Physics and scaling prospects of pH-based genome sequencers

Fig. 2 shows the temporal distribution of proton inside the well and the polymer layer (the diffusion coefficient of protons in electrolyte and polymer is D_free = 9.31×10^-5 and D_poly = 4×10^-8 cm²/s, respectively). The calculated voltage shift (ΔV) in MOSFET successfully explains the experiment data with two different sizes of wells and beads, as shown in Fig. 3. As we scale the microwells by a factor of k, the sensitivity remains constant: this is because the number of protons released from bead is proportional to the bead area (~k^-2) and the sensor surface area is also scaled by a factor of k^-2. Since the surface group charge on the oxide surface changes linearly with proton density while the amount excess protons is much smaller than that in the well, the voltage signal remains constant.