High Access Resistance Research Articles

Temperature dependence of current response to sub-terahertz radiation of AlGaN/GaN and graphene transistors

The current response to sub-terahertz radiation was studied experimentally over a wide range of temperatures for AlGaN/GaN and graphene transistors. It was found that the responsivity is enhanced at low temperatures by about an order of magnitude for AlGaN/GaN and more than two orders of magnitude for graphene transistors. However, the responsivity increase saturates at temperatures below 70–100 K. These results were explained by the temperature dependence of the charge carriers' mobility, which plays a dominant role in the current responsivity. It is also shown that in a realistic device, the access resistance may decrease the current responsivity, and in the case of high access resistance, the subthreshold swing may also affect the current responsivity. These results showed that extremely high carriers' mobility in graphene makes it very promising for high-frequency detectors.

AlGaN/GaN High Electron Mobility Transistors on Semi-Insulating Ammono-GaN Substrates with Regrown Ohmic Contacts.

AlGaN/GaN high electron mobility transistors on semi-insulating bulk ammonothermal GaN have been investigated. By application of regrown ohmic contacts, the problem with obtaining low resistance ohmic contacts to low-dislocation high electron mobility transistor (HEMT) structures was solved. The maximum output current was about 1 A/mm and contact resistances was in the range of 0.3–0.6 ·mm. Good microwave performance was obtained due to the absence of parasitic elements such as high access resistance.

In vivo whole-cell recording with high success rate in anaesthetized and awake mammalian brains.

As a critical technique for dissection of synaptic and cellular mechanisms, whole-cell patch-clamp recording has become feasible for in vivo preparations including both anaesthetized and awake mammalian brains. However, compared with in vitro whole-cell recording, in vivo whole-cell recording often suffers from low success rates and high access resistance, preventing its wide application in physiological analysis of neural circuits. Here, we describe experimental procedures for achieving in vivo amphotericin B-perforated whole-cell recording as well as conventional (breakthrough) whole-cell recording from rats and mice. The success rate of perforated whole-cell recordings was 70―80 % in the hippocampus and neocortex, and access resistance was 40―70 MΩ. The success rate of conventional whole-cell recordings was ~50 % in the hippocampus, with access resistance of 20―40 MΩ. Recordings were stable, and in awake, head-fixed animals, ~50 % whole-cell patched neurons could be held for > 1 hr. The conventional whole-cell recording also permitted infusion of pharmacological agents, such as intracellular blockers of Na+ channels and NMDA receptors. These findings open new possibilities for synaptic and cellular analysis in vivo.Electronic supplementary materialThe online version of this article (doi:10.1186/s13041-016-0266-7) contains supplementary material, which is available to authorized users.

High Yield Subcortical Patch Clamping in Vivo

Whole cell recording from subcortical structures has revealed much about the membrane properties of sensory systems. Since its introduction by Margrie et al in 2002, however, patch clamp recordings deep in the intact brain have suffered from low yield and high access resistance, primarily due to pipette contamination during descent to subcortical nuclei.We describe a novel algorithm and best practice for reducing pipette tip contamination and improving access resistance when targeting deep, subcortical neurons in mice (e.g., 1-3 mm deep). In this work, we introduce an automated method for dodging obstructions such as neurons and blood vessels during descent.Traditionally, pipettes are localized in the intact brain by blind, linear rapid descent to the subcortical region of interest (e.g., hippocampus, thalamus). During this descent, pipettes are often contaminated by debris that prevents gigasealing and increases access resistance. Using this new algorithm, pipette resistance is measured at 128 Hz during descent for a high spatial resolution. If an obstruction is encountered prior to the region of interest (as indicated by an increase in resistance >1 Mohm threshold), the pipette is stopped, retracted, and moved laterally in 20 um steps. Lateral steps are repeated until resistance decreases below threshold, indicating that the obstruction has been avoided and descent is resumed.Pipettes inserted using this method arrived at the region of interest without a significant change in resistance (<300 kohms at 3000 um) 69% of the time (n=46), while pipettes inserted using traditional blind, linear methods were inserted successfully 32% of the time (n=31); Whole cell recordings following successful descent resulted in lower access resistance recordings and smaller holding currents than those inserted with the traditional algorithm. This pipette localization method therefore greatly improves the quality and accessibility of subcortical tissues in the intact brain.

Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 1011 cm–2. The KFM measurements suggest that the device performance of the staggered C8–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures.

Direct evaluation of low-field mobility and access resistance in pentacene field-effect transistors

Organic field-effect transistors (OFETs) suffer from limitations such as low mobility of charge carriers and high access resistance. Direct and accurate evaluation of these quantities becomes crucial for understanding the OFETs properties. We introduce the Y function method (YFM) to pentacene OFETs. This method allows us to evaluate the low-field mobility without the access or contact resistance influence. The low-field mobility is shown to be more appropriate than the currently applied field-effect mobility for the OFETs’ performance evaluation. Its unique advantage is to directly suppress the contact resistance influence in individual transistors, although such contact resistance is a constant as compared to the widely accepted variable one with respect to the gate voltage. After a comparison in detail with the transmission-line method, the YFM proved to be a fast and precise alternative method for the contact resistance evaluation. At the same time, how the contact resistance affects the effective mobility and the field-effect mobility in organic transistors is also addressed.

Superconductivity in a single-C60 transistor

Single-molecule transistors have enabled studies of magnetism and other correlated nanoscale behaviour, but superconductivity has not been observed with this approach. It is now shown that superconducting junctions on both sides of a C60 molecule induce superconductivity across the whole device. Single-molecule transistors are currently attracting enormous attention as possible quantum information processing devices1,2,3,4,5. An intrinsic limitation to their prospects, however, is associated with the presence of a small number of quantized conductance channels, each channel with a high access resistance of at best RK/2=h/2e2=12.9 kΩ. However, when the contacting leads become superconducting, long-range correlations can extend throughout the whole system by means of the proximity effect. This not only lifts the resistive limitation of normal-state contacts, but further paves the way to probe electron transport through a single molecule. Here we demonstrate the realization of superconducting single-molecule transistors involving a single C60 fullerene molecule. In the past few years, we have seen gate-controlled Josephson supercurrents induced in the family of low-dimensional carbon structures such as flakes of two-dimensional graphene6 and portions of one-dimensional carbon nanotubes7. The present study, involving a full zero-dimensional fullerene, completes the picture.

Transconductance Linearity Improvement of Enhancement-Mode Pseudomorphic HEMT with High Gate Forward Turn-On Voltage

In this paper we discuss the sources of the transconductance reduction of enhancement-mode pseudomorphic high electron mobility transistor (E-pHEMT) at a high gate bias and methods of improving the transconductance at a high gate bias. E-pHEMT usually suffers from severe transconductance reduction at a high gate bias. Our simulation showed that this reduction is mainly due to the low channel carrier density and high access resistance of the ungated region rather than the parasitic MESFET phenomenon. An epi structure that facilitates electron transfer through the barrier layer of the ungated region and self align gate (SAG) process lead to an improvement in transconductance linearity.