Graphene based materials: Past, present and future

The linear dispersion relation in graphene gives rise to a surprising prediction: the resistivity due to isotropic scatterers, such as white-noise disorder or phonons, is independent of carrier density, n. Here we show that electron-acoustic phonon scattering is indeed independent of n, and contributes only 30 Omega to graphene's room-temperature resistivity. At a technologically relevant carrier density of 1 x1012 cm-2, we infer a mean free path for electron-acoustic phonon scattering of >2 microm and an intrinsic mobility limit of 2 x 105 cm2 V-1 s-1. If realized, this mobility would exceed that of InSb, the inorganic semiconductor with the highest known mobility ( approximately 7.7 x 104 cm2 V-1 s-1; ref. 9) and that of semiconducting carbon nanotubes ( approximately 1 x 105 cm2 V-1 s-1; ref. 10). A strongly temperature-dependent resistivity contribution is observed above approximately 200 K (ref. 8); its magnitude, temperature dependence and carrier-density dependence are consistent with extrinsic scattering by surface phonons at the SiO2 substrate and limit the room-temperature mobility to approximately 4 x 104 cm2 V-1 s-1, indicating the importance of substrate choice for graphene devices.

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Intrinsic and extrinsic performance limits of graphene devices on SiO2.

Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.

One-dimensional electrical contact to a two-dimensional material.

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

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Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 /spl mu/m gate length, an f/sub T/ of 30 GHz and an f/sub max/ of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

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Trapping effects and microwave power performance in AlGaN/GaN HEMTs

Several research groups have been actively pursuing antimonide-based electronic devices in recent years. The advantage of narrow-bandgap Sb-based devices over conventional GaAs- or InP-based devices is the attainment of high-frequency operation with much lower power consumption. This paper will review the progress on three antimonide-based electronic devices: high electron mobility transistors (HEMTs), resonant tunneling diodes (RTDs), and heterojunction bipolar transistors (HBTs). Progress on the HEMT includes the demonstration of Ka- and W-band low-noise amplifier circuits that operate at less than one-third the power of similar InP-based circuits. The RTDs exhibit excellent figures of merit but, like their InP- and GaAs-based counterparts, are waiting for a viable commercial application. Several approaches are being investigated for HBTs, with circuits reported using InAs and InGaAs bases.

Antimonide-based compound semiconductors for electronic devices: A review

The fabrication and characterisation of AlGaN/GaN HEMTs grown on SiC substrates are described. These devices have a transconductance of 70 mS/mm and a gate breakdown voltage in excess of 100V. The HEMTs have a hard pinch-off with nearly ideal subthreshold characteristics. An f/sub T/ of 6 GHz and an f/sub max/ of 11 GHz were measured for 1 /spl mu/m gate length devices.

AlGaN/GaN HEMTs grown on SiC substrates

GaN MESFETs have been fabricated with a 0.25 mu m thick channel on a high resistivity GaN layer grown by metal organic vapour phase epitaxy. These devices have a transconductance of 20 mS/mm. For a 0.7 mu m gate length device, the measured f/sub T/ and f/sub max/ were 8 and 17 GHz, respectively.

Microwave performance of GaN MESFETs

The current status of GaN-based FET technology and performance is reviewed. The fabrication details and the dc and microwave characteristics of GaN MESFETs that utilize Si-doped channels on semi-insulating buffer layers are presented. MESFETs with a 0.8 μm gate have exhibited an f T and f max of 6 and 14 GHz, respectively. These devices have excellent pinchoff characteristics and a source-drain breakdown voltage of over 85 V. A high-field current-collapse phenomenon is observed in these MESFETs in the absence of light. The characteristics of this current collapse as a function of temperature, illuminating wavelength, and time are described. A model describing the current collapse in terms of hot electron injection into the buffer layer is presented.

Walter Kruppa

Papers

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

Antimonide-based compound semiconductors for electronic devices: A review

AlGaN/GaN HEMTs grown on SiC substrates

Microwave performance of GaN MESFETs

Fabrication and Characterization of GaN FETs