# Gating Hysteresis as an Indicator for Silicon Nanowire FET Biosensors
## 2 Materials and Methods  
In order to fabricate such devices, a previously reported process was followed [32; 33; 42]. Nominally intrinsic, undoped silicon nanowires were grown by bottom-up means employing the vapor-liquid-solid (VLS) mechanism. SiO\({}_{2}\) coated Si wafers were plasma activated and functionalized with poly(diallyldimethylammonium chloride) (PDDA) followed by deposition of gold nanoparticles of an average 19 nm in diameter to be used as seeds for the nanowire growth. PDDA ensures a high gold nanoparticle density with no aggregation. An additional plasma cleaning was applied to remove organic residues, preventing contamination of the nanowires. The substrates were heated to 450 \({}^{\circ}\)C in a chemical vapor deposition reactor in hydrogen atmosphere at 65 mbar. A SiH\({}_{4}\)/H\({}_{2}\) (10/1 _p/p_) gas mixture was injected at 200 sccm for 15 min, which resulted in the growth of nanowires with 5-10 \(\upmu\)m length and 22 nm diameter. Figure 1: (**a**) Schematic representation of expected shift of the transfer curve upon thrombin binding on the surface. (**b**) Expected shrinkage of the hysteresis in presence of thrombin in solution. (**c**) Details of the biosensor including microelectrodes and nanowires. Bottom panel shows the formed nickel silicide after the annealing process. (**d**) Concept depiction of the fluidic channel on the sensing area, and (**e**) fluidic assembly including mechanical clamping system, tubings, and electrical connections. Using the nanowire growth substrate as donor, parallel arrangements of silicon nanowires were contact-printed onto a receiver Si wafer coated with a SiO\({}_{2}\) layer for further chip fabrication. Source and drain electrodes in meander shaped patterns were formed by UV photo-lithography, sputter-deposition of nickel and lift-off in order to integrate a plurality of nanowire channels in parallel. The native oxide shell was removed from the nanowires prior to the nickel deposition by etching in buffered HF. This step is necessary since the oxide shell could block the diffusion of nickel in to the wire during the next silicidation step. The contact resistance at the silicon-nickel interfaces was dramatically reduced via thermal annealing at 500 \({}^{\circ}\)C in N\({}_{2}\)/H\({}_{2}\) gas (10/1 _p/p_) and resulted in a NiSi\({}_{2}\) intrusion into the Si nanowires (see Figure 1c). A 15 nm-thick film of Al\({}_{2}\)O\({}_{3}\) was deposited using atomic layer deposition (ALD) as a gate dielectric and as an ion-sensitive interface. Devices with close to ideal Nernstian sensitivity were obtained using this oxide material [32; 43]. Each FET had up to 10\({}^{3}\) parallel nanowire channels. The sensing capabilities of similar devices in a liquid environment have been demonstrated recently by [PERSON] et al. [32] for pH detection. Microchips of approximately 2 \(\times\) 2 cm\({}^{2}\) were mounted onto a microfluidic device consisting of a PMMA (polymethylmethacrylate) capillary tubing holder and an embedded PDMS (polydimethylsiloxane) channel with width of about 800 \(\upmu\)m (see Figure 1d,e). After placing the PDMS channel above the sensitive NW region, the microchip was mechanically clamped to prevent leakage of the fluids. Ag/AgCl reference electrode (Microelectrodes Inc., USA) installed at the inlet tube assures it measures against a stable liquid potential. The transfer and output characteristics of the fabricated SiNW FET, exposed to a deionized (DI) water environment are represented in Figure 2a,b. The multi-silicon nanowire FET reveals p-type characteristics with a slight shift of the minimum current region towards positive gate voltages, which can be attributed to positive fixed charges arising by the stacking of Al\({}_{2}\)O\({}_{3}\) on top of SiO\({}_{2}\)[44]. Integration of devices into a portable fluidic system and the electrode isolation strategy described above permits long time robust sensing measurements in liquid environment.