Tingting Han

Graphene Amplifier MMIC on SiC Substrate

Low-noise amplifier is one of the most attractive applications of graphene transistors in the RF area. In this letter, a graphene amplifier MMIC is fabricated on the quasi-free-standing bilayer epitaxial graphene grown on SiC (0001) substrate. In order to realize both the high gain and low return loss, Au matching lines are designed as the input and output impedance match networks. The fabricated graphene amplifier MMIC shows a small-signal power gain of 3.4 dB at 14.3 GHz and a minimum noise figure of 6.2 dB. This letter is a significant step forward for graphene electronics in low-noise amplifier and demonstrates graphenes potential in RF applications for future high-speed electronics.

Enhanced effect of strain-induced polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

Using the measured Capacitance-Voltage and Current-Voltage characteristics of the rectangular AlN/GaN Heterostructure Field-Effect Transistors (HFETs) with different Schottky areas, we found that after device processing the polarization Coulomb field (PCF) scattering is induced and has an important influence on the two-dimensional electron gas electron mobility. Moreover, it was also found that PCF scattering has an enhanced influence on the mobility in AlN/GaN HFETs compared to that in AlGaN/AlN/GaN HFETs. This is attributed to the large lattice mismatch between AlN and GaN necessitating a thinner AlN barrier layer, which gives rise to a stronger converse piezoelectric effect.

DC and RF characteristics of enhancement-mode InAlN/GaN HEMT with fluorine treatment

We report an enhancement-mode InAlN/GaN HEMT using a fluorine plasma treatment. The threshold voltage was measured to be +0.86 V by linear extrapolation from the transfer characteristics. The transconductance is 0 mS/mm at VGS = 0 V and VDS = 5 V, which shows a truly normal-off state. The gate leakage current density of the enhancement-mode device shows two orders of magnitude lower than that of the depletion-mode device. The transfer characteristics of the E-mode InAlN/GaN HEMT at room temperature and high temperature are reported. The current gain cut-off frequency (fT) and the maximum oscillation frequency (fmax) of the enhancement-mode device with a gate length of 0.3 μm were 29.4 GHz and 37.6 GHz respectively, which is comparable with the depletion-mode device. A classical 16 elements small-signal model was deduced to describe the parasitic and the intrinsic parameters of the device.

70-nm-gated InAlN/GaN HEMTs grown on SiC substrate with f T / f max > 160 GHz

InAlN/GaN high-electron-mobility transistors (HEMTs) on SiC substrate were fabricated and characterized. Several techniques, consisting of high electron density, 70 nm T-shaped gate, low ohmic contacts and a short drain-source distance, are integrated to gain high device performance. The fabricated InAlN/GaN HEMTs exhibit a maximum drain saturation current density of 1.65 A/mm at V gs = 1 V and a maximum peak transconductance of 382 mS/mm. In addition, a unity current gain cut-off frequency ( f T ) of 162 GHz and a maximum oscillation frequency ( f max ) of 176 GHz are achieved on the devices with the 70 nm gate length.

Improvement of the Frequency Characteristics of Graphene Field-Effect Transistors on SiC Substrate

Analog applications attract increasing interest for graphene field-effect transistors (GFETs). GFET with a cutoff frequency of up to 427 GHz has been reported; however, the device suffered from the large parasitic parameters and poor drain current saturation, which made their maximum oscillation frequency lower than the cutoff frequency. In this letter, quasi-free-standing bilayer graphene transistors with a gate length of 60 nm and ultra-thin gate dielectric are fabricated by an improved, self-aligned, process. Good gate coupling is achieved, and parasitic parameters are suppressed to a significant extent. The as-measured extrinsic cutoff frequency reaches 70 GHz and the maximum oscillation frequency reaches 120 GHz, which are the highest extrinsic frequencies reported for graphene transistors to date. Our results show the application potential of graphene RF devices in future high-speed electronic systems.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates*

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors (GFETs) fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density IDS, improved transconductance g m , reduced sheet resistance R on , and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasi-free-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

The influence of the AlN barrier thickness on the polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

The electron mobility scattering mechanisms in AlN/GaN heterostuctures with 3 nm and 6 nm AlN barrier thicknesses were investigated by temperature-dependent Hall measurements. The effect of interface roughness (IFR) scattering on the electron mobility was found to be enhanced by increasing AlN barrier thickness. Moreover, using the measured capacitance-voltage and current-voltage characteristics of the fabricated heterostructure field-effect transistors (HFETs) with different Schottky areas on the two heterostuctures, the variations of electron mobility with different gate biases were investigated. Due to enhanced IFR scattering, the influence of polarization Coulomb field (PCF) scattering on electron mobility was found to decrease with increasing AlN barrier layer thickness. However, the PCF scattering remained an important scattering mechanism in the AlN/GaN HFETs.

High-Uniformity and High Drain Current Density Enhancement-Mode AlGaN/GaN Gates-Seperating Groove HFET

In this paper, we report on a novel E-mode AlGaN/GaN gates-seperating groove heterostructure field-effect transistor (GSG HFET). The current turn-on/off is controlled by changing gate voltage to regulate the horizontal energy band between the double gates. A threshold voltage of 0.23 V and a high drain current density of 851 mA/mm are obtained in GSG HFET. Compared with the proposed depletion-mode device, the maximum drain current of the GSG HFET deceases slightly (about 7%). It is noteworthy that the threshold voltage is less sensitive to the etching time. The devices show high-uniformity threshold voltage of 0.23 V within wide etching time range from 5 to 9 min.

High-frequency noise characterization of graphene field effect transistors on SiC substrates

Considering its high carrier mobility and high saturation velocity, a low-noise amplifier is thought of as being the most attractive analogue application of graphene field-effect transistors. The noise performance of graphene field-effect transistors at frequencies in the K-band remains unknown. In this work, the noise parameters of a graphene transistor are measured from 10 to 26 GHz and noise models are built with the data. The extrinsic minimum noise figure for a graphene transistor reached 1.5 dB, and the intrinsic minimum noise figure was as low as 0.8 dB at a frequency of 10 GHz, which were comparable with the results from tests on Si CMOS and started to approach those for GaAs and InP transistors. Considering the short development time, the current results are a significant step forward for graphene transistors and show their application potential in high-frequency electronics.