This article presents a comparative study of both active and passive upconverter frequency mixers, integrated in an amorphous indium gallium zinc oxide (a-IGZO) technology. These flexible circuits, based on n-type thin-film transistors (TFTs), operate within the radio frequency (RF) industrial, scientific and medical (ISM) band of 13.56 MHz. The fabricated passive ring mixer shows a conversion loss of 6 dB while the active designs—differential pair (DP) and Gilbert cell (GC)—are optimized for approximately +5-dB conversion gain (CG) at a target frequency of 13.56 MHz and have a very low dc power of 95 and 248 μ W, respectively. In addition, a degenerated GC is implemented and compared with the other designs. To the best of our knowledge, these are the first fully integrated mixers reported in this type of flexible technology.