Omid Habibpour

A Subharmonic Graphene FET Mixer

We demonstrate a subharmonic resistive graphene FET mixer utilizing the symmetrical channel-resistance versus gate-voltage characteristic. A down-conversion loss of 24 dB is obtained with fRF = 2 GHz, fLO= 1.01 GHz, and fIF= 20 MHz in a 50- Ω-impedance system. Unlike conventional subharmonic resistive FET mixers, this type of mixer operates with only one transistor and does not need any balun at the local oscillator (LO) port, which makes it more compact.

A 30-GHz Integrated Subharmonic Mixer Based on a Multichannel Graphene FET

A 30-GHz integrated subharmonic mixer based on a single graphene field-effect transistor (G-FET) has been designed, fabricated, and characterized. The mixer is realized in microstrip technology on a 250- μm-high-resistivity silicon substrate. In order to enhance the current on-off ratio, the G-FET utilizes a channel consisting of an array of bow-tie structured graphene, yielding a current on-off ratio of 7. A conversion loss (CL) of 19 ± 1 dB over the frequency range of 24-31 GHz is obtained with a local oscillator (LO) to RF isolation better than 20 dB at an LO power of 10 dBm. The overall minimum CL is 18 dB at 27 GHz. The mixer has a 3 GHz ± 1-dB IF bandwidth, which is achieved with a fixed LO signal of 15 GHz. The mixer linearity is characterized and the highest third-order intercept point is measured to be 12.8 dBm.

A Large-Signal Graphene FET Model

We propose a semi-empirical graphene field-effect-transistor (G-FET) model for analysis and design of G-FET-based circuits. The model describes the current-voltage characteristic for a G-FET over a wide range of operating conditions. The gate bias dependence of the output power spectrum is studied and compared with the simulated values. Good agreement between the simulated and the experimental power spectrums up to the third harmonic is demonstrated, which confirms the model validity. Moreover, S-parameter measurements essentially coincide with the results obtained from the simulation. The model contains a small set of fitting parameters, which can be straightforwardly extracted from S-parameters and dc measurements. The developed extraction method gives a more accurate estimation of the drain and source contact resistances compared with other approaches. As a design example, we use a harmonic balance load-pull approach to extract optimum embedding impedance values for a subharmonic G-FET mixer.

Graphene-Si Schottky IR Detector

This paper reports on photodetection properties of the graphene-Si schottky junction by measuring current-voltage characteristics under 1.55-μm excitation laser. The measurements have been done on a junction fabricated by depositing mechanically exfoliated natural graphite on top of the pre-patterned silicon substrate. The electrical Schottky barrier height is estimated to be (0.44-0.47) eV with a minimum responsivity of 2.8 mA/W corresponding to an internal quantum efficiency of 10%, which is almost an order of magnitude larger than regular Schottky junctions. A possible explanation for the large quantum efficiency related to the 2-D nature of graphene is discussed. Large quantum efficiency, room temperature IR detection, ease of fabrication along with compatibility with Si devices can open a doorway for novel graphene-based photodetectors.

Resistive Graphene FET Subharmonic Mixers: Noise and Linearity Assessment

We report on the first complete RF characterization of graphene field-effect transistor subharmonic resistive mixers in the frequency interval fRF=2-5 GHz . The analysis includes conversion loss (CL), noise figure (NF), and intermodulation distortion. Due to an 8-nm thin Al2O3 gate dielectric, the devices operate at only ~ 0 dBm of local oscillator (LO) power with an optimum measured CL in the range of 20-22 dB. The NF closely mimics the CL, thus determining the noise to be essentially thermal in origin, which is promising for cryogenic applications. The highest input third-order intercept point is measured to be 4.9 dBm at an LO power of 2 dBm.

Mobility Improvement and Microwave Characterization of a Graphene Field Effect Transistor With Silicon Nitride Gate Dielectrics

We report on the influence of a silicon nitride gate dielectric in graphene-based field-effect transistors (FETs). The silicon nitride is formed by a plasma-enhanced chemical vapor deposition method. The process is based on a low-density plasma at a high pressure (1 torr), which results in a low degradation of the graphene lattice during the top-gate formation process. Microwave measurements of the graphene FET show a cutoff frequency of 8.8 GHz for a gate length of 1.3 μm. A carrier mobility of 3800 cm2/V·s at room temperature was extracted from the dc characteristic.

Graphene FET Gigabit ON–OFF Keying Demodulator at 96 GHz

We demonstrate the demodulation of a multi-Gb/s ON-OFF keying (OOK) signal on a 96 GHz carrier by utilizing a 250-nm graphene field-effect transistor as a zero bias power detector. From the eye diagram, we can conclude that the devices can demodulate the OOK signals up to 4 Gb/s.

Wafer scale millimeter-wave integrated circuits based on epitaxial graphene in high data rate communication

In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes.

Noise figure characterization of a subharmonic graphene FET mixer

We report on the first room temperature noise figure measurement of a graphene FET subharmonic resistive mixer in the interval fRF = 2-5 GHz. Due to an 8 nm thin Al2O3 gate dielectric it can operate with a conversion loss in the range 20–22 dB at only 0 dBm of local oscillator power. The measurement yields a noise figure close to the conversion loss, thus determining the noise to be thermal in origin, which is promising for cryogenic applications. The general route to lower noise figure is an improvement of the conversion loss.

High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide

The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency fmax, cutoff frequency fT, ratio fmax/fT, forward transmission coefficient S21, and open-circuit voltage gain Av. All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained fmax/fT > 3, Av > 30 dB, and S21 = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance ~600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.