There are families of IC temperature sensors available today that produce linear current or voltage signals related to temperature over the -40°C to 150°C range. Such circuits could have been produced using discrete semiconductor components but that would have been of little advantage. Why place the entire circuit in the environment to be measured when you could put a thermistor there and keep the balance of the circuitry in a better regulated environment? The development of the first linear semiconductor IC to produce a linear current-to-temperature relationship (the Analog Devices AD590) was an achievement that was quickly recognized and copied. Contrary to the general practice of digitizing everything, these are analog circuits that linearize the Seebeck characteristics of thermistors over a limited temperature range.