Calibration of a pH sensitive buried channel silicon-on-insulator MOSFET for sensor applications

The threshold voltage characteristics of a buried channel silicon-on-insulator MOSFET is examined in solutions of varying acidity. Experiments utilizing an integrated micro-fluidic channel exhibit a variation in threshold voltage that appears approximately linear with pH in the range from pH 4 to pH 7, with a sensitivity of ∼1 V per pH unit. Charge configuration changes in the vicinity of the MOSFET inversion layer due to protonation/deprotonation of the device surface is proposed as an explanation for the observed shifts in threshold voltage. When the pH range is expanded we observe a non-linear relationship between pH and the threshold voltage of the device, this behavior is explained in terms of deprotonation of the different species of the native oxide surface, Numerical simulations of the MOSFET demonstrate that the threshold voltage sensitivity corresponds to an additional surface positive fixed charge density of ∼ 1 × 10¹⁰ cm ^-2 for each pH unit.