Naoki Shibata

High-power recessed-gate AlGaN-GaN HFET with a field-modulating plate

Recessed-gate AlGaN-GaN heterojunction field-effect transistors (FETs) with a field-modulating plate (FP) have been successfully fabricated for high-voltage and high-power microwave applications. The developed recessed-gate FP-FET with a gate length of 1 /spl mu/m exhibited an increased transconductance of 200 mS/mm. A series of current collapse measurements revealed that the recessed-gate FP-FET is highly desirable for collapse-free high-voltage power operation. Equivalent circuit analysis demonstrated that the gain loss due to the additional gate feedback capacitance associated with the FP electrode is considerably compensated by increasing the drain bias voltage to more than 30 V. A 48-mm-wide recessed-gate FP-FET biased at a drain voltage of 48 V exhibited a record saturated output power of 197 W with a linear gain of 10.1 dB and a power-added efficiency of 67% at 2 GHz.

Preparation of Polycrystalline Silicon by Hydrogen-Radical-Enhanced Chemical Vapor Deposition

The preparation of both amorphous and epitaxial crystalline silicon films by Hydrogen-Radical-Enhanced Chemical Vapor Deposition at variable hydrogen flow rates is discussed. The feasibility of fabricating polycrystalline Si at high growth rates and a low substrate temperature is demonstrated. Finally, the n-type characteristics of PH3 doping and p-type characteristic for BF3 doping are examined in terms of the conductivity and the Hall mobility of the films.

m-Plane GaInN Light Emitting Diodes Grown on Patterned a-Plane Sapphire Substrates

The fabrication of a blue m-plane GaInN light emitting diode (LED) grown on an m-plane GaN layer grown on a 3-in. patterned sapphire substrate is reported. The output power of the LED was approximately 3 mW at the wavelength of 461 nm, a driving current of 20 mA, and a forward voltage of 3.5 V. This is the first report of nonpolar or semipolar blue LEDs grown on hetero-substrates with milliwatt scale output power.

m-Plane GaN Films Grown on Patterned a-Plane Sapphire Substrates with 3-inch Diameter

Nonpolar m-plane GaN films have been grown by metalorganic vapor-phase epitaxy on patterned a-plane sapphire substrates (diameter: 3 in.) without dielectric masks made of materials such as SiO2. The m-plane GaN layer had a smooth and transparent surface over the entire area of the substrate. Furthermore, the epitaxial relationships between the m-plane GaN film and the patterned a-plane sapphire substrate were as follows: [0001]GaN∥[0001]Sapphire and [1120]GaN∥[1010]Sapphire. The full width at half maximum values of the X-ray rocking curves for (1010) GaN along [1120]GaN and [0001]GaN were found to be 396 and 565 arcsec, respectively.

The Role of Hydrogen Radicals in the Growth of a-Si and Related Alloys

A new method of the preparation of a-Si and its alloys is proposed. The role of atomic hydrogen in the growth of a-Si:H:(F) films by the glow-discharge of a mixture of SiF4 and H2 is clarified. Amorphous Si:H:(F) films have been prepared in a plasma-free environment by employing long-lifetime radicals, i.e., atomic hydrogen and SiFn radicals. Highly photoconductive a-Si:H:(F) films and related alloys, viz. a-SiGex:H:(F) and a-SiCx:H:(F), have been obtained at high deposition rates with the result that each radical is controlled independently.

A 149W recessed-gate AlGaN/GaN FP-FET

A recessed-gate AlGaN/GaN field-modulating plate (FP) FET was successfully fabricated on a SiC substrate. By employing recessed-gate structure for an FP-FET, the transconductance (gm) was increased from 130 mS/mm to 220 mS/mm, leading to an improvement in gain characteristics. The gate breakdown voltage (BV/sub gd/) was improved from 160V for the planar FP-FET to 200V for the recessed FP-FET, resulting from one-order reduction in gm and BV/sub gd/, current collapse was suppressed by introducing the recessed-gate structure. At 2GHz, a 32mm-wide recessed FP-FET exhibited an output power of 149 W (4.7W/mm) with 64% power-added efficiency and 8.7 dB linear gain with a drain bias of 47 V.

Hole Transport in a-Si:H(F) Prepared by Hydrogen-Radical-Assisted Chemical Vapor Deposition

The modes of hole transport in both non-doped and BF3-doped a-Si:H(F) films prepared by hydrogen-radical-assisted chemical vapor deposition (HRCVD) were investigated by the time-of-flight method. Non-dispersive transport was observed at temperatures above approximately 300 K, and dispersive transport below this temperature. The drift mobility conforms with a multiple-trapping model which assumes that the valence-band tail is distributed exponentially in terms of energy. The characteristic temperatures provided by this model, 310 K for non-doped a-Si:H(F) and 286 K for a-Si:H(F) doped using 30 ppm of BF3, are lower than the corresponding value for a-Si:H films prepared by an R.F. glow discharge of SiH4. This result shows that the distribution of valence-band tail states is narrower in the case of HRCVD. Sub-bandgap profiles based on the wavelength dependence of photoconductivity also reveal a decrease in the density of mid-gap states.

Hole Transport in Silicon Thin Films with Variable Hydrogen Content

The hole transport characteristics of Si:H(F) thin films with variable hydrogen contents, prepared by hydrogen-radical-enhanced chemical vapour deposition (HRCVD), were evaluated by the time-of-flight method. The drift mobility conforms with a multiple-trapping model derived from an exponential energy distribution of the valence band tail states, and increases as hydrogen contents decrease from 15 to 2 at.%. The characteristic temperatures provided by the above model show a significant decrease over the same compositional interval. These results were interpreted on the basis of the packing density of the films and the film growth processes during deposition.

Preparation of Highly Photoconductive a-SiGex from Fluorides by Controlling Reactions with Atomic Hydrogen

Highly photoconductive a-SiGex alloys with an optical gap of 1.2–1.7 eV are prepared by plasma-induced decomposition of fluorides (SiF4 and GeF4) and the introduction of atomic hydrogens (hydrogen radicals). Two new preparing methods are proposed to attain high alloy quality by controlling the reactions for making precursors using hydrogen radicals. Attempts have been made to deposit alloys on a substrate apart from the plasma to avoid damage of the films by bombardment of the particles. By this means, highly photoconductive a-SiGex:H(F) films with an optical gap of 1.4eV were obtained with high growth rates.

Optimum Rapid Thermal Activation of Mg-Doped p-Type GaN

In this paper, we describe the electrical properties of Mg-doped GaN annealed using rapid thermal annealing. Metal–organic vapor phase epitaxy (MOVPE)-grown samples with Mg concentrations ranging from 1.25 ×1019 to 1 ×1020 cm-3 were annealed at various temperatures in a nitrogen atmosphere. It was found that the maximum hole concentration achieved after rapid thermal annealing is limited to approximately 8 ×1017 cm-3 at room temperature and its optimum annealing temperature decreases with increasing Mg concentration. The temperature dependence of hole concentration suggested that the formation of donor complex defects is responsible for the suppressed electrical activation of Mg acceptors after annealing at high temperatures higher than the optimum annealing temperature.