JianJang Huang

Device technologies for RF front-end circuits in next-generation wireless communications

Next-generation high data rate wireless communication systems offer completely new ways to access information and services. To provide higher data speed and data bandwidth, RF transceivers in next-generation communications are expected to offer higher RF performance in both transmitting and receiving circuitry to meet quality of service. The semiconductor device technologies chosen will depend greatly on the tradeoffs between manufacturing cost and circuit performance requirements, as well as on variations in system architecture. It is hard to find a single semiconductor device technology that offers a total solution to RF transceiver building blocks in terms of system-on-chip integration. The choices of device technologies for each constituent component are important and complicated issues. We review the general performance requirement of key components for RF transceivers for next-generation wireless communications. State-of-the-art high-speed transistor technologies are presented to assess the capabilities and limitations of each technology in the arena of high data rate wireless communications. The pros and cons of each technology are presented and the feasible semiconductor device technologies for next-generation RF transceivers can be chosen upon the discretion of system integrators.

GaN nanorod light emitting diode arrays with a nearly constant electroluminescent peak wavelength

A practical process to fabricate InGaN/GaN multiple quantum well light emitting diodes (LEDs) with a self-organized nanorod structure is demonstrated. The nanorod array is realized by using nature lithography of surface patterned silica spheres followed by dry etching. A layer of spin-on-glass (SOG), which intervening the rod spacing, serves the purpose of electric isolation to each of the parallel nanorod LED units. The electroluminescence peak wavelengths of the nanorod LEDs nearly remain as constant for an injection current level between 25mA and 100mA, which indicates that the quantum confined stark effect is suppressed in the nanorod devices. Furthermore, from the Raman light scattering analysis we identify a strain relaxation mechanism for lattice mismatch layers in the nanostructure.

Selective area growth and characterization of AlGaN/GaN heterojunction bipolar transistors by metalorganic chemical vapor deposition

The selective area growth (SAG) and properties of AlGaN/GaN heterojunction bipolar transistors (HBTs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD) are described and analyzed. Transistors based on group III-nitride material are attractive for high-power and high-temperature applications. Much work has been focused on improving p-type material, as well as heterojunction interfaces. However, there have been very few reports on HBTs operating at room temperature, At this time, current gains for nitride-based HBTs have been limited to /spl sim/10. Selective area regrowth was applied to the growth of AlGaN/GaN HBTs to analyze its potential advantages as compared to more traditional growth techniques in order to realize improved electrical performance of the devices.

High performance InGaN/GaN nanorod light emitting diode arrays fabricated by nanosphere lithography and chemical mechanical polishing processes

We fabricated InGaN/GaN nanorod light emitting diode (LED) arrays using nanosphere lithography for nanorod formation, PECVD (plasma enhanced chemical vapor deposition) grown SiO(2) layer for sidewall passivation, and chemical mechanical polishing for uniform nanorod contact. The nano-device demonstrates a reverse current 4.77nA at -5V, an ideality factor 7.35, and an optical output intensity 6807mW/cm(2) at the injection current density 32A/cm(2) (20mA). Moreover, the investigation of the droop effect for such a nanorod LED array reveals that junction heating is responsible for the sharp decrease at the low current.

Improvement of External Extraction Efficiency in GaN-Based LEDs by

A practical approach to fabricate textured GaN-based light-emitting diodes (LEDs) by nanosphere lithography is presented. By spin coating a monolayer of SiO2 nanoparticles as the mask, textured LEDs can be fabricated. Both textured p-GaN and textured indium tin oxide LEDs show significant improvement over conventional LEDs without damaging the electrical characteristics. The results show that the method is promising for manufacturing low-cost high-efficient GaN-based LEDs.

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In this study, the process of nanosphere lithography was developed and applied to LED surface texturing and nanorod device fabrication. We observed a texture-size-dependent improvement of total light output. While the increase of output optical power from the textured LEDs can be attributed to surface roughening in the GaN-air surface and to the increase of internal quantum efficiency as the strain is relaxed with the surface texturing, the size-dependent device performance is related to the interaction of generated photons with the textured surface. We further etched through the p-GaN and quantum well region to form p-i-n nanorods on the sample. By inserting a spacer to prevent p-type contact from shorting the n-GaN, we successfully demonstrated nanorod LED arrays. For such a device, a narrower radiation profile was demonstrated from the nanorod LED array as compared with that from the planar LED. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods. Furthermore, the electroluminescence spectra showed a nearly constant peak wavelength of the nanorod LED arrays, which is due to the suppression of the effect of quantum confined Stark effect.

Nanosphere Lithography

An enhancement-mode (E-mode) high-electron mobility transistor (HEMT) was demonstrated by inserting a p-type GaN layer underneath the gate electrode. The effects of process flows and device structures on the electrical properties are investigated in this paper. We demonstrated a threshold voltage (Vth) of 4.3 V by adjusting the built-in voltage of the diode formed between the p-GaN and channel by the alloy temperature. Next, we found the existence of parallel conduction paths of the p-GaN layer and 2-D electron gas (2DEG) channel in such a HEMT structure. By removing p-GaN above the gate-source and gate-drain regions, current conduction migrates from p-GaN to 2DEG channel. The process window of the p-GaN residual thickness to ensure a steady forward current-voltage operation was estimated to be 10±5 nm in our case. Finally, with the p-GaN underneath the gate contact to deplete surface leakage current, an E-mode HEMT with a breakdown voltage (VBD) of 1630 V is achieved.

Application of Nanosphere Lithography to LED Surface Texturing and to the Fabrication of Nanorod LED Arrays

The authors report on the 394nm UV light emission from low-temperature sputtered n-ZnO∕SiO2 thin films on top of the p-GaN heterostructure. They compare samples with and without a SiO2 current blocking layer. With a SiO2 layer, electroluminescence spectrum shows a sharp emission peak at 394nm, which is attributed to the recombination of accumulated carriers between n-ZnO∕SiO2 and p-GaN∕SiO2 junctions. As for the sample without a SiO2 layer, a broadband ranging from 400to800nm is observed, which is due to Mg+ deep-level transition in the GaN along with defects in the ZnO layers.

Enhancement-Mode GaN-Based High-Electron Mobility Transistors on the Si Substrate With a P-Type GaN Cap Layer

Subthreshold swing (SS) is a key parameter in evaluating the power consumption and material properties of thin-film transistors (TFTs). In this letter, we report an amorphous indium gallium zinc oxide (a-IGZO) TFT with a high-κ SiO₂/HfO₂ gate insulator. The device shows a SS of 96 mV/decade and an on-to-off current ratio of 1.5 × 10¹⁰. The low SS was attributed to the fully depleted channel state, low interface defects, and efficient modulation of the device. With low defect states, the device demonstrates only 2.71% change of operating currents after 1.5 × 10⁴ s stress.

Observation of 394nm electroluminescence from low-temperature sputtered n-ZnO∕SiO2 thin films on top of the p-GaN heterostructure

The enhanced light extraction and collimated output beam profile from GaN-InGaN vertical-injection light-emitting diodes (VI-LEDs) are demonstrated utilizing high-aspect-ratio nanorod arrays. The nanorod arrays are patterned by self-assembled silica spheres, followed by inductively coupled-plasma reactive ion etching. The fabricated nanorod arrays not only provide an omnidirectional escaping zone for photons, but also serve as waveguiding channels for the emitted light, resulting in a relatively collimated beam profile. The light output power of the VI-LED with nanorod arrays is enhanced by 40%, compared to a conventional VI-LED. The measured far-field profiles indicate that the enhancement is mainly along the surface normal direction, within a view angle of 20deg.