Modeling of Low Concentrated Buffer DNA Detection with Suspend Gate Field-Effect Transistors (SGFET)

The experimental data of a suspend gate field-effect transistor (SGFET) have been analyzed with three different models. A SGFET is a MOSFET with an elevated gate and an empty space below it. The exposed gate-oxide layer is bio functionalized with single stranded DNA, which is able to hybridize with a complementary strand. Due to the intrinsic charge of the phosphate groups (minus one elementary charge per group) of the DNA, large shifts in the transfer characteristics are induced. Thus label-free, time-resolved, and in-situ detection of DNA is possible. It can be shown that for buffer concentrations below mmol/1 the Poisson-Boltzmann description it is not valid anymore. Because of the low number of counter ions at small buffer concentrations, the screening of the oligo-deoxynucleotides/DNA is more appropriately described with the Debye-Huckel model. Additionally we propose an extended Poisson-Boltzmann model which takes the closest possible ion distance to the oxide surface into account, and we compare the analytical solution of this model with the Poisson-Boltzmann and the Debye-Huckel model.