Drift Counteraction in Ion-selective Field Effect Transistors by Adjusting the Charge in the Semiconductor

In pH-sensitive ion-selective field effect transistors (ISFET’s) device instability, commonly known as drift, is manifested as a slow, monotonic, temporal increase in the threshold voltage of the device. A method for correction of threshold voltage drift in pH-sensitive ISFETs is presented. This method involves adjusting the charge in the semiconductor such that the variation in the flatband voltage is rendered independent of the temporal variation of the overall insulator capacitance. The adjustment of semiconductor charge can be accomplished by ion implantation to modify the surface charge density in the channel region. Application of this method for correction of ISFET drift is justified based on a physical model providing an accurate, quantitative description of the drift behavior in pH-sensitive ISFETs. The validity of the proposed method is verified by applying this model to demonstrate that drift can be reduced in an Al2O3-gate pH-sensitive ISFET using threshold voltage adjustment by ion implantation.

Keywords:

pH-sensitive ISFET , Drift , Instability , Continuous monitoring