Lin-An Yang

Yellow luminescence of polar and nonpolar GaN nanowires on r-plane sapphire by metal organic chemical vapor deposition.

We have grown horizontal oriented, high growth rate, well-aligned polar (0001) single crystalline GaN nanowires and high-density and highly aligned GaN nonpolar (11-20) nanowires on r-plane substrates by metal organic chemical vapor deposition. It can be found that the polar nanowires showed a strong yellow luminescence (YL) intensity compared with the nonpolar nanowires. The different trends of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure were discussed and proved by energy-dispersive X-ray spectroscopy, suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires.

Improved Negative Differential Mobility Model of GaN and AlGaN for a Terahertz Gunn Diode

This paper presents an improved negative differential mobility model for GaN and AlGaN to simulate GaN Gunn diodes at terahertz frequencies. Temperature-dependent parameters vsat, Ec, α, δ, and γ are proposed to improve the accuracy of the mobility model at high temperatures. In particular, an Al-composition-related coefficient fZ(x) and a random-alloy-potential-related factor falloy(p) are developed for an AlGaN mobility model. Simulation results show that notched doping GaN and AlGaN/GaN heterostructure Gunn diodes, both including 0.6-m transit and 0.2-m electron launching regions, have the capability of generating fundamental frequencies of 352-508 and 332-469 GHz, respectively, with a maximum radio-frequency (RF) power density of ~1010 W/cm3 and RF efficiency of over 2% accompanied with a shift of an oscillation mode from a dipole-domain mode toward an accumulation mode as the temperature rises from 300 to 500 K.

Modulation of Multidomain in AlGaN/GaN HEMT-Like Planar Gunn Diode

Terahertz oscillations of AlGaN/GaN HEMT-like planar Gunn diodes with the channel length ranging from 0.6 to 1.6 μm have been investigated for the first time by means of numerical simulations at the Silvacos ATLAS platform, with an emphasis on the mechanism of multiple domains coexisting in the 2-D electron gas (2-DEG) channel of planar Gunn diode. We found that the phenomenon of multidomain evidently influences the oscillation mode and the radio frequency performance of devices, and it is effective in redistributing the electric field throughout the 2-DEG channel by adjusting the doping concentration near the cathode side, which aims to modulate the formation of multidomain so as to enhance the output performance of planar Gunn diodes at terahertz frequencies.

Threading dislocation reduction in transit region of GaN terahertz Gunn diodes

An effect of the position of notch-doping layer in 1-μm GaN Gunn diode on threading dislocations (TDs) distribution is investigated by transmission electron microscopy. Compared with the top-notching-layer (TNL) structure, the bottom-notching-layer (BNL) structure can efficiently reduce the TDs density and improve the crystal quality in the transit region of GaN Gunn diode because it exhibits twice-transition of growth mode from atomic step flow to layer-by-layer nucleation and leads to a significant annihilation of TDs before penetrating into the transit region. X-ray diffraction and Raman spectroscopy reveal that the BNL structure has less compressive stress than the TNL structure.

A Novel Unequal Wilkinson Power Divider for Dual-Band Operation

This paper presents the design of a novel dual-band unequal Wilkinson power divider. The proposed power divider can operate at arbitrary two frequencies without reactive components. To achieve the dual-band operation, four groups of transmission lines called network 1, 2, 3 and 4 are introduced. The design and analysis of power divider are presented. Closed-form design equations are derived based on network theory. The validity of this analysis is confirmed through the design, simulation, and experimental results with a power divider for 1 and 2 GHz.

Influence of InGaN sub-quantum-well on performance of InAlN/GaN/InAlN resonant tunneling diodes

The resonant tunneling mechanism of the GaN based resonant tunneling diode (RTD) with an InGaN sub-quantum-well has been investigated by means of numerical simulation. At resonant-state, Electrons in the InGaN/InAlN/GaN/InAlN RTD tunnel from the emitter region through the aligned discrete energy levels in the InGaN sub-quantum-well and GaN main-quantum-well into the collector region. The implantation of the InGaN sub-quantum-well alters the dominant transport mechanism, increase the transmission coefficient and give rise to the peak current and peak-to-valley current ratio. We also demonstrate that the most pronounced negative-differential-resistance characteristic can be achieved by choosing appropriately the In composition of InxGa1−xN at around x = 0.06.

A Novel Design of Wideband Circular Polarization Antenna Array with High Gain Characteristic

A broadband of circularly polarized (CP) patch antenna fed by L-probes has been presented in this paper. In order to improve axial ratio bandwidth, wideband 90° hybrids with two L-probe feeds are used as feeding netwok, and cones act as radiator instead of common circular patch. A total of 6 array elements are used in a 2 × 3 configuration to achieve a maximum gain target of 14 dBi. However, it will enlarge the overall size of the antenna. The feeding network is located on the lower side of the substrate for compacting the size. The measured results agree well with simulations, indicating that the antenna array possesses an impedance (VSWR < 2) bandwidth of 42% and axial ratio (AR < 3 dB) bandwidth of 47.6%. In addition, the 8-dB gain bandwidth and peak gain are 55% and 14 dBi, respectively.

Use of AlGaN in the notch region of GaN Gunn diodes

The wurtzite gallium nitride (GaN) Gunn diodes with aluminum gallium nitride (AlGaN) as launcher in the notch region are investigated by negative-differential-mobility model based simulation. Under the operation of self-excitation oscillation with dipole domain mode, the simulations show that the diode with two-step-graded AlGaN launcher structure can yield the maximal rf power of 1.95 W and dc/rf conversion efficiency of 1.72% at the fundamental oscillation frequency of around 215 GHz. This kind of Gunn diode structure without the low doping process is convenient for accurately controlling the dopant concentration of GaN epitaxial growth.

Asymmetric quantum-well structures for AlGaN/GaN/AlGaN resonant tunneling diodes

Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39 A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates th...

Reproducibility in the negative differential resistance characteristic of In0.17Al0.83N/GaN resonant tunneling diodes—Theoretical investigation

We report on a simulation of gallium nitride (GaN) based resonant tunneling diode (RTD) at the Silvacos ATLAS simulation platform with indium aluminum nitride (InAlN) as barrier layer. Results show that an excellent reproducibility of negative-differential-resistance (NDR) characteristic can be achieved when experimentally obtained deep-level trapping centers at the activation energy of 0.351 and 0.487 eV, respectively, are introduced into the polarized InAlN/GaN/InAlN quantum well. Theoretical analysis reveals that the lattice-matched InAlN/GaN heterostructure with stronger spontaneous polarization and weaker piezoelectric polarization can reduce the activation energy level of trapping centers, suppress the probability of ionization of the trapping centers, and therefore minimize the degradation of NDR characteristics, which demonstrates a potential application of the GaN-based RTD in terahertz regime.