pH sensing and noise characteristics of Si nanowire ion-sensitive field effect transistors

We have fabricated Si-nanowire (Si-NW) based ion-sensitive field effect transistors (ISFETSs) for biosensing applications. The ability to prepare a large number of sensors on a wafer, standard silicon microfabrication techniques resulting in cost savings and potential sensitivity are significant advantages in favor of nanoscale ISFETs for future biosensor requirements. The Si-NW ISFETs were produced using a combination of oxide-grown Si-NWs and integrated Ag/AgCl reference electrodes. The Si-NWs were fabricated on a standard silicon-on-insulator wafer using electron-beam lithography and conventional semiconductor processing techniques. To form an Ag/AgCl reference electrode, a 250-nm thick Ag layer was deposited and later chlorinated in 100 mM KCl solution. SEM analysis reveals Si-NWs with a width of ~50 nm and a length of 10 μm. The DC characteristics were measured by placing an ISFET with 100 Si-NWs in parallel in a 0.1× PBS buffer solution. The measured I_D-V_G characteristics show an n-type FET behavior with a relatively high on/off current ratio, reasonable sub-threshold swing value, and low gate-leakage current. The pH responses of the ISFETs with different pH solutions were characterized at room temperature. A lateral shift of the I_D-V_G curve is clearly observed by changing the pH value of the solution. The low frequency noise characteristics have been performed in order to evaluate interface quality of the devices.