Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Transistors”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

The application of orthogonal photolithography to micro-scale organic field effect transistors and complementary inverters on flexible substrate

Micro-scale pentacene organic field effect transistors (OFETs) were fabricated on a flexible poly(ethylene terephthalate) (PET) substrate. By applying a highly fluorinated developing solvents and its compatible photoresist materials, it has become possible to make the micro-scale patterning for organic devices using standard photolithography without damaging the underlying polymer layers. The flexible pentacene OFETs with 3 μm-sized channel length exhibited stable electrical characteristics under bent configurations and under a large number of repetitive bending cycles. Furthermore, we demonstrated micro-scale organic complementary inverters on a flexible PET substrate using p-type pentacene and n-type copper hexadecafluorophthalocyanine materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Impact of graphene polycrystallinity on the performance of graphene field-effect transistors

We have used a multi-scale physics-based model to predict how the grain size and different grain boundary morphologies of polycrystalline graphene will impact the performance metrics of graphene field-effect transistors. We show that polycrystallinity has a negative impact on the transconductance, which translates to a severe degradation of the maximum and cutoff frequencies. On the other hand, polycrystallinity has a positive impact on current saturation, and a negligible effect on the intrinsic gain. These results reveal the complex role played by graphene grain boundaries and can be used to guide the further development and optimization of graphene-based electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Quantum mechanical solver for confined heterostructure tunnel field-effect transistors

Heterostructure tunnel field-effect transistors (HTFET) are promising candidates for low-power applications in future technology nodes, as they are predicted to offer high on-currents, combined with a sub-60 mV/dec subthreshold swing. However, the effects of important quantum mechanical phenomena like size confinement at the heterojunction are not well understood, due to the theoretical and computational difficulties in modeling realistic heterostructures. We therefore present a ballistic quantum transport formalism, combining a novel envelope function approach for semiconductor heterostructures with the multiband quantum transmitting boundary method, which we extend to 2D potentials. We demonstrate an implementation of a 2-band version of the formalism and apply it to study confinement in realistic heterostructure diodes and p-n-i-n HTFETs. For the diodes, both transmission probabilities and current densities are found to decrease with stronger confinement. For the p-n-i-n HTFETs, the improved gate control is found to counteract the deterioration due to confinement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effects of buffer layers on the performance and stability of flexible InGaZnO thin film transistors on polyimide substrates

We demonstrated the fabrication of flexible amorphous indium gallium zinc oxide thin-film transistors (TFTs) on high-temperature polyimide (PI) substrates, which were debonded from the carrier glass after TFT fabrication. The application of appropriate buffer layers on the PI substrates affected the TFT performance and stability. The adoption of the SiN{sub x}/AlO{sub x} buffer layers as water and hydrogen diffusion barriers significantly improved the device performance and stability against the thermal annealing and negative bias stress, compared to single SiN{sub x} or SiO{sub x} buffer layers. The substrates could be bent down to a radius of curvature of 15 mm and the devices remained normally functional.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Inkjet printed ambipolar transistors and inverters based on carbon nanotube/zinc tin oxide heterostructures

We report ambipolar field-effect transistors (FETs) consisting of inkjet printed semiconductor bilayer heterostructures utilizing semiconducting single-walled carbon nanotubes (SWCNTs) and amorphous zinc tin oxide (ZTO). The bilayer structure allows for electron transport to occur principally in the amorphous oxide layer and hole transport to occur exclusively in the SWCNT layer. This results in balanced electron and hole mobilities exceeding 2 cm{sup 2} V{sup −1} s{sup −1} at low operating voltages (<5 V) in air. We further show that the SWCNT-ZTO hybrid ambipolar FETs can be integrated into functional inverter circuits that display high peak gain (>10). This work provides a pathway for realizing solution processable, inkjet printable, large area electronic devices, and systems based on SWCNT-amorphous oxide heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Probing temperature gradients within the GaN buffer layer of AlGaN/GaN high electron mobility transistors with Raman thermography

We demonstrate the ability of confocal Raman thermography using a spatial filter and azimuthal polarization to probe vertical temperature gradients within the GaN buffer layer of operating AlGaN/GaN high electron mobility transistors. Temperature gradients in the GaN layer are measured by using offset focal planes to minimize the contribution from different regions of the GaN buffer. The measured temperature gradient is in good agreement with a thermal simulation treating the GaN thermal conductivity as homogeneous throughout the layer and including a low thermal conductivity nucleation layer to model the heat flow between the buffer and substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Single particle transport in two-dimensional heterojunction interlayer tunneling field effect transistor

The single particle tunneling in a vertical stack consisting of monolayers of two-dimensional semiconductors is studied theoretically, and its application to a novel Two-dimensional Heterojunction Interlayer Tunneling Field Effect Transistor (Thin-TFET) is proposed and described. The tunneling current is calculated by using a formalism based on the Bardeen's transfer Hamiltonian, and including a semi-classical treatment of scattering and energy broadening effects. The misalignment between the two 2D materials is also studied and found to influence the magnitude of the tunneling current but have a modest impact on its gate voltage dependence. Our simulation results suggest that the Thin-TFETs can achieve very steep subthreshold swing, whose lower limit is ultimately set by the band tails in the energy gaps of the 2D materials produced by energy broadening. The Thin-TFET is thus very promising as a low voltage, low energy solid state electronic switch.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Impact of contact and access resistances in graphene field-effect transistors on quartz substrates for radio frequency applications

High-frequency performance of graphene field-effect transistors (GFETs) has been limited largely by parasitic resistances, including contact resistance (R{sub C}) and access resistance (R{sub A}). Measurement of short-channel (500 nm) GFETs with short (200 nm) spin-on-doped source/drain access regions reveals negligible change in transit frequency (f{sub T}) after doping, as compared to ∼23% f{sub T} improvement for similarly sized undoped GFETs measured at low temperature, underscoring the impact of R{sub C} on high-frequency performance. DC measurements of undoped/doped short and long-channel GFETs highlight the increasing impact of R{sub A} for larger GFETs. Additionally, parasitic capacitances were minimized by device fabrication using graphene transferred onto low-capacitance quartz substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Examination of hot-carrier stress induced degradation on fin field-effect transistor

Degradation in fin field-effect transistor devices was investigated in detail under various hot-carrier stress conditions. The threshold voltage (V{sub TH}) shift, substrate current (I{sub B}), and subthreshold swing were extracted to determine the degradation of a device. The power-law time exponent of the V{sub TH} shift was largest at V{sub G} = 0.3 V{sub D}, indicating that the V{sub TH} shift was dominated by interface state generation. Although the strongest impact ionization occurred at V{sub G} = V{sub D}, the V{sub TH} shift was mainly caused by electron trapping resulting from a large gate leakage current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High performance transistors via aligned polyfluorene-sorted carbon nanotubes

We evaluate the performance of exceptionally electronic-type sorted, semiconducting, aligned single-walled carbon nanotubes (s-SWCNTs) in field effect transistors (FETs). High on-conductance and high on/off conductance modulation are simultaneously achieved at channel lengths which are both shorter and longer than individual s-SWCNTs. The s-SWCNTs are isolated from heterogeneous mixtures using a polyfluorene-derivative as a selective agent and aligned on substrates via dose-controlled, floating evaporative self-assembly at densities of ∼50 s-SWCNTs μm{sup −1}. At a channel length of 9 μm the s-SWCNTs percolate to span the FET channel, and the on/off ratio and charge transport mobility are 2.2 × 10{sup 7} and 46 cm{sup 2} V{sup −1} s{sup −1}, respectively. At a channel length of 400 nm, a large fraction of the s-SWCNTs directly span the channel, and the on-conductance per width is 61 μS μm{sup −1} and the on/off ratio is 4 × 10{sup 5}. These results are considerably better than previous solution-processed FETs, which have suffered from poor on/off ratio due to spurious metallic nanotubes that bridge the channel. 4071 individual and small bundles of s-SWCNTs are tested in 400 nm channel length FETs, and all show semiconducting behavior, demonstrating the high fidelity of polyfluorenes as selective agents and the promise of assembling s-SWCNTs from solution to create high performance semiconductor electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Non-hysteretic superconducting quantum interference proximity transistor with enhanced responsivity

This Letter presents fabrication and characterization of an optimized superconducting quantum interference proximity transistor. The present device, characterized by reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a function of magnetic flux piercing the superconducting loop.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Kelvin probe force microscopy study on operating In-Sn-O-channel ferroelectric-gate thin-film transistors

Surface potentials of an operating In-Sn-O channel ferroelectric-gate transistor (FGT) were mapped by Kelvin probe force microscopy. We clearly observed a gradual transition within the channel from linear potential profile to superlinear ones when drain voltage approaches and overcomes gate voltage, which is related to the physics of electronic transport under field-effect doping. The dependence of field-effect mobility on gate-bias and lateral field, as well as the effect of source/drain electrode materials on transport properties, was also elucidated. This study provides useful information for optimizing the FGT performance and for understanding its underlying physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of proton irradiation on AlGaN/GaN high electron mobility transistor off-state drain breakdown voltage

The effect of proton irradiation on the off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates was studied by irradiating protons from the backside of the samples through via holes fabricated directly under the active area of the HEMTs. There was no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for HEMTs irradiated with 275 keV protons, for which the defects created by the proton irradiation were intentionally placed in the GaN buffer. HEMTs with defects positioned in the 2 dimensional electron gas channel region and AlGaN barrier using 330 keV protons not only showed degradation of both drain current and extrinsic transconductance but also exhibited an improvement of the off-state drain breakdown voltage. Finite-element simulations showed the enhancement of the latter were due to a reduction in electric field strength at the gate edges by introduction of charged defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Selective epitaxial growth of monolithically integrated GaN-based light emitting diodes with AlGaN/GaN driving transistors

In this Letter, we report selective epitaxial growth of monolithically integrated GaN-based light emitting diodes (LEDs) with AlGaN/GaN high-electron-mobility transistor (HEMT) drivers. A comparison of two integration schemes, selective epitaxial removal (SER), and selective epitaxial growth (SEG) was made. We found the SER resulted in serious degradation of the underlying LEDs in a HEMT-on-LED structure due to damage of the p-GaN surface. The problem was circumvented using the SEG that avoided plasma etching and minimized device degradation. The integrated HEMT-LEDs by SEG exhibited comparable characteristics as unintegrated devices and emitted modulated blue light by gate biasing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Engineering the (In, Al, Ga)N back-barrier to achieve high channel-conductivity for extremely scaled channel-thicknesses in N-polar GaN high-electron-mobility-transistors

Scaling down the channel-thickness (t{sub ch}) in GaN/(In, Al, Ga)N high-electron-mobility-transistors (HEMTs) is essential to eliminating short-channel effects in sub 100 nm gate length HEMTs. However, this scaling can degrade both charge density (n{sub s}) and mobility (μ), thereby reducing channel-conductivity. In this study, the back-barrier design in N-polar GaN/(In, Al, Ga)N was engineered to achieve highly conductive-channels with t{sub ch} < 5-nm using metal organic chemical vapor deposition. Compositional-grading was found to be the most effective approach in reducing channel-conductivity for structures with t{sub ch} ∼ 3-nm. For a HEMT with 3-nm-thick-channel, a sheet-resistance of 329 Ω/◻ and a peak-transconductance of 718 mS/mm were demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of trap states under Schottky contact in GaN/AlGaN/AlN/GaN high electron mobility transistors

Forward gate-bias stress experiments are performed to investigate the variation of trap states under Schottky contact in GaN-based high electron mobility transistors. Traps with activation energy E{sub T} ranging from 0.22 eV to 0.31 eV are detected at the gate-semiconductor interface by dynamic conductance technique. Trap density decreases prominently after stressing, particularly for traps with E{sub T} > 0.24 eV. X-ray photoelectron spectroscopy measurements reveal a weaker Ga-O peak on the stressed semiconductor surface. It is postulated that oxygen is stripped by Ni to form NiO upon electrical stress, contributing to the decrease in O{sub N} donor sates under the gate contact.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Monochromatic light-assisted erasing effects of In-Ga-Zn-O thin film transistor memory with Al{sub 2}O{sub 3}/Zn-doped Al{sub 2}O{sub 3}/Al{sub 2}O{sub 3} stacks

We studied how electrical erasing of indium gallium zinc oxide-thin-film-transistor memory was improved by adding concurrent irradiation with monochromatic light (ML). At fixed gate bias, irradiating at wavelengths of ≤500 nm increased the erasing window (ΔV{sub th-e}) significantly: At a gate bias of −20 V and an erasing time of 5 min, ML irradiation at 400 nm increased ΔV{sub th-e} from 0.29 to 3.21 V. ΔV{sub th-e} increased incrementally with gate bias, erasing time, and ML power density, particularly at short ML wavelengths. Analyzing our experimental results, we discuss the underlying erasure mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Radio-frequency dispersive detection of donor atoms in a field-effect transistor

Radio-frequency dispersive read-out can provide a useful probe to nano-scale structures, such as nano-wire devices, especially, when the implementation of charge sensing is not straightforward. Here, we demonstrate dispersive “gate-only” read-out of phosphor donors in a silicon nano-scale transistor. The technique enables access to states that are only tunnel-coupled to one contact, which is not easily achievable by other methods. This allows us to locate individual randomly placed donors in the device channel. Furthermore, the setup is naturally compatible with high bandwidth access to the probed donor states and may aid the implementation of a qubit based on coupled donors.

36 MATERIALS SCIENCE↗