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This work presents an overview of recent research into integrated temperature sensors based on thermal diffusivity sensing. Such sensors make use of the fact that the thermal diffusivity of highly pure IC-grade silicon has a well-defined temperature dependence and is insensitive to process spread. To measure thermal diffusivity, an on-chip thermal delay can be defined and then used to define the frequency of a VCO. Alternatively, this delay can be directly digitized. Several proof-of-concept devices fabricated in 0.7mum CMOS technology demonstrate good accuracy and reproducibility. They achieve untrimmed inaccuracies in the order of plusmn0.6degC (3sigma) over the military temperature range (-55degC to 125degC), based on the measured performance of 16 samples per batch.