Stacked Top Gate Dielectrics in Dual Gate Ion Sensitive Field Effect Transistors: Role of Interfaces

Dual gate ion sensitive field effect transistors (DGISFETs) have elicited interest due to the capability to provide pH sensitivity values several times the Nernst limit. The interfaces in the device, the bottom gate (BG) dielectric–semiconductor, the semiconductor–top gate (TG) dielectric, the TG dielectric–electrolyte, play a crucial role in the performance parameters (such as sensitivity, drift, and hysteresis). While most of the works report use of a single TG dielectric, stacks of dielectrics have the potential to provide a better performance by taking advantage of the beneficial properties of the constituent layers. This has received scant attention and is the motivation of the present work. Four types of TG structure schemes were used using single dielectrics and layers of two dielectrics (Y2O3 and Ta2O5) in different sequences. The effects of the material properties and the processing of the dielectrics on the device performance characteristics were investigated, and particularly the interfacial effects utilizing X-ray photelectron spectroscopy depth profiling were studied. Metal oxide semiconductor (MOS) structures and thin film transistor (TFT) structures utilizing these dielectric schemes were fabricated and the leakage, stability, and TFT characteristics were investigated so as to elucidate the experimental results obtained with the DGISFET structures. The simulated and theoretical values of the pH sensitivities were utilized to benchmark the experimental results. The highlights of this detailed study include identification of the diffusion of yttrium into gallium to make the Y2O3/IGZO interface more stable than the Ta2O5/IGZO interface; and along with the higher pH sensitivity of Ta2O5, it is seen that the Ta2O5/Y2O3/IGZO/SiO2/Si is the best of the DGISFET structures investigated with a sensitivity of 454 mV/pH, hysteresis of 75 mV, and a drift of 250 mV/h. The learning can be utilized to fabricate DGISFETs with better performance.