Counteracting Threshold-Voltage Drift in Ion-Selective Field Effect Transistors (ISFETs) Using Threshold-Setting Ion Implantation

Ion-selective field effect transistors (ISFETs) exhibit instability, commonly known as drift, in the form of a slow, monotonic, temporal increase in the threshold voltage of the device. A method for counteracting instability inspired by a physical model for threshold voltage drift in pH-sensitive ISFETs is presented. This method involves adjusting the charge density at the insulator-semiconductor interface using threshold-setting ion implantation such that the net charge induced in the semiconductor at the operating point of the device is minimized. The proposed method is analytically validated based on characterization and modeling of drift in an Al₂O₃-gate pH-sensitive ISFET. Counteraction of ISFET drift by ion implantation is also demonstrated using TCAD simulations. The optimum donor-type implant dose of 3.25×10¹¹ cm^-2 determined based on ATLAS simulations is in good agreement with the corresponding dose of 6.58 × 10¹¹ cm^-2 obtained analytically.