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Cryogenic cirquits

  1. Jul 11, 2009 #1
    Cryogenic circuits

    Until recently I thought semiconductors break down in cryogenic environments, now I learned that this is not necessarily the case and there are amplifier chips that seem to work between 2K and 300K. Does someone know about this stuff or even work with it? What is the technology behind it?
    How about the supporting electronics? What capacitors and resistances can be used over such temperature ranges?
     
    Last edited: Jul 11, 2009
  2. jcsd
  3. Jul 11, 2009 #2

    f95toli

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    There is no big mystery here; the "trick" is simply to use III-V transistors (GaAs, GaN,InP etc); they work just fine at cryogenic temperatures since they do not rely on thermal excitations for charge carriers.
    There are also a few Si-based chips that work(even some op-amps); but they are based on overdoped Si.

    However; active semi-conductor based circuits are only really useful at relatively high frequencies (microwave frequencies) since all the abovementioned transistors have extremely high 1/f noise.
    There are designs that work at lower frequencies; but their noise performance is rather poor so most of the time it makes more sense to just keep the active electronics at RT.

    Most resistors work juse fine at low temperatures; but their resistance changes. There are no rules for how much it changes (except that high values tend to change more than low values).

    Capacitors are more unpredictable; some work and some don't. It obviously depends on the dielectric but also on their mechanical properties(some simply break when you cool them). It is usually better to use inductors for filter etc (i.e. use RL instead of RC circuits for filtering etc).
     
  4. Jul 11, 2009 #3
    Standard silicon CMOS technologies can be used at cryogenic temperatures. The circuit will not perform as expected from the model parameters generated for typical temperature ranges (~ 273 to 373 K) but some analog/mixed signal foundries do extract and provide model parameters for cryogenic temperature ranges so that ICs can be properly designed for low temperatures. Even if you take an analog chip designed for room temperatures it will probably still function at cryogenic temperatures but with different performance (better or worse depending on the circuit topology and the parameter of interest)

    Some types of optical detectors must be cooled to cryogenic temperatures to achieve acceptable dark current/noise performance and standard CMOS circuits are packaged with them to amplify and readout the signal. I've worked with standard CMOS at 77 K but I know that in physics experiments CMOS is also used at temperatures close to absolute zero. I don't know if those circuits were standard CMOS or if they required special processing.

    Some companies also make discrete R's and C's for cryogenic applications.
     
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