The effect of the interface between the semiconductor and the gate dielectrics have been studied in amorphous Indium-gallium Zinc-Oxide (a-IGZO) based dual gate thin film transistors (TFTs) for obtaining stable sensing characteristics of dual gate ion sensitive field effect transistors (DG-ISFETs). As tantalum pentoxide (Ta2O5) possesses a high dielectric constant (~25) with a low band gap (~3 eV), and yttrium oxide (Y2O3) has a higher bandgap (~5.5eV) but a lower dielectric constant (~13), combinations of both were investigated to utilize the possible advantage of both the materials. In this work, thermally grown silicon dioxide (SiO2) was used as the bottom gate dielectric on the silicon substrate (Si), and a-IGZO was used as the semiconductor material. Four combinations of the top gate (TG) dielectrics (i) Y2O3 only, (ii) Y2O3/Ta2O5, (iii) Ta2O5/Y2O3, (iv) Ta2O5 only were used to investigate the effect on the performance parameters and the effects on the interface between a-IGZO and the top gate dielectric. The effect of annealing was studied for minimizing the defects at the interface between the semiconductor and the dielectric. In the first case (Y2O3 only) a sensitivity of ~300mV/pH was obtained. The TFT characteristics were observed to improve by using the stacked Y2O3/Ta2O5 as the TG dielectric, where the sensitivity was ~450mV/pH, the threshold voltage was 3.8V, the sub-threshold swing was 1V/dec and hysteresis was zero. In the third case where the reverse stack (T2O5/Y2O3) was used, the obtained threshold voltage was 24V implying that Ta2O5/Y2O3 combination resulted in defects at the interface which reduced the device performance; thus the sensitivity test could not be performed. In the fourth case, where Ta2O5 acts as TG dielectric, the sensitivity of that device was observed to be 450 mV/pH but the drift and the hysteresis of the device were higher. The defect states, at the interfaces, were studied to understand the performance behavior of devices, using X-ray photon spectroscopy (XPS). All these DGISFTs require small volume (~2��L) of the analyte solution and thus can be used as chemical as well as biosensors.
Keywords: dual gate ISFETs, a-IGZO TFT, top gate dielectric, bottom gate dielectric