Micro-thermoelectric generators (µTEGs) are energy harvesting solutions to supply power for autonomous microsystems and other miniature applications. Based on a preliminary modeling we designed original all-Silicon µTEGs with a planar configuration in which the heat to harvest flows in-plane of a TE thin film. Such planar μΤΕGs were fabricated by CMOS compatible technology: they are ~1/3 cm² and are based on a polySi/Au Thermopile periodically distributed onto dielectric membranes; a heat-collector permits to concentrate the energy to harvest onto half the junctions of the thermopile. This planar topology allows the implementation of thermocouples with a high aspect ratio compared to classical vertical TEGs. This results into modules with high thermal resistances (can be tuned from 10 to 112 K/W) and that generate an important open circuit voltage which varies linearly with input heat: for instance, 6 to 10 Volt for 1W heat injected in the devices. Those experimental values correspond to a gradient of temperature at each thermocouple of 113°K, 74°K or 27°K for µTEG with respectively 2, 5 or 10 membranes. However, the weak point of these µTEGs is their high internal electrical resistance, which result into maximum output powers (P_max) that are moderate: up to 62µW/cm², 164µW/cm², and 86 µW/cm², for respectively 2, 5 or 10 membranes µTEGs and 1W injected. To better clarify further improvement to be carried out, the µTEG structure is modelled using COMSOL 3D thermal simulation for different new situations: higher power factor, modified thermal conductance, elimination of parasitic heat losses. In particular, we show that: i/ considering a thermopile integrating a constantan layer can double to triple P_max, ii/ the optimum membrane number is 4 (not 5), iii/ a modified heat-collector with corner bosses made of porous Silicon permit to generate up to 64% more power, iv/ using a better nanostructured polySi TE layer with a thermal conductivity reduced by a factor of 10 should up to double the output power.