Frank Schwierz

Graphene Transistors: Status, Prospects, and Problems

Graphene is a relatively new material with unique properties that holds promise for electronic applications. Since 2004, when the first graphene samples were intentionally fabricated, the worldwide research activities on graphene have literally exploded. Apart from physicists, also device engineers became interested in the new material and soon the prospects of graphene in electronics have been considered. For the most part, the early discussions on the potential of graphene had a prevailing positive mood, mainly based on the high carrier mobilities observed in this material. This has repeatedly led to very optimistic assessments of the potential of graphene transistors and to an underestimation of their problems. In this paper, we discuss the properties of graphene relevant for electronic applications, examine its advantages and problems, and summarize the state of the art of graphene transistors.

Low-field electron mobility in wurtzite InN

We report on the low-field electron mobility in bulk wurtzite InN at room temperature and over a wide range of carrier concentration calculated by the ensemble Monte Carlo (MC) method. All relevant phonon scatterings are included in the MC simulation. The scattering with ionized impurities is considered in the basic Brooks-Herring and Conwell-Weisskopf formulations. For the steady-state transport, the drift velocity attains a peak value of ∼5×107cm∕s at an electric field strength of 32kV∕cm. The highest calculated low-field mobility for undoped InN amounts to ∼14000cm2∕Vs at room temperature. We compare our theoretically calculated low-field mobilities with experimental data available in the literature and obtain a quite satisfactory agreement. Finally, an empirical low-field mobility model based on the MC simulation results and experimental mobility data is presented.

Modeling of graphene metal-oxide-semiconductor field-effect transistors with gapless large-area graphene channels

A quasianalytical modeling approach for graphene metal-oxide-semiconductor field-effect transistors (MOSFETs) with gapless large-area graphene channels is presented. The model allows the calculation of the I-V characteristics, the small-signal behavior, and the cutoff frequency of graphene MOSFETs. It applies a correct formulation of the density of states in large-area graphene to calculate the carrier-density-dependent quantum capacitance, a steady-state velocity-field characteristics with soft saturation to describe the carrier transport, and takes the source/drain series resistances into account. The modeled drain currents and transconductances show very good agreement with experimental data taken from the literature {Meric et al., [Nat. Nanotechnol. 3, 654 (2008)] and Kedzierski et al., [IEEE Electron Device Lett. 30, 745 (2009)]}. In particular, the model properly reproduces the peculiar saturation behavior of graphene MOSFETs with gapless channels.

Electronics: Industry-compatible graphene transistors

An innovative technique has been developed to manufacture graphene transistors that operate at radio frequencies and low temperatures. The process brings the devices closer to applications. See Letter p.74 Graphene, the one-atom-thick layered form of carbon, shows promise for use in high-frequency microelectronics devices. A team based at the IBM Thomas J. Watson Research Center in New York has now identified a diamond-like form of carbon, which is already well known in the semiconductor industry, as being particularly well suited for use as a substrate for graphene semiconductor devices. Graphene was grown on a copper film substrate by chemical vapour deposition (CVD) and then transferred to a wafer of diamond-like carbon. This was used to produce a high-performance graphene transistor with a cut-off frequency of 155 gigahertz at a gate length of 40 nanometres — the shortest length so far reported. This system not only achieves the highest operation speed so far for CVD-graphene transistors, but also is the smallest well-behaved transistor ever demonstrated on any graphene material.

On the suitability of DD and HD models for the simulation of nanometer double-gate MOSFETs

Abstract The drain currents of nanometer double-gate MOSFETs with gate lengths in the range from 100 to 5 nm are calculated using a hierarchy of simulation approaches. By comparing Monte Carlo (MC), drift-diffusion (DD), and hydrodynamic (HD) simulation results the suitability of the DD and HD models for the investigation of the on- and subthreshold currents of nano-scaled MOSFETs is tested. Modifications of the velocity-field characteristics in the DD simulations are suggested to improve the accuracy of the DD model.

Nanoelectronics: Flat transistors get off the ground

The presence of a large bandgap means that a single layer of molybdenum disulphide can be used to make field-effect transistors with high on/off ratios and reasonably high mobilities.

RF MOSFET: recent advances, current status and future trends

Abstract Recent advances in complementary metal oxide semiconductor (CMOS) processing, continuous scaling of gate length, and progress in silicon on insulator have stirred serious discussions on the suitability of metal-oxide semiconductor field-effect transistors (MOSFETs) for RF/microwave applications. This paper covers the recent advances and current status of mainstream CMOS as the dominating technology in very large scale integration, future trends of RF MOSFETs, and applications of MOSFETs in RF electronics. Aspects of RF MOSFET modeling are also addressed. Despite some lingering debates, the prospects for RF MOS with operating frequencies in the lower GHz range are very promising.

Gate-Recessed AlGaN/GaN Based Enhancement-Mode High Electron Mobility Transistors for High Frequency Operation

By combining a low damage chlorine based gate-recess etching and a sophisticated technology for AlGaN/GaN depletion-mode high electron mobility transistors (HEMTs) we fabricated high performance recessed enhancement-mode HEMTs. A comparative investigation of depletion- and enhancement-mode devices prepared by this technique shows excellent DC and RF properties. A transconductance of 540 mS/mm and cut-off frequencies fT of 39 GHz and fmax of 74 GHz were obtained for 0.25 µm gate enhancement-mode HEMTs. Large-signal power measurements at 2 GHz reveal an output power density of 4.6 W/mm at 68% PAE conclusively demonstrating the capability of our enhancement-mode devices.

Semiconductor devices for RF applications: evolution and current status

Abstract This paper reviews the history, evolution, current status, and applications of semiconductor devices for radio frequency (RF) applications. The most important developments and major milestones leading to modern high-performance RF transistors are presented. Heterostructures, which are key elements for some advanced RF transistors, are described, and an overview of the different transistor types and their figures of merit is given. Applications of RF transistors in civil RF systems with special emphasis on wireless communication systems are addressed, and the issues of transistor reliability are also briefly discussed.

Formation of two-dimensional electron gases in polytypic SiC heterostructures

The formation of two-dimensional electron gases (2DEGs) at polytypic (hexagonal/cubic) SiC heterojunctions (4H/3C SiC and 6H/3C SiC) is investigated by numerical self-consistent solutions of the Schrodinger and Poisson equations. The free-electron-density distributions and conduction-band profiles in the SiC heterostructures are calculated and compared to those occurring at AlGaN/GaN interfaces. Spontaneous and piezoelectric polarization effects in both SiC/SiC and AlGaN/GaN structures are taken into account. The combined effect of the polarization-induced bound charge and conduction-band offset between the hexagonal and cubic SiC polytypes results in the formation of 2DEGs with very high electron sheet concentration. 2DEG sheet densities about 20% larger than that in Al0.3Ga0.7N/GaN structures are calculated for 4H/3C SiC heterostructures. We also find that the 2DEG densities in the 4H/3C are much less sensitive to variations of the barrier layer thickness. The influence of the barrier doping layer on th...