 4.0    27m ago Copy Link History Edit Close Delete Flag

It seems that your question could be condensed into about 30 words plus a relevant data sheet link but, instead you write an essay and link to some very dubious pages of a person who, in his own words, has been banned by “orthodox wikipedians”. Very dubious posting indeed. — Andy aka‭ 1 day ago

This is a good question, but Andy is right. You really could simplify it a lot. — Olin Lathrop‭ 1 day ago

tlfong01, Congratulations on your interesting question! In addition to everything I wrote on the basis of thought experiments, here are two movies of real experiments – Tunnel diode 1 and Tunnel diode 2. It is interesting that I shot them exactly today, 3 years ago. As can be seen, the total resistance in series is quite high because the tunnel diode operates in a bistable mode (like a latch)… — Circuit fantasist‭ about 22 hours ago

… Next week I have a lab exercise with my students during which they will investigate all sorts of diodes with this experimental setup. We will try to see the region with negative resistance of a tunnel diode as well. — Circuit fantasist‭ about 22 hours ago

I agree with Olin and Andy. This isn’t intended to be a social media site or a place to preen. You could try facebook or reddit if that is what you are looking for. — Elliot Alderson‭ about 22 hours agoShow 12 more comments · Add a comment

ScoreActive+3−0

Apparently you want to measure the current/voltage relationship of a tunnel diode. The tricky part is that the voltage isn’t unique for currents over parts of the range. From your question:

Note that the current is still unique as a function of voltage. One solution is therefore to sweep the voltage and measure the current.

This needs to be done carefully, since small changes in the voltage can result in large changes in the current. You also need the voltage source to be very stiff (low impedance) so that the wildly changing current drawn by the device doesn’t cause control instability.

One way to achieve this is with high open loop gain and negative feedback. If you are only feeding back the output voltage, and the control signal to drive the pass element is reasonably insensitive to the resulting current, then the system will be stable.

I haven’t done this, but I’d probably start with an opamp driving an emitter follower, where the emitter voltage is fed back and is therefore ultimately what is regulated. The opamp output drives the base. The base voltage won’t need to change much, even with large changes in the load current.

You might also want to add some current limiting so that you don’t fry the device under test when the voltage goes a little too high. This limit needs to be above I1 in your diagram.

Another approach is to put a resistor in parallel with the diode under test. Make the resistor small enough so that the combined diode+resistor exhibits positive I/V slope over its whole operating range. You measure the I/V characteristics of the combined device, then do the math on the result to deduce the I/V characteristics of just the diode. This approach will require good accuracy, since the resistor is essentially adding noise to the desired signal. You still need to have sufficiently accuracy left after subtracting off the known “noise” signal.1 day agoOlin Lathrop‭ 1823CC BY-SA 4.0 Copy Link History Suggest edit Delete Flag

@Olin Lathrop, many thanks for your advice. (1) About this TLDR question – I agree that my question is too long. One way to shorten it is to summarize all the long paragraphs into a 2,000 words article, with perhaps 10 pictures and move it from Q&A section to the post/paper section, IF I finally arrive at something useful, either success or failure. Or I can move everything to a GitHub site and write up a summary here. / to continue, … — tlfong01‭ about 24 hours ago · edit · delete

(2) About using Op Amp for stability and low internal source, resolution, current limiting, etc – I am glad to let you know that I agree with everything you said. This test setup design is a long story, so let me give more details in the next comments. / to continue, … — tlfong01‭ about 24 hours ago · edit · deleteAdd a comment+2−0

## How to test a Tunnel diode in 25 words or less.

With DC bias = ~490mV with fine tuning and a small signal swing of 60mV you can generate a IV negative slope of -16 Ohms.

about 17 hours agoTonyStewart‭ 49CC BY-SA 4.0    ~17h ago Copy Link History Suggest edit Delete Flag

The title has the simulation link with sliders — TonyStewart‭ about 17 hours ago

The signal can be a sine, or triangle and at any frequency without flicker on a DSO, software scope or Falstad simulator. — TonyStewart‭ about 17 hours ago

Attractive experiment… If the load line could be shown… — Circuit fantasist‭ about 15 hours ago