This work demonstrates the thermoelectric (TE) performance of the cavity-free thermoelectric generators (TEGs) using GeSn and Si wires. In the Cavity-free structure, one side of the TEG is heated and the back of the substrate is cooled. Thus, the heat energy flows mainly in the vertical direction but also exudes horizontally. This horizontal heat flow forms a steep temperature gradient in the wires, which is ranged hundreds of nm. In this work, with the identical patterning design, the Seebeck coefficient, power-factor and TE performance of the generators were characterized. The obtained results were then used to compare the TEGs which prefer the superiority of the GeSn-TEG over the Si-TEG.