High-performance graphene transistors for radio frequency applications have received much attention and significant progress has been achieved. However, devices based on large-area synthetic graphene, which have direct technological relevance, are still typically outperformed by those based on mechanically exfoliated graphene. Here, we report devices with intrinsic cutoff frequency above 300 GHz, based on both wafer-scale CVD grown graphene and epitaxial graphene on SiC, thus surpassing previous records on any graphene material. We also demonstrate devices with optimized architecture exhibiting voltage and power gains reaching 20 dB and a wafer-scale integrated graphene amplifier circuit with voltage amplification.

A new mechanism for negative differential resistance (NDR) is discovered in three-terminal graphene devices based on a field-effect transistor configuration. This NDR effect is a universal phenomenon for graphene and is demonstrated in devices fabricated with different types of graphene materials and gate dielectrics. Operation of conventional NDR devices is usually based on quantum tunneling or intervalley carrier transfer, whereas the NDR behavior observed here is unique to the ambipolar behavior of zero-bandgap graphene and is associated with the competition between electron and hole conduction as the drain bias increases. These three terminal graphene NDR devices offer more operation flexibility than conventional two-terminal devices based on tunnel diodes, Gunn diodes, or molecular devices, and open up new opportunities for graphene in microwave to terahertz applications.

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The superior intrinsic properties of graphene have been a key research focus for the past few years. However, external components, such as metallic contacts, serve not only as essential probing elements, but also give rise to an effective electron cavity, which can form the basis for new quantum devices. In previous studies, quantum interference effects were demonstrated in graphene heterojunctions formed by a top gate. Here phase coherent transport behavior is demonstrated in a simple two terminal graphene structure with clearly resolved Fabry-Perot oscillations in sub-100 nm devices. By aggressively scaling the channel length down to 50 nm, we study the evolution of the graphene transistor from the channel-dominated diffusive regime to the contact-dominated ballistic regime. Key issues such as the current asymmetry, the question of Fermi level pinning by the contacts, the graphene screening determining the heterojunction barrier width, the scaling of minimum conductivity, and of the on/off current ratio are investigated.

Bubble domains have been observed in five composite garnet structures consisting of two epitaxial magnetic layers separated by a layer of nonmagnetic garnet. The static and ``ordering'' properties of the domains are described and the arrangement of the overlying domains is discussed as a function of bias field. It is shown qualitatively that the strength of the magnetostatic interaction between the two magnetic layers can be changed considerably by varying the relative film thicknesses in the composites.

Variable-field Hall measurements were performed on epitaxial graphene grown on Siface and C-face SiC. The carrier transport involves essentially a single-type of carrier in few-layer graphene, regardless of SiC face. However, in multi-layer graphene (MLG) grown on C-face SiC, the Hall measurements indicated the existence of several groups of carriers with distinct mobilities. Electrical transport in MLG can be properly described by invoking three independent conduction channels in parallel. Two of these are n-and p-type, while the third involves nearly intrinsic graphene. The carriers in this lightly doped channel have significantly higher mobilities than the other two.

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