This paper presents the modelling, design, fabrication and characterization of a planar micro thermoelectric generator (μTEG) which is able to convert waste heat into a few microwatts of electrical power. In order to get a better performance under a large variety of heat sources even if their thermal resistance is high, a planar μTEG with a large thermal resistance was designed and fabricated. It is built of two periodically etched silicon substrates that are respectively used as heat concentrator and heat evacuator, the whole embedding a multilayer membrane which includes a polysilicon-based thermopile with large thermoelement leg length. The thick air cavities etched in the substrates are effective in preventing the direct heat loss from concentrator to evacuator. 3D thermal simulations are carried out to improve the performance of the μTEG. A new definition of the “efficiency-factor” which involves the thermal input power instead of the temperature difference across the chip is suggested to evaluate the efficiency of this kind of μTEGs. The advantage of this new efficiency factor is that it takes the thermal resistance of the μTEG into consideration. With a thermal resistance of 78 K/W, the experimental results show that the μTEG can work under high temperature difference (up to 267 K). With an optimized structure, i.e. 5 membranes and annealed polySi as TE main material, the maximum output power of our μTEG is 138 μW/cm² when the input power is 4 W/cm² and its corresponding new efficiency factor is 865 μm²/W.