Video Cryogenic Experiments on Passive and Active Electronic Components - Page 1
Forum section: electronics / projects

author: Hugoneus
author_rank: Frequent Contributor
timestamp: July 22, 2012, 03:17:51 am
content: In this episode, Shahriar investigates the theory and experimental results of the impact of extreme low temperatures on passive and active components. Liquid Nitrogen in used in a transparent glass Dewar where different components can be fully submerged in the liquid. Various types of resistors are compared for their temperature stability. An electromagnet which uses Copper coils is used to generate a magnetic field at a constant power consumption at both extreme temperatures. The impact of liquid nitrogen on the junction voltage of an NPN device is measured as well as the frequency shift of a CMOS ring oscillator. Finally, the wavelength shift of an LED submerged in liquid nitrogen is studied. There is a puzzle at the end of this video, please share your thoughts in the comments section.Direct Link:http://thesignalpath.com/blogs/2012/07/22/cryogenic-experiments-on-passive-and-active-electronic-components/More Videos:http://www.TheSignalPath.com

author: TerminalJack505
author_rank: Super Contributor
timestamp: July 22, 2012, 08:39:26 am
content: Nicely done! Very interesting.

author: T4P
author_rank: Super Contributor
timestamp: July 22, 2012, 06:00:54 pm
content: Cryo a speaker!I would be happy

author: JorgeCarbajal
author_rank: Contributor
timestamp: July 22, 2012, 06:47:28 pm
content: You got featured at Hack a day! congrats

author: Hugoneus
author_rank: Frequent Contributor
timestamp: July 22, 2012, 07:02:04 pm
content: Quote from: JorgeCarbajal on July 22, 2012, 06:47:28 pmThanks! Several of my videos are on Hack a Day.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2012, 07:26:18 pm
content: I'm puzzled about the resistors.Before the experiment you predicted the resistance to go down at lower temperatures, but all of the resistances increased. This would have seemed like an obvious point to comment on, yet you didn't mention it at all.I did expect the carbon resistors to have a negative temperature coefficient so that result seemed to fit. Yet with metal (did you have a metal film resistor?) I would have expected a positive temperature coefficient.Anyway, I'm puzzled about why you completely skipped over this and went straight on to electromagnets?

author: SeanB
author_rank: Super Contributor
timestamp: July 22, 2012, 07:29:28 pm
content: Nice video, nice technique and well put together production wise. Would have been better to have shown the transistor with gain change with temperature as well.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: July 22, 2012, 07:39:20 pm
content: Quote from: IanB on July 22, 2012, 07:26:18 pmIan,I actually did explain the negative temperature coefficient of Carbon resistors, but during the editing I accidentally cut it out. I only realized that it was missing after some people raised the point. I added a little note to the video. I hope this helps.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: July 23, 2012, 04:01:39 am
content: Is there anyone who can explain the main reason behind the LED behavior at extreme low temperatures?

author: amspire
author_rank: Super Contributor
timestamp: July 23, 2012, 04:13:14 am
content: Quote from: IanB on July 22, 2012, 07:26:18 pmFor some reason, people seem to think metal film resistors have a positive temperature coefficients, but this is pure myth.The truth is they can be positive, negative, or near zero in their temperature coefficient. Negative temperature coefficient metal film resistors are very common. Different resistor values and different batches from the same manufacturer will have different measured temperature coefficients.

author: IanB
author_rank: Super Contributor
timestamp: July 23, 2012, 07:46:18 am
content: Quote from: amspire on July 23, 2012, 04:13:14 amThe reason for thinking this is very clear. Metals have a positive temperature coefficient. So if a resistive element is made out of metal it should also have a positive temperature coefficient.Do you have an explanation for how a metal film resistor can be made to have a negative coefficient?

author: Jad.z
author_rank: Regular Contributor
timestamp: July 23, 2012, 10:19:54 am
content: Hi Shahriar,After a little bit of Googling, I foundthis experimentwhere they study the behaviour of a green LED with a varying temperature.A bit more fancier/professional setup though.Anyway, they had the same opposite shift in wavelength and tried to explain it as follows:"This may be because the thermally excited particles have lower energiesand thus different transition energies than the primary charge carriers, which couldproduce a more spread out intensity – wavelength distribution at higher temperatures. Asthe temperature lowers, the thermal noise decreases, thus lowering the intensity due tothese extraneous wavelengths relative to the increasing peak intensity. The peak intensitywould be centered at the wavelength corresponding to the energy of the primary chargecarriers."

author: Hugoneus
author_rank: Frequent Contributor
timestamp: July 23, 2012, 03:18:02 pm
content: Dear Jad,Thanks for the information. I too found that document.Does anyone know of any theoretical work done to explain this behavior?

author: ejeffrey
author_rank: Super Contributor
timestamp: July 23, 2012, 08:32:10 pm
content: Quote from: IanB link=topic=9712.msg131465#msg131465Yeah. They aren't actually metals. Pure metals have resistivity approximately proportional to absolute temperature. This is because the density of conduction electrons stays constant but the number of phonons to scatter off of increases. Thus true metals have a TCR approximately 1/(300 kelvin). Resistive alloys are better than pure metals because defect scattering dominates over thermal scattering and is temperature independent. It still negative, but not as bad as metals.Metal film resistors are alloys and oxides that are somewhere between metals and semiconductors. They have defect states that can be thermally excited to become conduction electrons. In a metal film resistor you design it such that when you increase the temperature the increased number of electrons balances the increased scattering from thermal phonons.An interesting consequence is that the resistor is balanced for low TCR around room temperature. You take them to 800 K or 4 kelvin and it may be quite a bit worse than just a metal film.

author: caffeinatedbard
author_rank: Regular Contributor
timestamp: July 24, 2012, 01:16:23 am
content: Quote from: Hugoneus on July 23, 2012, 03:18:02 pmI believe part of the explanation is due to carrier freeze out in semiconductors. The concentration of the quasi-free electrons and occupied donor states changes as function of temperature. We assume for the most part complete ionization in the pn junction but at lower temperatures that is not the case. I would have to go back to my references for more information and someone please back me up on this, as I am a little rusty.Regards,-bard

author: IanB
author_rank: Super Contributor
timestamp: July 24, 2012, 01:22:36 am
content: Taking a purely intuitive approach to this, I would have to hypothesize that lower temperatures lead to lower energies and so emitted photons would have a lower frequency and longer wavelength. If we continued reducing the temperature all the way towards absolute zero would we expect gamma rays or low energy microwaves?

author: dda
author_rank: Contributor
timestamp: July 24, 2012, 01:53:15 am
content: longer wavelength/energy = radio, not gamma. Thank jeebus.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2012, 02:56:30 pm
content: The primary cause of the temperature dependent wavelength shift in an LED is due to the change in the band gap energy. Very roughly in an LED you inject electrons from one side and holes from the other and they recombine to release light. The band gap sets the scale of possible recombination energy, and thus the photon energy and wavelength. The bandgap gets larger at low temperature due to the thermal contraction of the lattice. Larger bandgap = shorter wavelength light.

author: Jad.z
author_rank: Regular Contributor
timestamp: July 24, 2012, 06:24:49 pm
content: Quote from: ejeffrey on July 24, 2012, 02:56:30 pmThis is right, here is your solid state physics A+, except in Shahriar's experiment the opposite happened

author: ejeffrey
author_rank: Super Contributor
timestamp: July 26, 2012, 10:05:20 am
content: Huh, so it did. When I first watched it the video cut out before the color test. I am not exactly sure what is causing this, or why it is such a large effect. I have certainly observed other (IR) LEDs that shift to the blue when cooled. The most obvious possibility is change in the relative energy of the dopant states relative to the intrinsic semiconductor. For heterojunction LEDs there may also be a change in the bandgap due to strain caused by mismatch of the thermal expansion coefficients.

author: caffeinatedbard
author_rank: Regular Contributor
timestamp: July 26, 2012, 11:32:26 am
content: Don't forget that there is phonon scattering present that might change the wave vector and thus the emitted wavelength when the bandgap changes.-bard

author: olsenn
author_rank: Frequent Contributor
timestamp: July 26, 2012, 11:42:59 am
content: Maybe our current sceintific model is wrong, and Shahriar prooved it.I applaud you sir for not just dismissing your emperical results as experimental error and moving on... I hope you discover the reason for this discrepancy.

author: digsys
author_rank: Unknown
timestamp: July 26, 2012, 12:01:06 pm
content: TO the BATCAVE Robin !! (CERN)

author: madires
author_rank: Super Contributor
timestamp: July 26, 2012, 09:15:54 pm
content: AFAIK the band gap doesn't change with the temperature. By cooling the transistor or the LED the thermal noise is reduced dramatically. That means that the electrons will move less in their band. At room temperature the electrons got more energy and move more. And it's more easy for them to move up to another band if external energy is applied. Therefore the breakthrough voltage of the np junction increases with decreasing temperature. When the LED is cooled down the energy of the electrons decreases. There are much less electrons with a little bit more energy than they need to stay in their band. If the average energy of the electrons decreases you'll get more and more light with longer wavelengths. LEDs aren't monochromatic!