Zongyang Hu

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.

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.

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...

600 V GaN vertical V-trench MOSFET with MBE regrown channel

GaN vertical power transistors have gained increasing interest in recent years due to the advantages over lateral transistors in high voltage/high current applications. To date, two major topologies have been studied most: gate-on-epi-surface (GoE) and gate-on-sidewall (GoS). The GoE devices include CAVET [1] and VDMOSFET-like transistors [2, 3]. The GoS devices include U-MOS or trench-MOSFETs with inversion channel [4, 5] or regrown AlGaN/GaN semi-polar channel [6], as well as depletion-mode MISFET [7]. The vertical MISFET is the simplest to fabricate, however, it does not have avalanche capabilities inherently besides being difficult to achieve sufficiently large Vth. It is easier for trench MOSFETs to achieve normally-off operation, high breakdown voltage (BV) and small footprint. However, it is challenging to achieve high mobility in the inversion channel. In contrast, CAVETs, VDMOS-like transistors and PolarMOS [3] utilize high mobility AlGaN/GaN channel to achieve low Ron, but the channel regrowth posts challenges in achieving low off-state leakage in un-gated regrowth interface. Recently, a novel design based on trench MOSFET is realized by MOCVD regrowth of a thin GaN interlayer [8]. Low Ron and high BV is achieved in the gated regrown channel. Similar to the other MOCVD regrown devices, the buried Mg-doped p-GaN needs to be re-activated by exposing the p-GaN surface during high temperature anneal. This leads to high thermal budget and poses limitations on device geometry. Furthermore, any incomplete activation of buried p-GaN leads to reduced BV. In this work, we design a V-shaped trench MOSFET with MBE regrown UID GaN channel. −600 V breakdown voltage with normally-off operation is demonstrated without the need for re-activation of the buried p-GaN. To our knowledge, this is the highest BV achieved in GaN vertical transistors with MBE regrown channel.

1.1-kV Vertical GaN p-n Diodes With p-GaN Regrown by Molecular Beam Epitaxy

High-voltage vertical regrown p-n junction diodes on bulk GaN substrates are reported in this letter with molecular-beam-epitaxy regrown p-GaN on metalorganic-chemical-vapor-deposition grown n-GaN drift region. The highest breakdown voltage is measured at 1135 V, and the differential on-resistance is 3.9 mOhm.cm2 at room temperature. The forward I–V show a turn-ON voltage near 3.9 V and an ideality factor of 2.5. Electroluminescence measurement of regrown p-n junctions shows ~30 times reduced emission intensity compared with as-grown p-n junctions, indicating presence of excessive non-radiative recombination centers introduced by the regrowth process. Temperature dependent reverse I–V measurements suggest that variable range hopping inside the depleted regrown p-GaN layer is likely the mechanism of the reverse leakage. This is the first high-voltage vertical regrown p-n junction ever reported in the GaN system.

Comparing buffer leakage in PolarMOSH on SiC and free-standing GaN substrates

GaN MOSHEMT or MOSFET on top of conducting (drift layer and drain electrode) layers is a building block for vertical GaN VDMOS power transistors. GaN MOSHEMTs incorporating a polarization-doped p-AlGaN layer as the back barrier on top of conducting layers is named as PolarMOSH. In this work, we present a comparative study of PolarMOSH fabricated on SiC and free-standing GaN substrates. PolarMOSH wafers epitaxially grown on SiC substrates are found to suffer from large leakage currents, with or without Mg doping in the back barrier. Much lower leakage currents are achieved when PolarMOSH wafers are grown on free-standing GaN substrates. The large reduction of buffer leakage current is attributed to the much reduced dislocation density brought by free-standing GaN substrates. The PolarMOSH fabricated on free-standing GaN substrates has a current On/Off ratio > 1010 thanks to the low leakage current.

Activation of buried p-GaN in MOCVD-regrown vertical structures

Thermal activation of buried p-type GaN is investigated in metal-organic chemical vapor deposition-regrown vertical structures, where the buried p-GaN is re-passivated by hydrogen during regrowth. The activation is performed by exposing the buried p-GaN through etched sidewalls and characterized by reverse breakdown measurements on vertical diodes. The effect of the n-type doping level on the activation has been observed. After 725 °C/30 min annealing in a dry air environment, the buried p-GaN with a regrown unintentionally-doped (UID) capping layer is sufficiently activated due to significant Mg-incorporation in the UID layer, allowing for hydrogen up-diffusion. With an additional regrown n+-GaN capping layer (i.e., in n+/i/p-n diodes), only lateral diffusion of H out of the exposed mesa sidewall is permitted. A critical lateral dimension between 10 and 20 μm is found for the n+/i/p-n diodes, under which the buried p-GaN is sufficiently activated. The diodes with activated buried p-GaN achieved up to 1200 V breakdown voltage, indicating that over 28% of the Mg dopants is activated. The study demonstrates the effectiveness of sidewall p-GaN activation in achieving high breakdown voltage pertinent to GaN vertical power devices, while providing guidelines on the required device geometry.Thermal activation of buried p-type GaN is investigated in metal-organic chemical vapor deposition-regrown vertical structures, where the buried p-GaN is re-passivated by hydrogen during regrowth. The activation is performed by exposing the buried p-GaN through etched sidewalls and characterized by reverse breakdown measurements on vertical diodes. The effect of the n-type doping level on the activation has been observed. After 725 °C/30 min annealing in a dry air environment, the buried p-GaN with a regrown unintentionally-doped (UID) capping layer is sufficiently activated due to significant Mg-incorporation in the UID layer, allowing for hydrogen up-diffusion. With an additional regrown n+-GaN capping layer (i.e., in n+/i/p-n diodes), only lateral diffusion of H out of the exposed mesa sidewall is permitted. A critical lateral dimension between 10 and 20 μm is found for the n+/i/p-n diodes, under which the buried p-GaN is sufficiently activated. The diodes with activated buried p-GaN achieved up to 120...

Breakdown mechanism in 1 kA/cm2 and 960 V E-mode β-Ga2O3 vertical transistors

A high current density of 1 kA/cm2 is experimentally realized in enhancement-mode Ga2O3 vertical power metal-insulator field-effect transistors with fin-shaped channels. Comparative analysis shows that the more than doubled current density over the prior art arises from a larger transistor channel width; on the other hand, a wider channel also leads to a more severe drain-induced barrier lowering therefore premature transistor breakdown at zero gate-source bias. The observation of a higher current density in a wider channel confirms that charge trapping in the gate dielectric limits the effective field-effect mobility in these transistor channels, which is about 2× smaller than the electron mobility in the Ga2O3 drift layer. The tradeoff between output-current density and breakdown voltage also depends on the trap density. With minimal trap states, the output current density should remain high while breakdown voltage increases with decreasing fin-channel width.

Wide-bandgap Gallium Nitride p-channel MISFETs with enhanced performance at high temperature

Wide-bandgap materials, particularly Gallium Nitride, have emerged as the platform underlying many of the most promising technologies in the high-power and high-frequency domain. However, GaN p-channel devices lag far behind their popular n-channel counterparts, due to lower mobilities as well as difficulties in doping and forming ohmic contacts. There is a strong need for wide-bandgap p-channel FETs — this missing piece would enable energy-efficient, high-voltage CMOS electronics, a critical technique for on-chip power conditioning and management.