Analytical Modeling of Surface Potential and Figure of Merit Computation for Planar Junctionless pH Sensing BioFET

Abstract

Herein this paper we propose a surface potential based analytical model for planar junctionless field effect transistor (JL-FET) for pH sensing. The electrolyte considered is phosphate buffer saline (PBS) solution which has been modeled as three layered stacked structure consisting of stern layer, ion-permeable membrane and bulk electrolyte. The proposed model has been deduced considering Poisson's equation in the channel region. Relative shift in threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Th</sub> ) and maximum drain current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS,max</sub> ) have been used as sensitivity metrics. The low concentrations of electrolyte (0.01), yielded higher V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Th</sub> sensitivity of 63 mV/pH and 59 mV/pH for bottom and liquid gate respectively as compared to higher molar concentrations of electrolyte. For 0.01 PBS the aggregate drain current shift has been found to be 52.8 μA/pH and is larger for liquid gate operation while as for bottom gate, shift of 18.9 μA/pH is observed. Further considering pH range of 1-14, we computed various figure of merits (FOMs) that include sensitivity, linearity and signal to noise ratio for the device. The FOMs were computed and analyzed for independent operation of liquid and bottom gate for three different molarities of PBS (1, 0.1, 0.01) each with pH range from 1 to 14. Signal to noise ratio of drain current is found maximum for low molar concentrations of electrolyte and also is highest at point of maximum transconductance. The results obtained from analytical model are in good coherence with the TCAD simulation model.