Pete Lefferts LED current source

Phil, my post on the Lambda "diode" got cut off here is the description:

Phil,

Connect the source of the N channel to the drain of the P channel then connect the gate of the N channel to the source of the P channel, then connect the gate of the P channel to the drain of the N channel.

The cathode is the P channel source.

Depending on the parts used, around 4 volts of bias you start to get a nice negative resistance, and with decent parts, much more current capacity then a TD.

Enjoy, Steve

Those are pretty interesting too--I've seen them discussed for use in protection circuits but never for anything else, mostly because the threshold voltages of JFETs are so very poorly controlled. What else are they good for?

BTW I looked up the 1N2929 tunnel diode (I_p = 1 mA), and was very surprised to see it listed with 150 pF of capacitance! Does anybody have a real datasheet for it?

Thanks,

Phil Hobbs

Phil Hobbs wrote:

Oh I'm only using the transistor for it's base emitter diode. There is no gain in the circuit. I measure the forward voltage drop with currents from 10nA to 1mA. When plotted semi-log you get a nice straight line of slope kT/e. You can do the same for the base collector voltage but the slope is further from ideal. And as you said previously, small signal diodes show even larger slopes. I've heard the base emitter junction is more aburpt and thought this might have something to do with the "better" behavior of the b-e junction. Perhaps less carrier recombination in the depletion region, but now I'm speaking of things of which I know very little. I would like to know what transistor specification might lead to more ideal V_BE behavior and I'll try some of the high beta's. Though I also wonder if higher speed transistors might be better. Oh, the emitter resistance is something I thought about only recently, at 10nA the "diode" has 25 Meg of source impedance, I must have had an opamp buffer in the circuit beofre the voltmeter else I would have noticed non-exponential behavior at the low currents. I hoping to use these down to 77K, but the plastic packages may not survive repeated trips into the liquid nitrogen and I'll have to "fall back" to just using the glass encapsulated diodes which seem to survive just fine. So finding a better transistor is not an issue until I know that the packages will work.

Thanks for the suggestion,

George Herold

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Say you have I =3D Is exp (qV/nkT), where n is the ideality constant.

See the GE Transistor Manual, seventh edition (1964), pp. 439-442. They mention that (for your kind of currents) n is about 2 for gold = doped=20 and can approach 1 for non-gold doped. The "impurity gradient" is also mentioned, but I don't see that they explain its effect.

Many years ago I did measurements similar to yours and found that diodes I'd expect to be gold doped (such as 1N4000 rectifiers and

Does your "error is a little less than 1%" mean that you are measuring n = of 1.01? If so, I'd be surprised if you could do closer to 1 than that.

--=20 Regards, Howard

PN junctions are going to be a problem at 77K, because the minority carriers freeze out.

When you said "diode-connected transistor" I thought you meant that you had the base and collector shorted, which is the usual usage AFAIK. If not, do that right away. Diode-connected transistors (my sense) make very much better logging devices and temperature sensors than diodes do--even when the diode in question is one junction of the same transistor.

Cheers,

Phil Hobbs

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"Does your "error is a little less than 1%" mean that you are measuring n of 1.01? If so, I'd be surprised if you could do closer to 1 than that."

Yes, Exactly. n < 1.01 for V_BE of 2N3904 transistor. Hmm, 1/(n-1) is suspiciously close to the Beta of the transistor. I definitely have to order some of the high Beta tranistors recommended by Phil H. I'll admit I don't really understand transistors and what factors determine the current gain. In fact it's only recently that I feel like I'm beginning to understand the humble diode.

George Herold

messagenews: snipped-for-privacy@x16g2000prn.googlegroups.com...

Laser noise cancellers can get up to 70 dB of cancellation (1 part in 3000 current error) with unmatched transistors running at very different current densities, which shows that BJTs follow the Ebers-Moll law very closely, until the extrinsic resistances become important. If your error is as much as 1%, you're not getting the performance available from your 2N3904s.

If you connect them as diodes (base and collector shorted), they actually run as normally-biased transistors--in fact that's how the noise canceller uses them. If your error were 0.1%, I'd begin to believe that it might be the transistors, but 1% is _way_ too large. That's definitely a circuit problem or perhaps a thermal leak--remember that the transistor leads are roughly

National has a useful temperature control handbook that talks a lot about the problem of temperature measurement, and admits in so many words that IC temperature sensors (and packaged transistors) measure the temperature of their leads.

BJTs are really amazing devices--they follow their simple models essentially exactly, unlike FETs.

Cheers,

Phil Hobbs

messagenews: snipped-for-privacy@x16g2000prn.googlegroups.com...

Ebers-Moll is not a law but merely model of how a transistor works. Spice programs have moved to Gummel-Poon models for BJTs and have gone through some 4 levels of MOS models.

Besides, ggherold should also try diode connected transistors, it would extend the plots by an order of magnitude or two, with better conformance. It is too bad that the V(be) reverse bias breakdown does not change, or this would be a more popular configuration.

The laws of thermodynamics are models as well. I'm really not intending to jail 2N3904s for logging across state lines. (Using 'law' to refer to a rule applying to any non-human thing is a metaphor in any case.)

Ebers-Moll is good to a couple of parts in 10**4 or better for small signal transistors if the CB bias is fixed, the transistor isn't saturating, and the collector current is low enough that the extrinsic resistances aren't important. (Even if they are, you can compensate for them accurately because they're real ohmic resistances.) This is the usual situation in logging applications, and is often true with diff pairs as well.

Phenomenological models with a bunch of adjustable parameters can fit just about anything you like, and for more general circuit simulations they are certainly superior. However, I rather doubt anyone has ever invented a circuit by using Gummel-Poon in his head.

One of the things I like best about BJTs is that they really really follow their simple models for noise and DC, so that I can toss them around however I like and be pretty confident how the circuit will work--generally to a fraction of a decibel. Accurate, analytically tractable models are key to all my best circuit hacks--I use about 10 times more algebra than I do SPICE. (I've used LTSPICE a couple of times, but my favourite SPICE is an ancient Windows 3.1 program, because I did a lot of work on its library back in the day and it has a nice schematic editor.)

If I were building SMPSes or discrete power amps I'd do a lot more modelling, of course. I care about the understanding and the results first of all--whether I use pliers or a Crescent wrench is a lot less important.

Cheers,

Phil Hobbs

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Thanks Phil, I'll look at your noise canceller circuit again, and check out the National temperature handbook. When I did these measurements I did not have the base and collector shorted! I was looking at both V_BE and V_BC as a function of current and temperature. So it looks like a made a circuit error. (well maybe not an error but I now have My ignorance of transistors fundementals is now reveled... I don't "quit" understand the difference between shorting the base and collector versus leaving the collector open. I now see that this is the same trick that is used in current mirrors. So is it true that in a transistor with the base and collector shorted most of the current is flowing through the collector?

The warning about the thermal path via the leads is something I learned long ago... I started my career doing low temperature physics.

But say if you think I might be able to get 0.1% accuracy that would be wonderful. I really start to be concerned with the accuracy of the sensor I'm comparing the transitor with!

George Herold

George,

I'm not a semiconductor expert at all, as I've already pointed out, so take the following with a good shake of salt.

The switchover from kT/e to 2kT/e behaviour in a diode comes from high-level injection, which happens at surprisingly low current densities. It's due to the bias current being so big that it modifies (or even dominates) the equilibrium carrier density.

Since the base of a normally-biased BJT works almost like the grid of a tube, there's no opportunity for the carrier concentration in the base to get up to those sorts of levels. When you use just the BE junction, you get into the high level injection condition at really low current densities, because there's a huge amount of stored charge in the base region.

Cheers,

Phil Hobbs

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Phil, Well your comment drove me to dig out my copy of "Solid State Electronic Devices" by Ben Streetman. (I should have done this days/weeks ago). In chapter 5.6 he discusses the PN junction deviations from simple theory. He introduces the "non-ideality" factor to account for the observation there can be two types of recombination current in a forward biased diode. In the simple model recombination in the transistion region is ignored and it is assumed that all the recombination occurs in what he calls the neutral region. I believe the neutral region extends for a diffusion length on each side of the transition region. The recombination in the neutral region is proportional to the minority carrier concentration, and this is proportional to the square of the intrinsic carrier concentration (ni)^2 which goes as exp(eV/kT). Streetman then states that recombination in the transistion region is a bit complicated becasue the carrier concnetration changes with distance, and then states (without a reference) that the current is proportional to the intrinsic carrier concnetration (ni) which goes as exp(eV/2kT). And thus the non-ideality factor!

So to answer my own question, very abrupt junctions, with minimal recombination in the transistion region will lead to diodes with ideality factor near one. Streetman discusses more including how the non-ideality factor will change with band gap and bias voltage..

Thanks for the kick into action.

George Herold

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George,

Thanks. I did a bit of digging myself: Sze 2nd Ed. says more or less the same thing but gives an equation. That's glory for you. Diode-connected transistors have almost no recombination in the base (i.e. they have high beta). I gather that's why they're kT/e to such high accuracy.

Cheers,

Phil Hobbs

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Dang, I was wondering if this was covered in Sze? It's one of the classic's that I don't own. I'll have to buy a used copy.

Streetman gives the ratio of the recombo in the neutral region divided by recombo in the transition region as; I (neut)/I (tran) ~ (ni^2*exp(qV/kT))/(ni*exp(qV/2kT)) ~ ni exp(qV/

It's only glory if the transistor packages survive repeated trips to

I'm still a bit confused by the diode connected transistor. I want to hang 1 ohm resistors in the base and collector arms and measure the current. But even if most of the current flows through the collector lead, doesn't it still have to flow through the base region inside the device?

George Herold

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Yes it does, but it doesn't _recombine_ there--it just shoots straight through into the collector. The base works like the grid of a vacuum tube, and the base current is just that fraction that does recombine in the base. The built-in field in the transistor makes it work fine with collector-base biases of zero or even a couple of hundred millivolts less than that. At V_CB = 0, the beta hasn't begun to drop off yet. (The base and collector aren't a voltage divider any more than the gate and drain of a FET.)

Pentodes work fine with the plate below screen potential, for the same reason.

Re Sze: Apparently the third edition (revised by somebody else) isn't as good as the second.

I'm still dubious about how well a minority-carrier device is going to work at 77K.

Cheers,

Phil Hobbs

Various experimenters have "hot rodded" PC's by sinking them in LN2. They cranked up the clock well over three times rated and ran it that as long as the LN2 lasted. And i expected mobility "freeze out" to render the units non-functional. I wonder what may be possible with GPU or Cell MP machines in LN2.

CMOS is all majority carriers. Diodes are bipolar devices.

Cheers,

Phil Hobbs

Okay, you're the cryogenic guy, not me. Transistors may be another story however.

Cheers,

Phil Hobbs

Excellent, with any luck I can get it cheaper than the newer edition.

Si diodes are used as temperature sensors down to 4 K. With forward voltages approaching 2 volts. (Reminds me of the 2.2V 'zero temperature' band gap you mentioned earlier in this thread.) I don't know if there is anything special about them.... (though they charge prices as if they were made of gold. The last time I looked Lakeshore Cryo. was charging $200.00 for an uncalibrated diode... most of this price may be the packaging) The garden variety small signal diodes work fine down to 77 K.

George