William Stillman

Low-frequency electronic noise in the double-gate single-layer graphene transistors

We report results of experimental investigation of the low-frequency noise in the topgate graphene transistors. The back-gate graphene devices were modified via addition of the top gate separated by ~20 nm of HfO2 from the single-layer graphene channels. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and Hooge parameter H  210 -3 . The analysis of noise spectral density dependence on the top and bottom gate biases helped us to elucidate the noise sources in these devices and develop a strategy for the electronic noise reduction. The obtained results are important for all proposed graphene applications in electronics and sensors.

Electrical and noise characteristics of graphene field-effect transistors: ambient effects, noise sources and physical mechanisms

Low frequency noise in virgin (not aged) graphene transistors might be relatively low (comparable to average Si MOSFETs), at least for high quality devices with the bottom gate configuration. Graphene channels are the dominant sources of noise, even though the contact resistances have an important effect on the noise magnitude due to the voltage re-distribution between the contacts and the channel. Gate voltage dependences of noise in graphene transistors reveal that the noise mechanism cannot be described by a conventional McWhorter model and might be linked to graphene mobility fluctuations. Aging in ambience causes a substantial degradation of device characteristics and increase of noise level. The temperature dependences of the current-voltage characteristics of graphene revealed a new effect of a “memory step” near the charge neutrality voltage. Further studies of low frequency noise under such conditions might help in understanding of this novel phenomenon.

Enhanced Plasma Wave Detection of Terahertz Radiation Using Multiple High Electron-Mobility Transistors Connected in Series

We report on enhanced room-temperature detection of terahertz radiation by several connected field-effect transistors. For this enhanced nonresonant detection, we have designed, fabricated, and tested plasmonic structures consisting of multiple InGaAs/GaAs pseudomorphic high electron-mobility transistors connected in series. Results show a 1.63-THz response that is directly proportional to the number of detecting transistors biased by a direct drain current at the same gate-to-source bias voltages. The responsivity in the saturation regime was found to be 170 V/W with the noise equivalent power in the range of 10-7 W/Hz0.5. The experimental data are in agreement with the detection mechanism based on the rectification of overdamped plasma waves excited by terahertz radiation in the transistor channel.

Terahertz response of field-effect transistors in saturation regime

We report on the broadband terahertz response of InGaAs/GaAs high electron mobility transistors operating at 1.63 THz and room temperature deep in the saturation regime. We demonstrate that responses show linear increase with drain-to-source voltage (or drain-bias current) and might reach very high values up to 170 V/W. We also develop a phenomenological theory valid both in the Ohmic and in the saturation regimes.

Nanometer Scale Complementary Silicon MOSFETs as Detectors of Terahertz and Sub-terahertz Radiation

We demonstrate, for the first time, THz detection by Si CMOS, i.e. by both p-channel and n-channel Si MOS devices. Previous work demonstrated that Si n-MOS devices detect THz and sub-THz radiation via the excitation of plasma waves in the device channel, with responsivity and Noise Equivalent Power on the order of commercial pyroelectric detectors but capable of operating at much greater speed as shown by a theory of temporal response predicting the maximum operating frequency. The CMOS responsivity is a strong increasing function of the drain current. Our experimental data and modeling results allow us to understand the effects of device geometry and bias on the Si CMOS THz detector performance.

Detection and Homodyne Mixing of Terahertz Gas Laser Radiation by Submicron GaAs/AlGaAs FETs

We demonstrated detection and homodyne mixing of laser modes of optically pumped terahertz gas laser at 2.52 THz by submicron AlGaAs/GaAs field-effect transistors (FETs). The mechanism of the detection of terahertz radiation is based on the phenomena of non-linear excitations of plasma waves in a channel with 2D electron gas. According to theoretical models, the response time associated with this detection mechanism can be as fast as picoseconds, which makes these plasma wave devices attractive for real time detection and heterodyne mixing applications in the terahertz frequency range.

Resonant Detection and Modulation of Terahertz Radiation by 2DEG Plasmons in GaN Grating-Gate Structures

We observed, for the first time, resonant absorption and modulation of THz radiation by 2DEG plasmons in GaN-based large area grating-gate structures. Transmission spectra of the fabricated grating-gate devices were measured by a commercial FTIR system using a blackbody emitter as broadband radiation source and a CW FIR gas laser. Obtained spectra clearly showed the absorption peaks, which were attributed to the resonance plasmon excitation in the 2DEG at the AlGaN/GaN heterointerface. The results demonstrate the potential of using GaN-based plasmonic device for THz applications.

Low-frequency noise in graphene field-effect transistors

The low frequency noise has been studied in mechanically exfoliated single- and bilayer graphene deposited on Si/SiO2 substrates. Measurements were performed in 2- and 4-probe configuration schemes. The analysis of the gate voltage dependences of noise showed that noise in graphene transistors does not comply with the McWhorter model. Aging of graphene transistors due to exposure to ambient resulted in increased noise attributed to the decreasing mobility of graphene and increasing contact resistance. The model linking noise in graphene to the mobility fluctuations is discussed.

Subwavelength detection of terahertz radiation using GaAs HEMTs

We have designed and fabricated THz detectors based on excitation and rectification of radiation induced overdamped THz plasmons in InGaAs/GaAs HEMT structures. These plasma wave detectors were used to image the beam profile of THz gas laser operating at 1.63 THz with FWHM of 140 µm. The images were recorded with deep sub-wavelength spatial resolution. This allowed to resolve a sub-wavelength shifts in the detected THz responses from adjacent transistors with equivalent separation distances of 12 µm and 30 µm. Our results motivate the utilization of plasma wave detectors for near-field terahertz imaging and suggest the possibility of designing focal plane arrays with subwavelength pixel pitch.

LOW-FREQUENCY ELECTRONIC NOISE IN GRAPHENE TRANSISTORS: COMPARISON WITH CARBON NANOTUBES

We report results of the experimental investigation of the low-frequency noise in graphene transistors. The graphene devices were measured in three-terminal configuration. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and the Hooge parameter αH ~10-3. Both top-gate and back-gate devices were studied. The analysis of the noise spectral-density dependence on the gate biases helped us to elucidate the noise sources in these devices. We compared the noise performance of graphene devices with that of carbon nanotube devices. It was determined that graphene devices works better than carbon nanotube devices in terms of the low-frequency noise. The obtained results are important for graphene electronic, communication and sensor applications.