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The output power of a thin-film thermoelectric generator consisting of a Cu-20- μm nanowire (NW) array (NWA)-Si bulk-Cu sandwich with Si or Si0.8Ge0.2 NWs is measured and compared to Cu-Si bulk-Cu for small temperature differences around room temperature. The array of NWs is made by metal-assisted chemical etching that retains the Ge concentration in the wires. The conversion of one surface of Si bulk into an array of short-relative to the remaining bulk-Si NWs improves the maximum output power by a factor of two. Using an array of Si NWs in combination with Si0.8Ge0.2 increases the maximum output power by a factor of 20. The increased output power, under the same heating power, is due to the lower thermal conductivity and contact resistance of the NWA and SiGe compared to Si bulk. A matrix model is developed to solve the coupled thermal transport equations for an arbitrary number of layers. Fits to the measurement allow the extraction of the electrical contact resistance and the effective internal temperature drop across the semiconductor composite.