Performance optimization of single-layer and double-layer high-k gate nanoscale ion-sensitive field-effect transistors

The use of high-k dielectric materials for the gate insulator of nanoscale Ion-Sensitive Field-Effect Transistor (ISFET) has been demonstrated to generally improve the ISFET performance. High-k gate insulator reduces the gate leakage current and increases the gate capacitance, which lead to higher trans-conductance and higher current sensitivity. The best high-k material for ISFET sensors is not simply the material of highest dielectric constant because high-k ISFET performance depends on many conflicting factors such as dissociation properties with electrolytes and linearity of voltage/current variation with pH change. In this work, a comparative simulation study is presented for six high-k gate insulators (Si3N4, Al2O3, ZrO2, Ta2O5, HfO2, and TiO2), along with SiO2 for comparison, for nanoscale ISFET sensors, comparing their sensitivity and linearity. The simulation tool used (NIST) is based on the numerical solution of nanoscale ballistic MOSFET equations along with Gouy-Chapman-Stern model equations. The study presents a guideline for the determination of the optimum insulator material and insulator thickness used for nanoscale ISFETs. The study included single-layer as well as double-layer insulators in which two layers of different dielectric materials are used.