We report the radio-frequency performance of carbon nanotube array transistors that have been realized through the aligned assembly of highly separated, semiconducting carbon nanotubes on a fully scalable device platform. At a gate length of 100 nm,
we observe output current saturation and obtain as-measured, extrinsic current gain and power gain cut-off frequencies, respectively, of 7 GHz and 15 GHz. While the extrinsic current gain is comparable to the state-of-the-art the extrinsic power gain is improved. The de-embedded, intrinsic current gain and power gain cut-off frequencies of 153 GHz and 30 GHz are the highest values experimentally achieved to date. We analyze the consistency of DC and AC performance parameters and discuss the requirements for future applications of carbon nanotube array transistors in high-frequency electronics.
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 ca
vity, 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.
This letter reports the impact of surface morphology on the carrier transport and RF performance of graphene FETs formed on epitaxial graphene films synthesized on SiC substrates. Such graphene exhibits long terrace structures with widths between 3-5
{mu}m and steps of 10pm2 nm in height. While a carrier mobility above 3000 cm2/Vs at a carrier density of 1e12 cm-2 is obtained in a single graphene terrace domain at room temperature, the step edges can result in a vicinal step resistance of ~21 k{Omega}.{mu}m. By orienting the transistor layout so that the entire channel lies within a single graphene terrace, and reducing the access resistance associated with the ungated part of the channel, a cut-off frequency above 200 GHz is achieved for graphene FETs with channel lengths of 210 nm, which is the highest value reported on epitaxial graphene thus far.
Variable-field Hall measurements were performed on epitaxial graphene grown on Si-face 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.
Up to two layers of epitaxial graphene have been grown on the Si-face of two-inch SiC wafers exhibiting room-temperature Hall mobilities up to 1800 cm^2/Vs, measured from ungated, large, 160 micron x 200 micron Hall bars, and up to 4000 cm^2/Vs, from
top-gated, small, 1 micron x 1.5 micron Hall bars. The growth process involved a combination of a cleaning step of the SiC in a Si-containing gas, followed by an annealing step in Argon for epitaxial graphene formation. The structure and morphology of this graphene has been characterized using AFM, HRTEM, and Raman spectroscopy. Furthermore, top-gated radio frequency field effect transistors (RF-FETs) with a peak cutoff frequency fT of 100 GHz for a gate length of 240 nm were fabricated using epitaxial graphene grown on the Si face of SiC that exhibited Hall mobilities up to 1450 cm^2/Vs from ungated Hall bars and 1575 cm^2/Vs from top-gated ones. This is by far the highest cut-off frequency measured from any kind of graphene.
High-performance graphene field-effect transistors have been fabricated on epitaxial graphene synthesized on a two-inch SiC wafer, achieving a cutoff frequency of 100 GHz for a gate length of 240 nm. The high-frequency performance of these epitaxial
graphene transistors not only shows the highest speed for any graphene devices up to date, but it also exceeds that of Si MOSFETs at the same gate length. The result confirms the high potential of graphene for advanced electronics applications, marking an important milestone for carbon electronics.
A dual-gate graphene field-effect transistors is presented, which shows improved RF performance by reducing the access resistance using electrostatic doping. With a carrier mobility of 2700 cm2/Vs, a cutoff frequency of 50 GHz is demonstrated in a 35
0-nm gate length device. This fT value is the highest frequency reported to date for any graphene transistor, and it also exceeds that of Si MOSFETs at the same gate length, illustrating the potential of graphene for RF applications.
We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-e
ffect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis, and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.
Top-gated graphene transistors operating at high frequencies (GHz) have been fabricated and their characteristics analyzed. The measured intrinsic current gain shows an ideal 1/f frequency dependence, indicating an FET-like behavior for graphene tran
sistors. The cutoff frequency fT is found to be proportional to the dc transconductance gm of the device. The peak fT increases with a reduced gate length, and fT as high as 26 GHz is measured for a graphene transistor with a gate length of 150 nm. The work represents a significant step towards the realization of graphene-based electronics for high-frequency applications.
We report the experimental observation of conductance quantization in graphene nanoribbons, where 1D transport subbands are formed due to the lateral quantum confinement. We show that this quantization in graphene nanoribbons can be observed at tempe
ratures as high as 80 K and channel lengths as long as 1.7 $mu$m. The observed quantization is in agreement with that predicted by theoretical calculations.
