Sensor for very low temperature (liquid nitrogen)
Answer is simple: thermocouple.

Thermocouple type K (chromel - alumel) works from -200°C and has very linear characteristics. It's very popular and cheap, many multimeters use it as a temperature sensor. Type T will be better for such low temperatures, but it is hard to buy and not as cheap as K.

Another solution is a platinum resistive sensor, like Pt100, but it is not as linear as thermocouple.