Molecular gated transistors: Role of self-assembled monolayers

In order to understand the biosensing mechanism of field-effect based biosensors and optimize their performance, the effect of each of its molecular building block must be understood. In this work the gating effect of self-assembled linker molecules on field-effect transistor was studied in detail. We have combined Kelvin probe force microscopy, current–voltage measurements, capacitance–voltage measurements, equivalent circuit modeling and device simulations in order to trace the mechanism of silicon-on-insulator biological field-effect transistors. The measurements were conducted on the widely used linker molecules (3-aminopropyl)-trimethoxysilane (APTMS) and 11-aminoundecyl-triethoxysilane (AUTES), which were self-assembled on ozone activated silicon oxide surface covering the transistor channel. In a dry environment, the work function of the modified silicon oxide decreased by more than 1.5 eV, and the transistor threshold voltage increased by about 30 V following the self-assembly. A detailed analysis indicates that these changes are due to negative induced charges on the top dielectric layer, and an effective dipole due to the polar monolayer. However, the self-assembly did not change the silicon flat-band voltage when in contact with an electrolyte. This is attributed to electrostatic screening by the electrolyte.