The effects of carrier concentrations in the front and back channels were comparatively studied in Hf-In-Zn-O thin film transistors by controlling the concentration of In. Although the field effect mobility is mainly controlled by the front channel composition, V th is dominantly affected by the composition of the back channel. For instance, the use of an In-rich back channel that contains a large carrier concentration results in negatively shifted V th while a relatively In-poor back channel that contains a small carrier concentration doesnt. Also, the V th of an In-rich front channel device was near 0 V by applying an In-poor back channel. These results indicate that the adjustment of back channel carrier concentration and conductivity allows the control of V th in Hf-In-Zn-O devices.