It has been observed that the forward voltage drop, at constant forward current, of a GaAs diffused p‐n junction varies almost linearly with temperature from 2.0°K to above 300°K. Since carrier ``freeze‐out'' at low temperatures is not observed, these junctions make excellent cryometers. In addition, these diodes exhibit good repeatability on temperature cycling and an insensitivity to magnetic fields. The sensitivity of the devices measured near room temperature is &Dgr;V/&Dgr;T≈−3.5 mV/°C,If=0.1 &mgr;A; &Dgr;V/&Dgr;T≈−2.0 mV/°C,If=1.0 mA. The sensitivity decreases slowly toward lower temperatures and at liquid helium is −1.5 mV/°C and is essentially independent of current. This high and nearly uniform sensitivity over such a wide temperature range is unique among low temperature thermometers. The shape of the temperature‐voltage characteristic, with current as a parameter, is explained quite satisfactorily forT>50°K by assuming that the device conducts primarily recombination‐generation currents, according to the theory of Sah, Noyce, and Shockley. Below 50°K certain deviations from this theory occur, but the applicability of the devices to thermometry is not affected. It appears likely that these devices are capable of a temperature resolution of at least ±0.01°C over the entire range investigated, using ordinary current sources and precision potentiometers.