5 stage Peltier will get you down to about 200K, depending on your hot-side temperature Below that you're into cooling engine territory. I've used 4-5 stage to cool HgCdTe devices. Boy are these greedy.
I had a two-stage Peltier for a germanium APD design, before InGaAs took off.
This is current state-of-the art
Brian
-- Brian Howie
I don't know about special doping but they have "special" prices. I've used garden variety diodes down to 77K, no problem. I don't have (easy) access to lower temps, helium is damn expensive these days.
George H.
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