What sort of diode are you using. DOn't forget that some are not optically well shielded and might pick up harmonics from the ambient light driven from your 50 Hz mains. Try shielding it in a totally black box.

Thank you for your feedback, Dr. Maine.

Diodes are SiC schottky barrier devices measured from a wafer (a wafer piece actually). The lab is as dark as it can be, all lights of and the probe station used to perform the measurements is inside a shielding box (although it is handcrafted, I tried measurements with lights both on and off and the observed difference is very small compared to the effects of different ground connections).

I've preformed measurements on small MOS capacitors and the coupling of line harmonics is substantially smaller (measurements are comprehensible as the coupled harmonics are well below a 1/f noise characteristic).

As an example, I'm attaching the captured waveforms from a measurement of a SiC diode , slightly forward biased (current around 1nA). TIA gain is set at 1x10^6 A/V while an AC-coupled voltage amplifier stage at its output adds 100 times gain (total system gain is roughly 1x10^8 A/V).

Additionally, I've tried using a commercial transimpedance amplifier with similar gain, but that limits the bandwith to 20Hz at such gains. Obtained spectrum is very similar but, of course, with the harmnics being supressed by the amplifier's transfer characteristic.

Any further advice will be largely appreciated.

Best regards.

Hi Sebastian,

You are working at quite low power levels, so shielding from external fields becomes extremely important - but you know that! It is interesting that there isn't much 50 Hz in your output, it's multiples of 100 Hz, which certainly suggests a power related interference not a voltage or current related interference.

Are you sure that your bias current is completely free from 100 Hz? It would only need picoamps to produce the effect noticed. And the experiment might also be susceptible to stray magnetic fields from mains cabling - but the absence of 50 Hz component probably rules that out.

Is the device sensitive to mechanical vibration? If you tap it does the TIA output jump?

Can you send me a photograph of your experiment and a simple circuit showing power supplies, how they are grounded, how they are connected? It might help in finding the source of the interference.

You might have to make the entire experiment battery powered and link it to the outside world using a non-electrical process e.g. fibre optics; but this is a last resort.

Sebastian,

I had another thought after looking more closely at your waveforms. It is also possible that the device is susceptible to RF interference such as you might get from things like microwave ovens, Mobile phones, nearby radars and the like. This is not fantasy - radars can easily scramble commercial electronics if precautions aren't taken. Again, you wouldn't need much, and a potential rectifier at the input is all that is needed.

Have you noticed a change of behaviour if you use a phone nearby? On the other hand, phone signalling is not at mains frequencies, so it probably isn't that.

RF interference would mean putting the device in a screened metal box.Is that feasible?

Best wishes with your research!