The sensitivity of planar multijunction thermal converters (PMJTCs) was significantly increased by filling the housing with noble gases, preferably xenon or krypton. The performance was observed for almost than two years and no degradation of the sensitivity was found. Also the frequency response of the AC-DC transfer difference remains unchanged at high frequencies and even improves below 40 Hz. Using the lowest at present manufacturable heater resistance of 90 with the noble gas filling, AC-DC transfer down to almost 70 mV compared to about 100 mV in air becomes possible.