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This paper analyzes the nonidealities of temperature sensors based on substrate pnp transistors and shows how their influence can be minimized. It focuses on temperature measurement using the difference between the base-emitter voltages of a transistor operated at two current densities. This difference is proportional to absolute temperature (PTAT). The effects of series resistance, current-gain variation, high-level injection, and the Early effect on the accuracy of this PTAT voltage are discussed. The results of measurements made on substrate pnp transistors in a standard 0.5-μm CMOS process are presented to illustrate the effects of these nonidealities. It is shown that the modeling of the PTAT voltage can be improved by taking the temperature dependency of the effective emission coefficient into account using the reverse Early effect. With this refinement, the temperature can be extracted from the measurement data with an absolute accuracy of ±0.1°C in the range of -50 to 130°C.