Kazuki Nomoto

1.9-kV AlGaN/GaN Lateral Schottky Barrier Diodes on Silicon

In this letter, we present AlGaN/GaN lateral Schottky barrier diodes on silicon with recessed anodes and dual field plates. A low specific ON-resistance R_ON,SP (5.12 mQ · cm²), a low turn-ON voltage (<; 0.7 V), and a high reverse breakdown voltage (BV) (>1.9 kV) were simultaneously achieved in devices with a 25-μm anode/cathode distance, resulting in a power figure-of-merit BV²/R_ON,SP of 727 MW · cm^-2. The record high BV of 1.9 kV is attributed to the dual field-plate structure.

Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown

Textbook-like device characteristics are demonstrated in vertical GaN p-n diodes grown on bulk GaN substrates. These devices show simultaneously an avalanche breakdown voltage (BV) of >1.4 kV under reverse bias, an ideality factor plateau of ∼2.0 in a forward bias window followed by a near unity ideality factor of 1.1, which are consistently achieved over a temperature range of 300–400 K. At room temperature (RT), the diode with a mesa diameter of 107 μm showed a differential on-resistance Ron of 0.12 mΩcm2, thus resulting in a record figure-of-merit BV2/Ron of ∼16.5 GW/cm2, which is the highest ever demonstrated in any semiconductors. Analytical models are used to fit experimental I-Vs; based on the recombination current with an ideality factor of ∼2.0, a Shockley-Read-Hall lifetime of 12 ns is extracted at RT with an estimated recombination center concentration of 3 × 1015 cm−3.

High-Breakdown-Voltage and Low-Specific-on-Resistance GaN p–n Junction Diodes on Free-Standing GaN Substrates Fabricated Through Low-Damage Field-Plate Process

In this letter, we describe the characteristics of Gallium Nitride (GaN) p–n junction diodes fabricated on free-standing GaN substrates with low specific on-resistance Ron and high breakdown voltage VB. The breakdown voltage of the diodes with the field-plate (FP) structure was over 3 kV, and the leakage current was low, i.e., in the range of 10-4 A/cm2. The specific on-resistance of the diodes of 60 µm diameter with the FP structure was 0.9 mΩcm2. Baligas figure of merit (VB2/Ron) of 10 GW/cm2 is obtained. Although a certain number of dislocations were included in the device, these excellent results indicated a definite availability of this material system for power-device applications.

GaN-on-GaN p-n power diodes with 3.48 kV and 0.95 mΩ-cm2: A record high figure-of-merit of 12.8 GW/cm2

We report GaN p-n diodes on free-standing GaN substrates: a record high Baligas figure-of-merit (V<;sub>B<;/sub><;sup>2<;/sup>/ Ron) of 12.8 GW/cm<;sup>2<;/sup> is achieved with a 32 μm drift layer and a diode diameter of 107 μm exhibiting a BV > 3.4 kV and a R<;sub>on<;/sub> <; 1 mΩ-cm<;sup>2<;/sup>. The leakage current density is low: 10<;sup>-3<;/sup> - 10<;sup>-4<;/sup> A/cm<;sup>2<;/sup> at 3 kV. A record low ideality factor of 1.1-1.3 is signature of high GaN quality. These are among the best-reported GaN p-n diodes.

High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm2 is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm2 is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

Ultralow-Leakage AlGaN/GaN High Electron Mobility Transistors on Si With Non-Alloyed Regrown Ohmic Contacts

Without employing gate dielectrics, AlGaN/GaN high-electron mobility transistors (HEMTs) on Si with non-alloyed regrown ohmic contacts exhibit record-low leakage currents ~10-12 A/mm, high ON/OFF current ratios 1011. Compared with HEMTs with conventional alloyed ohmic contacts, HEMTs with non-alloyed contacts show a reduction of 106 in leakage current, a steeper subthreshold slope, and 50% improvement in breakdown voltage. These observations indicate that avoiding high-temperature alloyed ohmic processes can lead to improved device performance.

Strained GaN Quantum-Well FETs on Single Crystal Bulk AlN Substrates

We report the first realization of molecular beam epitaxy (MBE) grown strained GaN quantum well field-effect transistors on single-crystal bulk AlN substrates. The fabricated double heterostructure FETs exhibit a two-dimensional electron gas (2DEG) density in the excess of 2 × 1013/cm2. The ohmic contacts to the 2DEG channel were formed by the n+ GaN MBE regrowth process, with a contact resistance of 0.13 Ω · mm. The Raman spectroscopy using the quantum well as an optical marker reveals the strain in the quantum well and strain relaxation in the regrown GaN contacts. A 65-nm-long rectangular-gate device showed a record high DC drain current drive of 2.0 A/mm and peak extrinsic transconductance of 250 mS/mm. Small-signal RF performance of the device achieved the current gain cutoff frequency fT∼120 GHz. The DC and RF performances demonstrate that bulk AlN substrates offer an attractive alternative platform for strained quantum well nitride transistors for the future high-voltage and high-power microwave ap...

Dispersion-free operation in InAlN-based HEMTs with ultrathin or no passivation

The origin and management of DC-RF dispersion in InAlN-based GaN high electron mobility transistors (HEMTs) is examined, in conjunction with consideration of the implications for device speed. This study, in which GaN HEMTs with alloyed and non-alloyed ohmic contacts are compared, renders the following observations and hypotheses: 1) We show and explain that dispersion free operation can be achieved without passivation. 2) The root cause of dispersion associated with surface states is often introduced during device processing; in particular, unintentional or un-optimized oxidation of the HEMT surface. 3) These undesired surface states also lead to gate extension (virtual gate), which decreases device speed but increases the breakdown voltage. In addition, the function and efficacy of a plasma-based ultrathin passivation is evaluated.

Single-crystal N-polar GaN p-n diodes by plasma-assisted molecular beam epitaxy

N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk wafers by plasma-assisted molecular beam epitaxy growth. The current-voltage characteristics show high-quality rectification and electroluminescence characteristics with a high on currents ∼10 kA/cm2, low off currents 109, and interband photon emission. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level luminescence is greatly suppressed. A very low dislocation density leads to a high reverse breakdown electric field of ∼2.2 MV/cm without fields plates—the highest reported for N-polar epitaxial structures. The low leakage current N-polar diodes open up several potential applications in polarization-engineered photonic and electronic devices.

Electron mobility in polarization-doped Al0-0.2GaN with a low concentration near 1017 cm−3

In this letter, carrier transport in graded AlxGa1-xN with a polarization-induced n-type doping as low as ∼1017 cm−3 is reported. The graded AlxGa1-xN is grown by metal organic chemical vapor deposition on a sapphire substrate, and a uniform n-type doping without any intentional doping is realized by linearly varying the Al composition from 0% to 20% over a thickness of 600 nm. A compensating center concentration of ∼1017 cm−3 was also estimated. A peak mobility of 900 cm2/V·s at room temperature is extracted at an Al composition of ∼7%, which represents the highest mobility achieved in n-Al0.07GaN with a carrier concentration of ∼1017 cm−3. A comparison between experimental data and theoretical models shows that, at this low doping concentration, both dislocation scattering and alloy scattering are significant in limiting electron mobility and that a dislocation density of <107 cm−2 is necessary to optimize mobility near 1016 cm−3. The findings in this study provide insights into key elements for achievi...