High mobility bottom gate thin film transistors (TFTs) with an amorphous gallium tin zinc oxide (a-GSZO) channel layer have been produced by rf magnetron cosputtering using a gallium zinc oxide (GZO) and tin (Sn) targets. The effect of postannealing temperatures (200, 250, and 300 degrees C) was evaluated and compared with two series of TFTs produced at room temperature (S1) and 150 degrees C (S2) during the channel deposition. From the results, it was observed that the effect of postannealing is crucial for both series of TFTs either for stability as well as for improving the electrical characteristics. The a-GSZO TFTs (W/L=50/50 mu m) operate in the enhancement mode (n-type), present a high saturation mobility of 24.6 cm(2)/V s, a subthreshold gate swing voltage of 0.38 V/decade, a turn-on voltage of -0.5 V, a threshold voltage of 4.6 V, and an I-on/I-off ratio of 8 x 10(7), satisfying all the requirements to be used as active-matrix backplane. (C) 2008 American Institute of Physics.