Mathias Wipf

Nernst Limit in Dual-Gated Si-Nanowire FET Sensors

Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.

Understanding the Electrolyte Background for Biochemical Sensing with Ion-Sensitive Field-Effect Transistors

Silicon nanowire field-effect transistors have attracted substantial interest for various biochemical sensing applications, yet there remains uncertainty concerning their response to changes in the supporting electrolyte concentration. In this study, we use silicon nanowires coated with highly pH-sensitive hafnium oxide (HfO(2)) and aluminum oxide (Al(2)O(3)) to determine their response to variations in KCl concentration at several constant pH values. We observe a nonlinear sensor response as a function of ionic strength, which is independent of the pH value. Our results suggest that the signal is caused by the adsorption of anions (Cl(-)) rather than cations (K(+)) on both oxide surfaces. By comparing the data to three well-established models, we have found that none of those can explain the present data set. Finally, we propose a new model which gives excellent quantitative agreement with the data.

True Reference Nanosensor Realized with Silicon Nanowires

Conventional gate oxide layers (e.g., SiO(2), Al(2)O(3), or HfO(2)) in silicon field-effect transistors (FETs) provide highly active surfaces, which can be exploited for electronic pH sensing. Recently, great progress has been achieved in pH sensing using compact integrateable nanowire FETs. However, it has turned out to be much harder to realize a true reference electrode, which--while sensing the electrostatic potential--does not respond to the proton concentration. In this work, we demonstrate a highly effective reference sensor, a so-called reference FET, whose proton sensitivity is suppressed by as much as 2 orders of magnitude. To do so, the Al(2)O(3) surface of a nanowire FET was passivated with a self-assembled monolayer of silanes with a long alkyl chain. We have found that a full passivation can be achieved only after an extended period of self-assembling lasting several days at 80 °C. We use this slow process to measure the number of active proton binding sites as a function of time by a quantitative comparison of the measured nonlinear pH-sensitivities to a theoretical model (site-binding model). Furthermore, we have found that a partially passivated surface can sense small changes in the number of active binding sites reaching a detection limit of δN(s) ≈ 170 μm(-2) Hz(-1/2) at 10 Hz and pH 3.

Selective sodium sensing with gold-coated silicon nanowire field-effect transistors in a differential setup.

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (e.g., Al2O3 or HfO2) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na(+)) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na(+) response of ≈-44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H(+)), potassium (K(+)), and chloride (Cl(-)) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements.

High mobility graphene ion-sensitive field-effect transistors by noncovalent functionalization

Noncovalent functionalization is a well-known nondestructive process for property engineering of carbon nanostructures, including carbon nanotubes and graphene. However, it is not clear to what extend the extraordinary electrical properties of these carbon materials can be preserved during the process. Here, we demonstrated that noncovalent functionalization can indeed delivery graphene field-effect transistors (FET) with fully preserved mobility. In addition, these high-mobility graphene transistors can serve as a promising platform for biochemical sensing applications.

Sensing with Advanced Computing Technology: Fin Field-Effect Transistors with High-k Gate Stack on Bulk Silicon

Field-effect transistors (FETs) form an established technology for sensing applications. However, recent advancements and use of high-performance multigate metal-oxide semiconductor FETs (double-gate, FinFET, trigate, gate-all-around) in computing technology, instead of bulk MOSFETs, raise new opportunities and questions about the most suitable device architectures for sensing integrated circuits. In this work, we propose pH and ion sensors exploiting FinFETs fabricated on bulk silicon by a fully CMOS compatible approach, as an alternative to the widely investigated silicon nanowires on silicon-on-insulator substrates. We also provide an analytical insight of the concept of sensitivity for the electronic integration of sensors. N-channel fully depleted FinFETs with critical dimensions on the order of 20 nm and HfO2 as a high-k gate insulator have been developed and characterized, showing excellent electrical properties, subthreshold swing, SS ∼ 70 mV/dec, and on-to-off current ratio, Ion/Ioff ∼ 10(6), at room temperature. The same FinFET architecture is validated as a highly sensitive, stable, and reproducible pH sensor. An intrinsic sensitivity close to the Nernst limit, S = 57 mV/pH, is achieved. The pH response in terms of output current reaches Sout = 60%. Long-term measurements have been performed over 4.5 days with a resulting drift in time δVth/δt = 0.10 mV/h. Finally, we show the capability to reproduce experimental data with an extended three-dimensional commercial finite element analysis simulator, in both dry and wet environments, which is useful for future advanced sensor design and optimization.

Silicon-Based Ion-Sensitive Field-Effect Transistor Shows Negligible Dependence on Salt Concentration at Constant pH

wherethe interface between the tran-sistor channel and the solutionwas made from silica, we demonstrate here that FETs coveredwith a thin alumina layer are almost insensitive to ions at dif-ferent concentrations except for hydrogen ions. Such FETs aretherefore ideal pH sensors, only responding to protons andnot to other ions. We also demonstrate that this result canonly be obtained if the liquid gate potential is carefully appliedthrough a counter electrode and potentiometrically sensed bya reference electrode.Sensing silicon FETs were produced according to the previ-ously reported protocol

High-k dielectric FinFETs towards sensing integrated circuits

This work presents a well-defined electronic device, namely a n-channel high-k dielectric FinFET (Fin Field Effect Transistor) as new label-free sensor for enhanced sensing integrated circuits. Metal gate FinFETs on bulk Si have been successfully electrically characterized, showing excellent SS (Subthreshold Slope) and high Ion/Ioff ratio. Exposed n-channel FinFETs, integrated on the same die, have been demonstrated pH sensitive with high current variation per pH unit. Herein, we also describe the fabrication process, assisted by FEA (Finite Element Analysis) simulations, and the HfO2 characterization.

Implementing Silicon Nanoribbon Field-Effect Transistors as Arrays for Multiple Ion Detection

Ionic gradients play a crucial role in the physiology of the human body, ranging from metabolism in cells to muscle contractions or brain activities. To monitor these ions, inexpensive, label-free chemical sensing devices are needed. Field-effect transistors (FETs) based on silicon (Si) nanowires or nanoribbons (NRs) have a great potential as future biochemical sensors as they allow for the integration in microscopic devices at low production costs. Integrating NRs in dense arrays on a single chip expands the field of applications to implantable electrodes or multifunctional chemical sensing platforms. Ideally, such a platform is capable of detecting numerous species in a complex analyte. Here, we demonstrate the basis for simultaneous sodium and fluoride ion detection with a single sensor chip consisting of arrays of gold-coated SiNR FETs. A microfluidic system with individual channels allows modifying the NR surfaces with self-assembled monolayers of two types of ion receptors sensitive to sodium and fluoride ions. The functionalization procedure results in a differential setup having active fluoride- and sodium-sensitive NRs together with bare gold control NRs on the same chip. Comparing functionalized NRs with control NRs allows the compensation of non-specific contributions from changes in the background electrolyte concentration and reveals the response to the targeted species.

Finfet with fully PH-responsive HFO 2 as highly stable biochemical sensor

In this work, highly scaled FinFETs (Fin Field Effect Transistors) are proposed as both sensing and circuit units of a lab-on-a-chip platform. The FinFET-based sensors with an HfO₂ gate oxide demonstrate full pH-response with ΔV_th ≈ 56 mV/pH. High readout sensitivity S_out = ΔI_d/I_d ≈ 43% is achieved in combination with excellent device electronic properties, i.e. SS = 77 mV/dec and I_on/I_off =1.5×10⁶. High long-term stability is proven over 4.5 days with a drift in time limited at 0.14 mV/h.