Sensors that use the base emitter junction voltage of a bipolar junction transistor to measure temperature are quietly displacing sensors that measure resistance variations. Unlike thermistors, whose resistance varies exponentially with temperature, these semiconductor sensors provide a linear output, which makes for higher accuracy over a wider temperature range; they need less space on a board because they don’t need the resistive network that thermistors do, and they dissipate less power, which avoids self-heating errors. Also, these analog sensors can easily interface to other semiconductors for signal conditioning, monitoring and display.
Thermistors still hold a cost and size advantage, but not for long. The inexorable march of semiconductor-manufacturing technology is driving down the size and cost of analog temperature sensors. New, tiny sensors—some as small as 1 mm2 — are now viable as sensing elements in probes.
With this new low-cost, low-power alternative to precision temperature measurement, engineers are exploring several thermometry applications such as wirelessly interconnected remote sensor nodes that use harvested energy, or battery-powered, high-precision medical or industrial instrumentation.
Particularly promising applications are bio-patches — wearable sensors for untethered, continuous monitoring of physiological parameters. This temperature patch reference design, for example, can be worn on the skin and monitored wirelessly: A semiconductor sensor measures body temperature that is digitized by a 16-bit ADC, stored by a microcontroller and sent via an NFC transponder to a NFC reader in a smartphone or tablet. The patch is powered by harvesting the NFC RF field, which means it works only within close proximity of the reader.