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The gate conductor material affects the threshold voltage of metal-oxide-semiconductor (MOS) transistors through the influence of the electrochemical work function and electric charge. Measurements of the threshold voltage from current voltage characteristics may therefore provide a method to estimate the electronic properties of biomolecules located on the gate electrode. We have deposited DNA from the corn genome onto the gate oxide of Si nMOS transistors and measured the effects on the current-voltage characteristics. We found that the DNA decreased the drain-source current compared to devices with clean gate oxides and pure buffer solutions. The threshold voltage was extracted by current-voltage measurements in the linear operating region and was found to increase by +1.9 volts after application of the DNA specimen, a value consistent with the expected negative charge density. This large change suggests that MOS devices may be useful as sensitive bioelectronic detectors.