High electron mobility InAs nanowire field-effect transistors

Single-crystal InAs nanowires (NWs) are synthesized using metal-organic chemical vapor deposition (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO(2)/n(+)-Si substrate with a global n(+)-Si back-gate and sputtered SiO(x)/Au underlap top-gate. For top-gate NWFETs, we have developed a model that allows accurate estimation of characteristic NW parameters, including carrier field-effect mobility and carrier concentration by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temperature field-effect mobility as high as 6580 cm(2) V(-1) s(-1), which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.     

Label‐Free Attomolar Detection of Proteins Using Integrated Nanoelectronic and Electrokinetic Devices

High‐sensitivity screening of biomarkers is critical to areas ranging from early disease detection and diagnosis to bioterrorism surveillance. Here the development of integrated nanoelectronic and electrokinetic devices for label‐free attomolar detection of proteins is reported. Electrically addressable silicon nanowire field‐effect transistors and electrodes for electrokinetic transport are integrated onto a common sensor chip platform, and the nanowire devices are subsequently functionalized with receptors for selective biomarker detection. Nanowire devices modified with monoclonal antibody for prostate specific antigen exhibit close to a 10⁴ increase in sensitivity due to streaming dielectrophoresis and corresponding electrostatic contribution to the binding affinity after application of an AC electric field. The devices are also modified with receptors for cholera toxin subunit B and achieve a similar enhancement. These results show general applicability of this method, and could open up opportunities in early stage disease detection and the analysis of proteins from single cells.     

Schottky barrier-based silicon nanowire pH sensor with live sensitivity control

We demonstrate a pH sensor based on ultrasensitive nanosize Schottky junctions formed within bottom-up grown dopant-flee arrays of assembled silicon nanowires. A new measurement concept relying on a continuous gate sweep is presented, which allows the straightforward determination of the point of maximum sensitivity of the device and allows sensing experiments to be performed in the optimum regime. Integration of devices into a portable fluidic system and an electrode isolation strategy affords a stable environment and enables long time robust FET sensing measurements in a liquid environment to be carried out. Investigations of the physical and chemical sensitivity of our devices at different pH values and a comparison with theoretical limits are also discussed. We believe that such a combination of nanofabrication and engineering advances makes this Schottky barrier-powered silicon nanowire lab-on-a-chip platform suitable for efficient biodetection and even for more complex biochemical analysis.     

Facile and Clean Release of Vertical Si Nanowires by Wet Chemical Etching Based on Alkali Hydroxides

A simple method to release Si nanowires (SiNWs) from a substrate, with their original length almost intact, is demonstrated. By exploiting the unique chemistry involved for the fabrication of vertical arrays of SiNWs in metal‐assisted chemical etching (MaCE) based either on HF/AgNO₃ or HF/H₂O₂ chemistries, wet etching with alkali hydroxides such as NaOH or KOH preferentially attacks the bottom part of the vertical SiNWs. A protective layer of Si oxide is found to exist on the outer wall of the SiNWs and to play the key role of etch mask during the release‐etching by alkali hydroxides. The clean release of SiNWs also enables the repeated use of the Si substrate for the fabrication of vertical SiNW arrays by MaCE. The released SiNWs are further used for the fabrication of field‐effect transistors on a flexible plastic substrate. The method developed here, when combined with a suitable assembling technique, can be very useful in implementing flexible electronics, or in the fabrication of SiNW composites with other functional materials.     

Thin‐Film Transistors

Thin‐film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin‐film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.     

Tuning the Electrical Properties of Si Nanowire Field‐Effect Transistors by Molecular Engineering

Exposed facets of n‐type silicon nanowires (Si NWs) fabricated by a top‐down approach are successfully terminated with different organic functionalities, including 1,3‐dioxan‐2‐ethyl, butyl, allyl, and propyl‐alcohol, using a two‐step chlorination/alkylation method. X‐ray photoemission spectroscopy and spectroscopic ellipsometry establish the bonding and the coverage of these molecular layers. Field‐effect transistors fabricated from these Si NWs displayed characteristics that depended critically on the type of molecular termination. Without molecules the source–drain conduction is unable to be turned off by negative gate voltages as large as ?20 V. Upon adsorption of organic molecules there is an observed increase in the “on” current at large positive gate voltages and also a reduction, by several orders of magnitude, of the “off” current at large negative gate voltages. The zero‐gate voltage transconductance of molecule‐terminated Si NW correlates with the type of organic molecule. Adsorption of butyl and 1,3‐dioxan‐2‐ethyl molecules improves the channel conductance over that of the original SiO₂? Si NW, while adsorption of molecules with propyl‐alcohol leads to a reduction. It is shown that a simple assumption based on the possible creation of surface states alongside the attachment of molecules may lead to a qualitative explanation of these electrical characteristics. The possibility and potential implications of modifying semiconductor devices by tuning the distribution of surface states via the functionality of attached molecules are discussed.