I have previously asked a question about using thermocouples to maintain a temperature range of 36-39° inside an insulated and dry environment.

The more enlightened engineers advised I look into using either a semiconductor device or an RTD instead.

Parts availability is limited where I'm currently located in the third world and mail order is unreliable at best.

I have a lot of bipolar transistors as well as MOSFETs at my disposal.

I've seen some examples of using 2N3904/3906 as thermal sensors.

The BS270 looks a bit more appealing to me because of the positive temperature coefficient and lower currents required.

My one concern is the inherent D-S resistance warming the device. But if the current is low enough I think the resistance curve with respect to junction temperature looks easier to calibrate than something based on bipolar transistors.

Am I travelling down the wrong path here? Bipolars seem to be more popular for this sort of hack job sensor.

The alternative is for me to manufacture an RTD which is indeed a possibility if that's a better option.