# Gating Hysteresis as an Indicator for Silicon Nanowire FET Biosensors
## 1 Introduction  
The latest advances in microfluidics and nanotechnology has made possible the transfer of medical diagnostics and health monitoring equipment from the laboratory to the patient's home [1; 2]. Naturally, these developments have triggered the fast evolution of personalized medicine that currently more and more relies on so-called point-of-care (POC) devices. The latter ones can be adapted for prevention, diagnosis and the treatment of diseases, and are especially useful in the context of population ageing [3] or, for example, in developing countries, offering a cheap, simple and rapid molecular detection of pathogens [4], blood protein analysis [5], concentration of blood electrolytes [6], amongst others. The current trend in POC devices for medical applications is to continue the enhancement of the internal technical complexity and high sensitivity on one hand, while keeping a simple user interface [7]. On the other hand they continuously become more cost and energy efficient [8; 9; 10]. To bring state-of-the-art diagnostic devices to the hands of the people in need, a crucial task is to reduce the price of the devices and increase their high-volume delivery at the same time. In these regards, printable silicon nanowire field-effect transistor (SiNW FET) sensors represent a great potentialfor POC biosensors, because they offer the possibility of a cost-efficient wafer-scale production, CMOS compatibility [11], miniaturized integration on a chip, and detection process in real-time [12]. Since the key component of such devices are 1D nanoscale sized elements, the advantages of ultrahigh sensitivity of the measurements down to the fM level [13; 14] given by the channel full depletion capability, in combination with the remarkably high signal-to-noise ratio, are achievable. Biodetection is typically realized within the distance of approximately one Debye length in the liquid, which is comparable to the nanowires' radial dimensions [15; 16; 17; 18]. Charged molecular species approaching the surface influence the spatial distribution of charges around the wires and alter the surface potential in the semiconductor. The surface potential change will lead to a strong change in current when integrating the semiconductor nanowire into a field effect transistor device, because the channel can be depleted more efficiently compared to a bulk semiconductor [19]. Since the conventional receptors; i.e., antibodies--are relatively large and Debye length is strongly dependent on the ionic strength of the environment [20], an efficient sensing requires altering at least one of these parameters. Therefore, either a significant buffer dilution to increase the Debye layer thickness or the use of small receptors and short linker molecules during functionalization, is necessary for this sensor type [18; 20; 21]. As low buffer concentrations can destabilize the formation of three-dimensional biomolecular structures, buffers should only be diluted moderately. To overcome this issue, artificial antibodies; i.e., highly specific aptamers (short oligonucleotides)--synthesized _in vitro_[22] can be used as a functional receptor element. They offer many advantages compared to antibodies including higher stability under various conditions or the potential for facile labeling with functional groups and dyes [23; 24]. The benefits of the combination of aptamers with FETs for biosensing has been reported by a number of groups, where the FETs are adapted for the detection of various analytes including thrombin [25; 26], potassium [27], dopamine [28], and vascular endothelial growth factor [29], among others. Specifically, the detection of human alpha-thrombin has a high clinical relevance as there are diseases correlated with coagulation irregularities with pathological levels in the upper pM and healthy levels in the nM-\(\upmu\)M range [30]. Thus, it is important to cover such wide dynamic range with a biosensor able to offer a stable signal readout in both regions [31].