It is known that thermometers of good sensitivity and excellent reproducibility for low temperatures can be based on the temperature dependence of extrinsic electrical conductivity in a semiconductor. Uniformly doped germanium single crystals are customarily used for this purpose; however, the present paper reports on the performance of experimental semiconductor thermometers made by diffusion techniques. Thermometer elements were made both by diffusion of arsenic into germanium, and by diffusion of boron into silicon. For both of these systems, the conductance‐temperature characteristic of a diffused layer was qualitatively similar to the conductivity‐temperature characteristic of a uniformly doped thermometer bar. However, diffused thermometers suffered appreciable changes of resistance on thermal cycling, to an extent corresponding with temperature errors as large as 0.1° at 4.2°K. This compares unfavorably with the submillidegree reproducibility attainable with uniformly doped bridge shaped bars. Attempts to improve the stability of diffused layers are discussed.