We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

AlGaN/GaN HEMTs-an overview of device operation and applications

Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

The turn-on process and steady state current-voltage characteristics have been investigated in 4H-SiC thyristors with a forward breakover voltage close to 700 V and pulse switch current density of . The current rise constant decreases monotonically with increasing temperature: ns at and ns at T = 500 K at high bias V. The value of ns is the lowest observed value for SiC thyristors. At very high current density the residual voltage drop on 4H-SiC thyristors is lower than on identically rated Si thyristors. The voltage drop decreases monotonically with increasing temperature.

Turn-on process in high voltage 4H-SiC thyristors

High performance 15 kV n-GTOs were demonstrated for the first time in 4H-SiC. The device utilized a 140 μm thick, lightly doped n-type drift layer, with 1450°C lifetime enhancement oxidation, which resulted in a carrier lifetime of 17.5 μs. The p+ backside injector layer was thinned to minimize parasitic resistances. A room temperature forward voltage drop of 5.18 V was observed at a current density of 100A/cm2. A 1 cm2 device showed a leakage current of 0.17 μA at 15 kV. The 4H-SiC n-GTO showed latching characteristics, and showed a turn-off time of 170 ns in a resistive load switching setup, which represents about a factor of 45 improvement in turn-off speed over 4H-SiC p-GTOs with comparable voltage and current ratings.

15 kV n-GTOs in 4H-SiC

Low frequency noise has been studied in forward biased 4H‐SiC p+‐n diodes at current densities from 10−4to10A∕cm2. At small current densities j⩽10−3A∕cm2, the spectral noise density SI follows the law SI∝1∕f3∕2. At 10−3A∕cm2 <j<10−2A∕cm2, the generation-recombination (GR) noise predominates. The amplitude of this GR noise nonmonotonically depends on current. At j⩾10−2A∕cm2, the 1∕f (flicker noise) dominates. It has been shown that the recombination time in the space charge region of the p+‐n junction, τR, is about 70ns. This value is approximately one order of magnitude larger than that reported earlier for SiC p‐n structures. A model of GR noise in forward biased p‐n junctions has been proposed. The model links the GR noise with fluctuations of the charge state of a trap in the space charge region.

Generation-recombination noise in forward biased 4H-SiC p-n diodes

1000 V Bipolar Junction Transistor and integrated Darlington pairs with high current gain have been developed in 4H-SiC. The 3.38 mm x 3.38 mm BJT devices with an active area of 3 mm x 3 mm showed a forward on-current of 30 A, which corresponds to a current density of 333 A/cm2, at a forward voltage drop of 2 V. A common-emitter current gain of 40 was measured on these devices. A specific on-resistance of 6.0 mW-cm2 was observed at room temperature. The onresistance increases at higher temperatures, while the current gain decreases to 30 at 275°C. In addition, an integrated Darlington pair with an active area of 3 mm x 3 mm showed a collector current of 30 A at a forward drop of 4 V at room temperature. A current gain of 2400 was measured on these devices. A BVCEO of 1000 V was measured on both of these devices.

1000 V, 30 A SiC Bipolar Junction Transistors and Integrated Darlington Pairs

In this work, we report our recently developed 16 kV, 1 cm2, 4H-SiC PiN diode results. The SiC PiN diode was built on a 120 µm, 2×1014/cm3 doped n-type SiC drift layer with a device active area of 0.5175 cm2. Forward conduction of the PiN diode was characterized at temperatures from 20°C to 200°C. At high injection-current density (JF) of 350 ~ 400 A/cm2, the differential on-resistance (RON,diff) of the SiC PiN diode decreased from 6.08 mΩ·cm2 at 20°C to 5.12 mΩ·cm2 at 200°C, resulting in a very small average temperature coefficient of –5.33 µΩ·cm2/°C, while the forward voltage drop (VF) at 100 A/cm2 reduced from 4.77 V at 20°C to 4.17 V at 200°C. This is due to an increasing high-level carrier lifetime with an increase in temperature, resulting in reduced forward voltage drop. We also observed lower RON,diff at higher injection-current densities, suggesting that a higher carrier lifetime is needed in this lightly doped n-type SiC thick epi-layer in order to achieve full conductivity modulation. The anode to cathode reverse blocking leakage current was measured as 0.9 µA at 16 kV at room temperature.

John W. Palmour

Papers

Turn-on process in high voltage 4H-SiC thyristors

15 kV n-GTOs in 4H-SiC

Generation-recombination noise in forward biased 4H-SiC p-n diodes

1000 V, 30 A SiC Bipolar Junction Transistors and Integrated Darlington Pairs

16 kV, 1 cm2, 4H-SiC PiN Diodes for Advanced High-Power and High-Temperature Applications