I What is the temperature dependence of the NIRCam sensor's responsivity?

AI Thread Summary
The NIRCam sensor utilizes HgCdTe detectors, which are sensitive to infrared photons, including those at 3.5 microns. The discussion centers on the temperature dependence of these sensors and the implications of the second law of thermodynamics on photon detection. It is debated whether a 3.5 micron photon from a 299K object can be detected by a 300K sensor, given that the sensor's temperature is equal to or higher than that of the emitting object. Some participants argue that the sensor can detect photons regardless of their source temperature, while others emphasize the challenge of distinguishing signals from thermal noise at similar temperatures. Ultimately, the conversation highlights the complexities of thermal dynamics and photon detection in infrared imaging.
  • #51
Fig 3(b) shows photocurrent density with zero bias voltage scaling down linearly with decreasing irradiance from 1 W/cm^2 all the way down to 1/1000th W/cm^2 for 20 micron IR…

c8ra07683a-f3_hi-res.gif

https://pubs.rsc.org/en/content/articlehtml/2018/ra/c8ra07683a#imgfig4
 
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  • #52
Why can’t they avoid the need for cooling the telescope to cryogenic temperatures by operating the detectors with zero bias voltage (photovoltaic mode)? As long as the telescope is the same temperature as the sensor, say 300k telescope and 300k sensor, how can there be any noise-generating dark current? The Carnot efficiency between the temp of the telescope and the temp of the sensor would be 0% if they are both the same temperature. Wouldn’t that reduce the cost and complexity if the need for cryogenic cooling was eliminated by eliminating the dark current?
 
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  • #53
Why does Figure 2 show 1mA/cm^2 of DC dark current with 0v bias voltage at 161k? It shows the dark current in amps being directly proportional to the surface area of the imaging sensor & non-zero with 0v bias & no photocurrent. For a sensor 31.7cm x 31.7cm that’s more than a full amp of DC current.

If I have a 0v bias voltage, 161k photovoltaic IR sensor in a 161k telescope with the lens cap on, I shouldn’t be able to generate any useful DC current because the Carnot efficiency is 0%…

https://www.researchgate.net/publication/345174470/figure/fig2/AS:1022731785080842@1620849667815/Dark-current-density-versus-applied-bias-measured-at-several-temperatures-for-the-T2SL.png

Fig. 2: “Dark current density versus applied bias measured at several temperatures for the T2SL MWIR 15 Â 15 detector array of 15 μm pitch and a 4.6 μm cut-off wavelength.”

https://www.researchgate.net/figure/Dark-current-density-versus-applied-bias-measured-at-several-temperatures-for-the-T2SL_fig2_345174470

https://www.researchgate.net/publication/345174470_1f_Noise_and_Dark_Current_Correlation_in_Midwave_InAsGaSb_Type-II_Superlattice_IR_Detectors/fulltext/609c2ee8299bf1259ecd763c/1-f-Noise-and-Dark-Current-Correlation-in-Midwave-InAs-GaSb-Type-II-Superlattice-IR-Detectors.pdf?origin=publication_detail
 
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  • #54
Also this paper Fig 4 shows about 0.4 amps dark current at 300k in a 300k enclosure for a 31.6cmx31.6cm sized detector…

“Figure 4(a) shows the dark current density vs applied bias voltage characteristic of the nBn photodetector at different temperatures ranging from 120 to 300 K. During the measurement, the device was covered by a cold shield and cooled by a temperature-controlled stream of nitrogen in the vapor cryostat. The cold shield has the same tempera- ture as the device. “

https://www.researchgate.net/profile/Arash-Dehzangi/publication/335005180_Demonstration_of_mid-wavelength_infrared_nBn_photodetectors_based_on_type-II_InAsInAs_1-x_Sb_x_superlattice_grown_by_metal-organic_chemical_vapor_deposition/links/5d4a53afa6fdcc370a80ec0c/Demonstration-of-mid-wavelength-infrared-nBn-photodetectors-based-on-type-II-InAs-InAs-1-x-Sb-x-superlattice-grown-by-metal-organic-chemical-vapor-deposition.pdf?origin=publication_detail

CD6CA3B5-F2F6-4780-9128-22F6951A3A37.jpeg
 
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  • #55
Devin-M said:
Why can’t they avoid the need for cooling the telescope to cryogenic temperatures by operating the detectors with zero bias voltage (photovoltaic mode)? As long as the telescope is the same temperature as the sensor, say 300k telescope and 300k sensor, how can there be any noise-generating dark current?
"the telescope" is a mirror. The camera isn't trying to avoid detecting the mirror it is trying to avoid detecting itself. If it isn't cooled, the whole apparatus is trying to image a background that's colder than it is.

The issue you raise and are hyper-focused on feels a lot like a typical perpetual motion machine problem: people "invent" PMMs that are just complicated enough that they don't understand them well enough to be able find the flaw and instead assume they work.

I don't understand the quantum mechanics of how my cameras work, but I do know that without cooling they operate above ambient temperature, meaning they are net emitters of heat, not absorbers, much less 2nd law violators/spontaneous coolers.
 
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  • #56
russ_watters said:
"the telescope" is a mirror. The camera isn't trying to avoid detecting the mirror it is trying to avoid detecting itself. If it isn't cooled, the whole apparatus is trying to image a background that's colder than it is.

The issue you raise and are hyper-focused on feels a lot like a typical perpetual motion machine problem: people "invent" PMMs that are just complicated enough that they don't understand them well enough to be able find the flaw and instead assume they work.

I don't understand the quantum mechanics of how my cameras work, but I do know that without cooling they operate above ambient temperature, meaning they are net emitters of heat, not absorbers, much less 2nd law violators/spontaneous coolers.
See Fig 4a in post #54.

“During the measurement, the device was covered by a cold shield and cooled by a temperature-controlled stream of nitrogen in the vapor cryostat. The cold shield has the same tempera- ture as the device.”

I also assume the 2nd Law is inviolable, and I also don’t understand the quantum mechanics (I was hoping someone would explain the quantum part), but the current measurement with 0v bias voltage and everything 300k (sensor & surroundings) was roughly 4*10^-3 amps “dark current” per cm^2 of sensor area which works out to about 0.4 amps with a 31.6cm x 31.6cm sensor size. I was hoping someone would explain the quantum mechanical part because the Carnot efficiency with same temp sensor and surroundings should be 0%.

I’m trying to understand the quantum dynamics, not make a perpetual motion machine or violate the 2nd Law.
 
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  • #57
Devin-M said:
Why does Figure 2 show 1mA/cm^2 of DC dark current with 0v bias voltage at 161k? It shows the dark current in amps being directly proportional to the surface area of the imaging sensor & non-zero with 0v bias & no photocurrent. For a sensor 31.7cm x 31.7cm that’s more than a full amp of DC current.

If I have a 0v bias voltage, 161k photovoltaic IR sensor in a 161k telescope with the lens cap on, I shouldn’t be able to generate any useful DC current because the Carnot efficiency is 0%…

https://www.researchgate.net/publication/345174470/figure/fig2/AS:1022731785080842@1620849667815/Dark-current-density-versus-applied-bias-measured-at-several-temperatures-for-the-T2SL.png

Fig. 2: “Dark current density versus applied bias measured at several temperatures for the T2SL MWIR 15 Â 15 detector array of 15 μm pitch and a 4.6 μm cut-off wavelength.”

https://www.researchgate.net/figure/Dark-current-density-versus-applied-bias-measured-at-several-temperatures-for-the-T2SL_fig2_345174470

https://www.researchgate.net/publication/345174470_1f_Noise_and_Dark_Current_Correlation_in_Midwave_InAsGaSb_Type-II_Superlattice_IR_Detectors/fulltext/609c2ee8299bf1259ecd763c/1-f-Noise-and-Dark-Current-Correlation-in-Midwave-InAs-GaSb-Type-II-Superlattice-IR-Detectors.pdf?origin=publication_detail

Measurement error given the circuit configuration?

Keep in mind when the data was measured for the plots, the photodetector was placed in an active circuit. The bias voltage was then adjusted, maybe by adjusting a potentiometer in a voltage divider circuit, or perhaps by adjusting the output voltage of a benchtop power supply -- a power supply attached to the circuit specifically for bias voltage adjustments, where the rest of the circuit was powered by a separate supply. Something like that.

By tweaking the bias voltage in the active circuit, one can sort-of simulate a passive circuit when the bias voltage is at 0 V. But any error in the method, or any minor, non-ideal property of the circuit that still provides power to some parts of the circuit, could manifest itself as non-zero measurement when the bias voltage is supposedly "zero."

But what I can all but guarantee is that they didn't yank out the photodetector and place it in a passive circuit just for the special situation of dark current at 0 bias voltage.

You can assume though that if the sensor was actually placed in a passive circuit, such that the photodetector, other components in the circuit, its enclosure, and everything else in the vicinity is all at the same temperature, the dark current density would measure 0.
 
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  • #58
Devin-M said:
It shows the dark current in amps being directly proportional to the surface area of the imaging sensor

I'm not sure this is supported by the graph, it may be, but if the data was collected on a single device (or multiple identical devices) then one cannot see the shape of the curve for the dark current vs active area. Dividing the measured current by the device area all by itself and using this as the Y axis scale does not provide good insight to the area vs dark current shape. Having multiple curves, as we see with the changing temperature does provide this (for temp, and would, for area, if available). Labelling the Y axis this way does imply a linear relationship, and if I were involved in a review of this particular graph, I'd poke at that. As you say, it gives pretty suprising results when one scales it a bit.

I agree with @collinsmark . In my experience reviewing and interpreting semi-conductor measurement data, when one sees a result that looks to be in conflict with physics, one starts from an assumption of measurement non-ideality and tries to identify where its coming from, then one assesses whether it matters enough to go and spend time and $ to attempt an improved measurement. At some point, improved measurements are judged to no longer be value-added, and one calls it a day.

Edit:

Considering the graph a bit more, I speculate that the author was wanting to communicate current density specifically as the quantity of interest in the device being measured, and not intending to imply that it scales linearly with area, and this why the Y axis is reported as it is.
 
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  • #59
Here’s another one I found, for which the graph shows more dark current per cm^2 at 300k with 0.0v applied bias voltage than 170k with -0.8v bias voltage:
C5B13BA1-EE95-4D1A-A13C-3A70D7EF9E15.jpeg

Fig. 3. (a) Dark current density versus applied bias voltage char- acteristic of the photodetectors as a function of temperature.
https://www.researchgate.net/profile/Arash-Dehzangi/publication/322833826_nBn_extended_short-wavelength_infrared_focal_plane_array/links/5a73e8f1a6fdcc53fe148ed3/nBn-extended-short-wavelength-infrared-focal-plane-array.pdf?origin=publication_detail
 
  • #60
More dark current per cm^2 at 0v applied bias voltage at 300k is indicated in this graph than at 120k, -1.0v bias voltage…
37F02D1E-C63D-4FAB-B17B-6176D501FBBC.jpeg

https://www.researchgate.net/profile/Junqi-Liu-3/publication/267340100_Room_temperature_quantum_cascade_detector_operating_at_43_m/links/579709df08aec89db7b86bd8/Room-temperature-quantum-cascade-detector-operating-at-43-m.pdf?origin=publication_detail
 
  • #62
Devin-M said:
In this one, the dark current decreases as the forward bias voltage increases above 0v:

[...]

https://opg.optica.org/oe/fulltext.cfm?uri=oe-28-16-23660&id=433783

C'mon, @Devin-M. Please. The paper discusses the discrepancies in the text and addresses some of the problems such as the imperfection of the cold shield. I mean, it's right there.

You've been repeating a lot of similar figures. Possible non-ideal behaviors and measurement errors have already been addressed several posts back by myself and others.

I'm tempted to repeat some of those points on the subject. Can you guess which points I'm tempted to repeat?
 
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  • #64
Devin-M said:
at 110k, the dark current decreases as the bias voltage increases just above 0v…

How do you interpret this data (what do you think it implies)?

For myself, the first thing I would check is pixelation, I'd ask the presenter if they could show the table of raw data to make sure its not just that (pixelation). Its a little hard to say for sure, but I think the 180K line is showing the same thing, and to my eye, one can't tell about the other temp lines, because they all overlap too much around 0V bias.
 
  • #65
  • #66
@Devin-M, let me point out something else that you may be running into besides measurement errors and non-deal aspects of circuit setups.

The emf point at which a photodetector exhibits zero [dark] current is not necessarily at exactly 0 Volts. It's highly dependent upon the circuit in which it is placed. This is due to the fact that photodetectors are non-linear devices and have junction emfs. The P-doped and N-doped material in the photodetectors act as dissimilar metals (or in this case, dissimilar semiconductors) and a residual emf maybe present across its terminals, even though no current flows.

Certainly, if the photodetector is by itself, not connected to anything, this emf will exist. It can also exist if placed in a circuit, particularly when other non-linear devices, such as a diode, are wired in parallel.

As an analogy, this is similar to how you wire two batteries in parallel, and no current will flow between them. Don't take this analogy too far though -- batteries have significant energy stored within them, but diodes do not. (And photodetectors do not contain significant stored energy when they and their sourroundings are all at the same temperature.)

So the idea that the voltage across the terminals of a photodetector is not precisely 0 when the minimum current point is reached, should not come as a complete surprise. Again though, it all depends on the circuit details.

-------------------

But let me repeat my main point again. If you place a photodetector in a passive circuit, and the photodetector, the other circuit components, and the surroundings are all at the same temperature (that also means no external light sources -- only the thermal background within the dark enclosure), the current through the photodetector will be zero. A this point, the voltage across the terminals of the photodetector might not be exactly 0, but I can guarantee you that the steady-state, DC current through the photodetector will be 0.
 
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  • #67
@Devin-M To circle back on post #64, your post #65 is clearly not pixelation around the zero point, my question is answered on that.
 
  • #71
Sorry I was responding to this comment:
collinsmark said:
You can assume though that if the sensor was actually placed in a passive circuit, such that the photodetector, other components in the circuit, its enclosure, and everything else in the vicinity is all at the same temperature, the dark current density would measure 0.
I was wondering what powers the dark current measured in the paper when there is zero bias voltage, in the dark, and the sensor and surroundings at the same temperature? From Ohm's law we have P=I^2R
Devin-M said:
View attachment 312635
https://apps.dtic.mil/sti/pdfs/ADA429637.pdf
1-jpg.jpg
 

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