Gil Shalev

Requirement of ATM-Dependent Monoubiquitylation of Histone H2B for Timely Repair of DNA Double-Strand Breaks

The cellular response to DNA double-strand breaks (DSBs) is mobilized by the protein kinase ATM, which phosphorylates key players in the DNA damage response (DDR) network. A major question is how ATM controls DSB repair. Optimal repair requires chromatin relaxation at damaged sites. Chromatin reorganization is coupled to dynamic alterations in histone posttranslational modifications. Here, we show that in human cells, DSBs induce monoubiquitylation of histone H2B, a modification that is associated in undamaged cells with transcription elongation. We find that this process relies on recruitment to DSB sites and ATM-dependent phosphorylation of the responsible E3 ubiquitin ligase: the RNF20-RNF40 heterodimer. H2B monoubiquitylation is required for timely recruitment of players in the two major DSB repair pathways-nonhomologous end-joining and homologous recombination repair-and optimal repair via both pathways. Our data and previous data suggest a two-stage model for chromatin decondensation that facilitates DSB repair.

Standard CMOS Fabrication of a Sensitive Fully Depleted Electrolyte-Insulator-Semiconductor Field Effect Transistor for Biosensor Applications

Microfabricated semiconductor devices are becoming increasingly relevant for detection of biological and chemical components. The integration of active biological materials together with sensitive transducers offers the possibility of generating highly sensitive, specific, selective and reliable biosensors. This paper presents the fabrication of a sensitive, fully depleted (FD), electrolyte-insulator-semiconductor field-effect transistor (EISFET) made with a silicon-on-insulator (SOI) wafer of a thin 10-30 nm active SOI layer. Initial results are presented for device operation in solutions and for bio-sensing. Here we report the first step towards a high volume manufacturing of a CMOS-based biosensor that will enable various types of applications including medical and environmental sensing.

Tunable diameter electrostatically formed nanowire for high sensitivity gas sensing

We report on an electrostatically formed nanowire (EFN)-based sensor with tunable diameters in the range of 16 nm to 46 nm and demonstrate an EFNbased field-effect transistor as a highly sensitive and robust room temperature gas sensor. The device was carefully designed and fabricated using standard integrated processing to achieve the 16 nm EFN that can be used for sensing without any need for surface modification. The effective diameter for the EFN was determined using Kelvin probe force microscopy accompanied by threedimensional electrostatic simulations. We show that the EFN transistor is capable of detecting 100 parts per million of ethanol gas with bare SiO2.

Gain optimization in ion sensitive field-effect transistor based sensor with fully depleted silicon on insulator

The sensitivity in bulk silicon (Si) and in silicon-on-insulator (SOI) ion sensitive field-effect transistor (ISFET) is determined according to its manufacturing process, geometry, and the selected materials. However, in SOI ISFETs the back gate biasing plays a major part in device sensitivity. It is shown that in fully depleted SOI ISFET the existing charge coupling between the front and back interfaces allows for gain optimization in terms of both gain increase and widening of the conventional gain peak. This stands in contrast with bulk Si ISFET where only a single channel exists. Here we report gain increase in ∼40% and increase in gain peak width of ∼250%.

Electrostatic Limit of Detection of Nanowire-Based Sensors

Scanning gate microscopy is used to determine the electrostatic limit of detection (LOD) of a nanowire (NW) based chemical sensor with a precision of sub-elementary charge. The presented method is validated with an electrostatically formed NW whose active area and shape are tunable by biasing a multiple gate field-effect transistor (FET). By using the tip of an atomic force microscope (AFM) as a local top gate, the field effect of adsorbed molecules is emulated. The tip induced charge is quantified with an analytical electrostatic model and it is shown that the NW sensor is sensitive to about an elementary charge and that the measurements with the AFM tip are in agreement with sensing of ethanol vapor. This method is applicable to any FET-based chemical and biological sensor, provides a means to predict the absolute sensor performance limit, and suggests a standardized way to compare LODs and sensitivities of various sensors.

Bioorganic nanodots for non-volatile memory devices

In recent years we are witnessing an intensive integration of bio-organic nanomaterials in electronic devices. Here we show that the diphenylalanine bio-molecule can self-assemble into tiny peptide nanodots (PNDs) of ∼2 nm size, and can be embedded into metal-oxide-semiconductor devices as charge storage nanounits in non-volatile memory. For that purpose, we first directly observe the crystallinity of a single PND by electron microscopy. We use these nanocrystalline PNDs units for the formation of a dense monolayer on SiO2 surface, and study the electron/hole trapping mechanisms and charge retention ability of the monolayer, followed by fabrication of PND-based memory cell device.

Surface chemical modification induces nanometer scale electron confinement in field effect device

Design, preparation, and study of physicochemical properties of molecular assemblies are extremely challenging multidisciplinary research fields. Understanding the elementary principles that correlate these properties with molecular level of electronic behavior will enable us to control basic properties of molecule-based compounds as well as of classical semiconductors. In particular, chemical modification of field effect sensor devices where the metal gate is replaced with organic molecular layer, projects a crucial impact upon the electrical properties of the sensor. In these cases it is important to control the effects in order to ensure that the organic gate is optimized for sensing. Here we used fully depleted silicon-on-insulator (SOI) ion sensitive field effect transistor in order to analyze the projection of surface chemical modification on electronic performance. We suggest that surface activation and the application of 3-aminopropyltrimethoxysilane on top of the gate dielectric introduces negative charge at the Si/SiO(2) interface or/and on top of the gate dielectric and consequently an accumulation layer that confines the electrons to the bottom of the SOI channel. The transistor gain postmodification is characteristic of volume inversion, and therefore suggests that, following modification, the channel electrons are confined to SOI thickness of <10 nm. Finally, measurements of pH sensitivity indicate that the pH sensitivity post-UV/O(3) treatment is maximized suggesting that the negative charge is introduced during the activation process, where the density of the negatively charged amphoteric sites maximized.

Control of the Intrinsic Sensor Response to Volatile Organic Compounds with Fringing Electric Fields

The ability to control surface-analyte interaction allows tailoring chemical sensor sensitivity to specific target molecules. By adjusting the bias of the shallow p-n junctions in the electrostatically formed nanowire (EFN) chemical sensor, a multiple gate transistor with an exposed top dielectric layer allows tuning of the fringing electric field strength (from 0.5 × 107 to 2.5 × 107 V/m) above the EFN surface. Herein, we report that the magnitude and distribution of this fringing electric field correlate with the intrinsic sensor response to volatile organic compounds. The local variations of the surface electric field influence the analyte-surface interaction affecting the work function of the sensor surface, assessed by Kelvin probe force microscopy on the nanometer scale. We show that the sensitivity to fixed vapor analyte concentrations can be nullified and even reversed by varying the fringing field strength, and demonstrate selectivity between ethanol and n-butylamine at room temperature using a single transistor without any extrinsic chemical modification of the exposed SiO2 surface. The results imply an electric-field-controlled analyte reaction with a dielectric surface extremely compelling for sensitivity and selectivity enhancement in chemical sensors.

The Electrostatically Formed Nanowire: A Novel Platform for Gas-Sensing Applications

The electrostatically formed nanowire (EFN) gas sensor is based on a multiple-gate field-effect transistor with a conducting nanowire, which is not defined physically; rather, the nanowire is defined electrostatically post-fabrication, by using appropriate biasing of the different surrounding gates. The EFN is fabricated by using standard silicon processing technologies with relaxed design rules and, thereby, supports the realization of a low-cost and robust gas sensor, suitable for mass production. Although the smallest lithographic definition is higher than half a micrometer, appropriate tuning of the biasing of the gates concludes a conducting channel with a tunable diameter, which can transform the conducting channel into a nanowire with a diameter smaller than 20 nm. The tunable size and shape of the nanowire elicits tunable sensing parameters, such as sensitivity, limit of detection, and dynamic range, such that a single EFN gas sensor can perform with high sensitivity and a broad dynamic range by merely changing the biasing configuration. The current work reviews the design of the EFN gas sensor, its fabrication considerations and process flow, means of electrical characterization, and preliminary sensing performance at room temperature, underlying the unique and advantageous tunable capability of the device.