Optimization of Signal to Noise Ratio in Silicon Nanowire ISFET Sensors

Inversion-mode (IM) and depletion-mode (DM) ion-sensitive field effect transistors (ISFETs) are investigated in terms of dc characteristics, pH response and low-frequency noise (LFN) characteristics. The dc characteristics show a low threshold voltage (V-TH) of 28 mV for the DM ISFETs, which is preferred for the long lifetime of the pseudo-reference electrode. The DM ISFETs exhibit an enhanced pH response in the subthreshold region, which comes from the lower sub-threshold swing. The LFN analysis for both devices shows similar level of noise equivalent current (I-n.RMS) near V-TH; otherwise, a reduction of I-n.RMS is obtained in the DM ISFETs in the linear region. In addition, the signal-to-noise ratio of the DM ISFETs is improved by 82.9% compared with the IM ISFETs in the sub-threshold region. Consequently, the DM ISFETs can be a better sensor platform for low-power, portable, and high-precision performance.