Highly sensitive pH sensors based on double-gate silicon nanowire field-effect transistors with dual-mode amplification

This paper presents a method of achieving high sensitivity for silicon nanowire (SiNW) field-effect transistor (FET) pH sensor by operating the FETs in both capacitive amplification mode and differential amplification mode. The capacitive amplification is implemented by employing an inversed double-gate configuration that uses the electrode immersed in a solution well etched from the backside of the sensor chip as the top gate and the SiNW metal gate as the back gate. This enables the threshold voltages of the FETs to be amplified through the ratio of the top gate capacitance to the back gate capacitance. The differential amplification is achieved by a readout circuit chip that is vertically integrated with the sensor chip using flip-chip bonding. This allows the back gate to be controlled independently by the circuits, such that the differential amplification that is originally applicable to single-gate FETs can be applied to double-gate FETs. By operating the dual amplifications simultaneously, a sensitivity of 720.7 mV/pH has been achieved in the pH range of 4−10. Small deviations between the SiNW pH sensor and a commercial pH meter in measuring phosphate buffer saline, bovine serum, and tap water demonstrate the capability of the SiNW pH sensor in measuring practical samples.