BJT behaviour at ridiculously low current levels

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Alas, it's a region that has high dependence on collector leakage currents (and cosmic rays, etc). Some of the space-rated research does get into this, because radiation damage moves the effects into higher-current parts of the characteristic. Beta is 'high' because the collector leakage is adding to the external base wire's contribution. It takes negative base bias to get an 'off' state, through Rbb resistance (base spreading resistance).

There is a niche of building oscillators that run at very low supply voltages, ballpark 10 mV. I think jfets do pretty good.

A depletion PHEMT might be good too.

Yes, there are several interesting examples. The lowest voltage I know of is 20mV. But in this off-time research application related to the recent CeraCharge purchase I am trying to minimize current consumption, not voltage. It can stop oscillating at 1.2V, no problem with that. But if the current could be as low as 20nA, that would be something.

Best regards, Piotr

The AOE X-chapter book has some data on using BJTs at pA levels. Section 2x.9

2N4401 holds up pretty well at Ic of 100 pA. I'd expect some very small RF transistors to be pretty good.

Lots of small BJTs work fine in the low nanoamps. I've used BFT25As as transdiodes down to 5 nA or so. They crapped out about 1 nA.

The transconductance is a lot better than FETs down there!

Cheers

Phil Hobbs

The f_T goes in the tank--at 1 nA, even a BFT25A is a < 1-MHz transistor.

Cheers

Phil Hobbs

This is getting really interesting. I didn't know that BJTs have useful subthreshold region too.

Indeed, Spice appears to say the same. I need to build a JFET probe in order not to disturb the single electron microcosmos down there with a dingy default 1Meg probe. And grab some radial resistors in the range of

47Meg+, as I don't fancy the idea of connecting hordes of 4.7Meg ones to get there.

Best regards, Piotr

Well, 1 pA into 1 pF gets you 1 volt per second. Speed takes power.

Big savings on heat sinks!

I was theorizing that an LED, at low current, wouldn't have enough voltage across it to make visible photons. A typical green LED runs about 80 mV per decade current.

I tried some green LEDs at 1 nA and they made visible light.

I conjecture (ie guess) that base current makes collector current down at the single-electron level. 1 pA is just 6 milion electrons per second.

I have some 1T ohm 0805 resistors. High values are available.

I remember seeing a sci-fi illustration when I was a kid of an alien species discovering the Voyager golden record many millions of years in the future - their little scout-ships the size of what would be 1:1000 scale toys of a Space Shuttle touching down on its surface to investigate.

Those guys really got it good with respect to the energy requirements of "manned" space travel.

I also recall an alternative interpretation of general relativity (shape dynamics) says that time isn't relative and instead the sizes of objects in the Universe are what's relative.

I have 1G in 1206. I have quite a lot of 10M-33M in 0603 too, but they make prototyping difficult. Wires help, hence this purchase of their THT equivalents. Should arrive today, so no problem here.

Best regards, Piotr

AFAIK, it's not the base current but base voltage that controls the collector current. The base current is just an unfortunate side effect. The relation between them happens to be rather constant, hence the idea that hfe is the main factor. Ideally, the base 'leakage' could be so low that base current electrons can be counted. In that case, would the much higher collector current come in packets?

Arie

Certainly it a dark room you will see the die light up. I'd be surprised if it did it with less voltage across the junction than energy of the the photons being emitted (give or take a bit of slop for thermal phonon interactions boosting the odd one).

These days at 5uA current drive you can see the white LED dies lit up glowing a faint blue white under normal lab conditions.

I suspect that there was a good reason why we used 10^11 ohm precision resistors on Faraday amplifiers in mass specs but I never thought to ask. They would sometimes put a 10^10 or 10^9 resistor on one channel that was expected to have a higher beam current for some applications.

"Resistors" in that very high value regime don't behave at all ideally either (maybe this has improved - I don't know). Considerable effort went into correcting their non-ideal behaviour in post processing.

When the guy making them retired the first few batches with just his apprentice at the helm unsupervised were unusable. There was an art to cooking them just right to have the best combination of properties.

It isn't just the active components that behave oddly at ultra low currents so does the pcb and the passive components. High value resistors tend to behave like a distributed capacitance at >10^10.

I did one photodiode amp that was all surface-mount but I couldn't find suitable high-value thinfilm resistors, so I had to fly-over a series pair of axial thinfilms.

The high value surfmounts are all thickfilm, at least the ones I could get easily.

I don't like very low current design. Measurements take ages to settle. Picosecond stuff happens right now.

Ditto thermal measurements. Tedious.

On a sunny day (Wed, 11 May 2022 12:17:51 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in snipped-for-privacy@4ax.com:

Have a JFET LC running from a thermocouple:

Circuit diagram: did not design for very low current.

Some PIC microprocessors take very little current The 18LF1XK22 uses 34 nA in sleep mode, and 650 nA @32 kHz clock (says dataheet) and can do a lot more than just oscillate...

Temperature range would be interesting.

Yup, a BJT is a transconductance device with a loss mechanism (recombination in the base region) that produces leakage current. IIRC Mikko Kiviranta or somebody said that the beta of a BFP650 goes up to above 10000 at low temperature.

Cheers

Phil Hobbs

Good point. You must be within the active region to use the hybrid model to ignore transconductance.

Danke,

Don't ever ignore transconductance.

Cheers

Phil Hobbs