In building an ultrasensitive instrument, we're always fighting to improve our signal-to-noise ratio (SNR).  The SNR is the ratio of signal power to noise power in the measurement bandwidth, and is limited by noise in the instrument itself and the noise of any background signals, such as the shot noise of the background light or the slight hiss of a microphone. 

James Elbert (Jim) Thompson was a well-known chip designer who used to be a regular on sci.electronics.design.  He last posted in July 2018.  As he was very sick at the time, we presume that he has died, but no obituary has so far turned up.  He was born on February 29th, 1940, and used to say that he was looking forward to his 21st birthday in 2024.

InGaAs heterojunction FETs are magic parts—fast, strong, and extremely quiet.  They're also called pseudomorphic high electron-mobility transistors (pHEMTs), because they use a 2D quantum well to to force the conduction electrons to move in a plane without much scattering.  My fave Avago ATF38143 pHEMT was discontinued, but luckily Mini-Circuits stepped into the breach with their very nice SAV-551+ and its siblings, which are similar enough that the ATF SPICE model can be hacked up to work with them.  (RF companies like Mini-Circuits never seem to supply SPICE models for some reason.)  In one post on the 'purpose of precision' thread on sci.electronics.design, I noted that the Avago ATF38143 model I had posted awhile back predicted way, way too much low frequency noise. The real pHEMTs tend to have a pretty accurately 1/f PSD with corner frequencies between 10 and 50 MHz and flatband noise of around 0.3 nV/√Hz, about 10 dB quieter than the best JFETs, as well as being 20 times faster.

There are widespread shortages of electronics parts at the moment, especially passives.  Quoted factory lead times are 40 weeks or thereabouts, and since the industry is capacity-limited, it isn't clear that the situation is going to get better any time soon, so everybody's starting to panic.   Given all this churn I've been spending an unconscionable amount of time lately finding suitable replacements for out-of-stock parts. 

Measuring temperature is surprisingly subtle.  There are lots of sensors out there; Digikey sells thermistor sensors interchangeable to +- 0.1 C from several vendors for about $3 in onesies.  IC sensors tout good accuracy and linearity, and come in both analogue and digital versions for way under a buck.  So what's the issue?