Differential On-resistance Research Articles

High-current, high-voltage AlN Schottky barrier diodes

AlN Schottky barrier diodes with low ideality factor (<1.2), low differential ON-resistance (<0.6 mΩ cm2), high current density (>5 kA cm−2), and high breakdown voltage (680 V) are reported. The device structure consisted of a two-layer, quasi-vertical design with a lightly doped AlN drift layer and a highly doped Al0.75Ga0.25N ohmic contact layer grown on AlN substrates. A combination of simulation, current–voltage measurements, and impedance spectroscopy analysis revealed that the AlN/AlGaN interface introduces a parasitic electron barrier due to the conduction band offset between the two materials. This barrier was found to limit the forward current in fabricated diodes. Further, we show that introducing a compositionally-graded layer between the AlN and the AlGaN reduces the interfacial barrier and increases the forward current density of fabricated diodes by a factor of 104.

(Invited) Towards GaN-on-GaN High-Power Electronic Devices

Application of gallium nitride (GaN) substrates in electronic and optoelectronic industries is constantly increasing. In order to fabricate wafers, GaN crystals of the highest structural quality and desired electrical (and sometimes optical) properties must be grown. Today, there are three main GaN crystallization methods: i/ halide vapor phase epitaxy (HVPE) with its derivatives: halide-free VPE and oxide VPE; ii/ sodium-flux; and iii/ ammonothermal. The last approach can be basic or acidic depending on what mineralizer is used to increase the solubility of GaN in the feedstock zone. In this paper we will focus on HVPE and basic ammonothermal growth of GaN [1]. Not only bulk growth will be presented. The HVPE method will also be discussed as the best method to crystallize the drift layers necessary for high-power vertical electronic devices (FET transistors, Schottky diodes).One of the best methods for introducing dopants into semiconductors is ion implantation. The introduced structural damage can be removed by a proper annealing process. The high-temperature treatment enables also electrical and/or optical activation of the implanted dopants. In the case of GaN, annealing at high temperature (~1300°C - 1400°C) seems difficult. This compound loses its thermodynamic stability slightly above 800°C at atmospheric pressure. At higher temperature the crystal will decompose. One of the solutions is to anneal GaN at high nitrogen (N2) pressure. Such technology is called ultra-high-pressure annealing (UHPA) [2]. In this paper, application of UHPA for GaN crystals and layers implanted by different ions (acceptors and donors) will be presented. The latest results of the implantation with magnesium (Mg), beryllium (Be), zinc (Zn), and calcium (Ca) ions into GaN in order to obtain p-type conductivity will be discussed [3,4]. Silicon (Si) implantation into GaN for n-type doping will also be analyzed. Structural, electrical and optical properties of implanted GaN after UHPA will be discussed in terms of application for GaN-based devices.A kV class, low ON-resistance, vertical GaN junction barrier Schottky (JBS) diode with selective-area p-regions formed via Mg implantation followed by high-temperature, ultra-high pressure post-implantation activation anneal without a capping layer was already demonstrated [5]. The forward characteristics of the JBS diode showed ideality factor (n) 1.03, turn-on voltage (VON) 0.75 V, current ON/OFF ratio (at ±3 V) ~1011, and specific differential ON-resistance (RON) 0.6 mΩ·cm2. The breakdown voltage of the JBS diode was 915 V.

Enhancing performance of β-Ga2O3 diodes through a NixO/SiNx/Ga2O3 sandwich structure

In this study, we propose and investigate a novel sandwich structure composed of NixO/SiNx/Ga2O3 for the purpose of enhancing the forward current density and reducing the on-resistance in β-Ga2O3 diodes. The fabricated sandwich Ga2O3 diode demonstrates an impressive forward current density exceeding 1 kA/cm2 under a forward bias of 2 V, surpassing previously reported Ga2O3 Schottky diodes. Furthermore, the sandwich diode exhibits a remarkably low differential specific on-resistance (Ron,sp) of 1.1 mΩ·cm2. These findings present a promising avenue for the design of high-current Ga2O3 power electronics.

2.5 kV Vertical Ga2O3 Schottky Rectifier With Graded Junction Termination Extension

This work demonstrates vertical Ga2O3 Schottky barrier diodes (SBDs) with a novel junction termination extension (JTE) comprising multiple layers of sputtered p-type nickel oxide (NiO). The NiO layers have the varied lengths to enable a graded decrease in effective charge density away from the main junction. The fabrication of this JTE obviates the etch or implantation and shows good throughput. The fabricated Ga2O3 SBDs exhibit a forward voltage below 1.9 V at the current density of 100 A/cm2, a differential specific on-resistance of 5.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> cm2, and a breakdown voltage over 2.5 kV. The Baliga’s figure of merit (FOM) exceeds 1 GW/cm2 and is among the highest in multi-kilovolt Ga2O3 SBDs. Besides, the capacitance of the JTE region is extracted, allowing for evaluation of the capacitance, charge, and switching FOM of 1.7 kV-class Ga2O3 SBDs with varied current ratings. The results show good promise of Ga2O3 SBDs for kilovolt power electronics.

Interface-state suppression of AlGaN/GaN Schottky barrier diodes with post-anode-annealing treatment

Owing to the high density and high mobility of two-dimensional electron gas (2DEG) induced by strong spontaneous polarization and piezoelectric polarization effect, AlGaN/GaN Schottky barrier diodes (SBDs) with high output current density and low on-resistance have proved to be a promising candidate. Anode of GaN SBD is the core structure, which affects the device performance such as turn-on voltage, reverse current, on-resistance, and breakdown voltage. Therefore, idealized Schottky junction with low interface state density is very important in achieving high-performance GaN SBD. In this work, AlGaN/GaN SBD with low work-function metal W as anode is fabricated, and the post-anode-annealing (PAA) treatment is found to be effective in promoting the bonding reaction between anode metal and GaN in the anode region. Comparing with GaN SBDs without PAA treatment, the interface state density decreases from 9.48×10&lt;sup&gt;15&lt;/sup&gt; eV&lt;sup&gt;–1&lt;/sup&gt;·cm&lt;sup&gt;–2&lt;/sup&gt; to 1.77×10&lt;sup&gt;13&lt;/sup&gt; eV&lt;sup&gt;–1&lt;/sup&gt;·cm&lt;sup&gt;–2&lt;/sup&gt; after PAA treatment. The reverse leakage current is reduced by two orders, which ascribes to the idealized anode interface with low interface state density. Meanwhile, the influence of interface state on carriers in the forward conduction process is also suppressed, and the differential on-resistance of the fabricated GaN SBDs decreases from 17.05 Ω·mm to 12.57 Ω·mm. It is obvious that the PAA process proves to be an effective method to suppress the interface states density at M/S interface, thus significantly improving the performance of GaN SBD, which is the key technology in fabricating the high-performance GaN device.

Pt/ZnGa₂O₄ Schottky Barrier Diodes Fabricated by Using Single Crystal n-ZnGa₂O₄ (111) Substrates

In this letter, we fabricated Schottky barrier diodes (SBDs) using n-type zinc gallium oxide (n-ZnGa2O4) single-crystal substrates, which were grown from the melt by the vertical gradient freeze (VGF) method. X-ray diffraction rocking curve and Laue diffraction results indicate the good crystal quality. The electrical properties of SBDs fabricated on substrates with two doping concentrations are systematically investigated. Heavily-doped (HD) SBDs exhibit lower differential specific on-resistance (~1.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> cm2) and lower turn-on voltage (~0.16 V) while the lightly-doped (LD) SBDs exhibit better ideality factor (1.17), rectification characteristics (on/off ratio ~1011) and lower reverse leakage. In addition, we separately tested the electrical properties of the two devices at different temperatures (298 K-473 K). The results of varying temperature show that LD-SBDs have better temperature stability in smaller variation of ideality factor and barrier height with temperature. Based on these experimental results, this work demonstrates the great promise of ZnGa2O4 materials to fabricate high-performance power devices.

Contact resistance in sub-micron and nanoscale organic thin-film transistors

Two improved methods for evaluating contact resistance in organic thin-film transistors (TFTs) are presented. These methods can be applied to TFTs with any arbitrary geometry and are also suitable for retroactive analysis of current-voltage characteristics. The first method is based on the analysis of the differential conductance of TFTs and in separating the conductivity contributions from the contacts and the channel. This method provides accurate results when the contact resistance is approximately 0.1–10 times the channel resistance. The second method is especially applicable to nanoscale and short channel length TFTs. The minimum value of differential on-resistance as a function of drain bias is used to separate the contributions of the channel and contact resistances at various drain voltages. This method is accurate when the contact resistance is approximately 0.3–10 times the channel resistance, as verified by simulations. It works even when there is no distinct saturation in current-voltage characteristics with increasing drain voltage. Both methods have been applied to experimental data from organic TFTs with channel lengths in the range 10 nm to 5 μm. • Two novel methods of extracting contact resistance in OTFTs are introduced. • Contact resistance of various channel lengths OTFTs are extracted and analyzed. • Effects of contact resistance in short channel pentacene TFTs are discussed. • New design of ultra-short channel TFTs is proposed to improve carrier injection.

Vertical GaN junction barrier Schottky diodes with near-ideal performance using Mg implantation activated by ultra-high-pressure annealing

We report a kV class, low ON-resistance, vertical GaN junction barrier Schottky (JBS) diode with selective-area p-regions formed via Mg implantation followed by high-temperature, ultra-high pressure (UHP) post-implantation activation anneal. The JBS has an ideality factor of 1.03, a turn-on voltage of 0.75 V, and a specific differential ON-resistance of 0.6 mΩ·cm2. The breakdown voltage of the JBS diode is 915 V, corresponding to a maximum electric field of 3.3 MV cm−1. These results underline that high-performance GaN JBS can be realized using Mg implantation and high-temperature UHP post-activation anneal.

(Invited, Digital Presentation) Exploring the Potential and Limits of Gallium Oxide Electronics: In-Situ Dielectrics, Heterointegration and High-k Field Plates

The intrinsic properties of beta phase Gallium Oxide such as the high breakdown field strength makes it a very attractive material for next generation electronics. Rapid progress has been made over the last few years in pushing the performance of Gallium Oxide-based vertical Schottky diodes and lateral field effect transistors and impressive figure of merit has been reported. However, the device performance is farther from the expected performance based on intrinsic properties. Exploration of the intrinsic potential is rather limited by several factors such as the quality of dielectrics and dielectric/semiconductor interfaces, electric field management challenges due to the lack of p-type material, to name a few. In this work, we will highlight a few approaches to tackle some of these challenges.(1) In-situ dielectrics: High quality dielectric-semiconductor interfaces are critical for reliable high-performance transistors. We report in situ metal–organic chemical vapor deposition of Al2O3 on β-Ga2O3 as a potentially better alternative to the most commonly used atomic layer deposition (ALD) [1]. The growth of Al2O3 is performed in the same reactor as Ga2O3 using trimethylaluminum and O2 as precursors without breaking the vacuum at a growth temperature of 600 °C. The fast and slow near interface traps at the Al2O3/β-Ga2O3 interface are identified and quantified using stressed capacitance–voltage (CV) measurements on metal oxide semiconductor capacitor (MOSCAP) structures. The density of shallow and deep level initially filled traps (Dit) are measured using ultraviolet-assisted CV technique. The average Dit for the MOSCAP is determined to be 6.4 × 1011 cm−2eV−1. The conduction band offset of the Al2O3/ Ga2O3 interface is also determined from CV measurements and found out to be 1.7 eV which is in close agreement with the existing literature reports of ALD Al2O3/Ga2O3 interface. The current–voltage characteristics are also analyzed and the average breakdown field is extracted to be approximately 5.8 MV cm−1.(2) Field management using High-K field Plate: We report a vertical (001) β-Ga2O3 field-plated (FP) Schottky barrier diode (SBD) with a novel extreme permittivity dielectric field oxide [2] . A thin drift layer of 1.7μm was used to enable a punch-through (PT) field profile and very low differential specific on-resistance (Ron-sp) of 0.32 mΩ-cm2. The extreme permittivity field plate oxide facilitated the lateral spread of the electric field profile beyond the field plate edge and enabled a breakdown voltage (Vbr) of 687 V. The edge termination efficiency increases from 13.2%for non-field plated structure to 61% for high permittivity field plate structure. The high permittivity field plated SBD demonstrated a record high Baliga's figure of merit (BFOM) of 1.47 GW/cm2 showing the potential of Ga2O3 power devices for multi-kilovolt class applications.(3) p-type III-nitride/Ga2O3 integration for field management: This work presents the simulation and analysis of a novel Ga 2 O 3 vertical Schottky diode with three different guard ring (GR) configurations to reduce the peak electric field at the metal edges [3]. Highly doped p-type GaN, p-type nonpolar AlGaN, and polarization-doped graded p-AlGaN are simulated and analyzed as the GR material, which forms a heterojunction with the Ga 2 O 3 drift layer. GR with nonpolar graded p-AlGaN with a bandgap larger than Ga 2 O 3 is found to show the best performance in terms of screening the electric field at the metal edges. The proposed GR configuration is also compared with a reported Ga 2 O 3 Schottky diode with no GR and a structure with high-resistive nitrogen-doped GR. The optimized design is predicted to have a breakdown voltage as high as 6.2 kV and a specific ON-resistance of 3.55 mΩ-cm 2, which leads to an excellent power figure of merit of 10.8 GW/cm 2.The approaches discussed here can offer a pathway towards understanding the true potential and limitations of Gallium Oxide for extreme electric field devices. We acknowledge funding support from NSF (DMR-1931652) and the AFOSR MURI program (FA9550-18-1-0507).

Distributed polarization-doped GaN p–n diodes with near-unity ideality factor and avalanche breakdown voltage of 1.25 kV

Polarization-induced (Pi) distributed or bulk doping in GaN, with a zero dopant ionization energy, can reduce temperature or frequency dispersions in impurity-doped p–n junctions caused by the deep-acceptor-nature of Mg, thus offering GaN power devices promising prospects. Before comprehensively assessing the benefits of Pi-doping, ideal junction behaviors and high-voltage capabilities should be confirmed. In this work, we demonstrate near-ideal forward and reverse I–V characteristics in Pi-doped GaN power p–n diodes, which incorporates linearly graded, coherently strained AlGaN layers. Hall measurements show a net increase in the hole concentration of 8.9 × 1016 cm−3 in the p-layer as a result of the polarization charge. In the Pi-doped n-layer, a record-low electron concentration of 2.5 × 1016 cm−3 is realized due to the gradual grading of Al0-0.72GaN over 1 μm. The Pi-doped p–n diodes have an ideality factor as low as 1.1 and a 0.10 V higher turn-on voltage than the impurity-doped p–n diodes due to the increase in the bandgap at the junction edge. A differential specific on-resistance of 0.1 mΩ cm2 is extracted from the Pi-doped p–n diodes, similar with the impurity-doped counterpart. The Pi-doped diodes show an avalanche breakdown voltage of ∼1.25 kV, indicating a high reverse blocking capability even without an ideal edge-termination. This work confirms that distributed Pi-doping can be incorporated in high-voltage GaN power devices to increase hole concentrations while maintaining excellent junction properties.

Metal-nitride dual-anode AlGaN/GaN heterostructure Schottky barrier diodes with tunable turn-on voltage and reverse leakage current

AlGaN/GaN heterostructure lateral Schottky barrier diodes (SBDs) with TiN and NiN dual anode (DA) on sapphire substrates are investigated in this letter. The NiN anode with its high work-function leads to low leakage current and high breakdown voltage, while TiN anode with its low work-function determines the low turn-on voltage of the DA SBDs. Tunable turn-on voltage and leakage current are obtained in the DA SBDs by varying the radius of the TiN anode. As the radius of the TiN anode decreases from 80 to 8 μm, the turn-on voltage increases from 0.64 to 0.94 V, while the reverse leakage current decreases from 1 × 10−2 to 1 × 10−4 mA mm−1 at a reverse bias of −10 V and cathode–anode distance of 20 μm. The differential specific on-resistance at 100 mA mm−1 is 4.5 mΩ cm2 and barely changes with various radius of the TiN anode. A high breakdown voltage of 1.49 kV is achieved in the AlGaN/GaN DA SBDs with the radius of 80 μm of the TiN anode, obtaining a power Baliga’s figure of merit of 0.48 GW cm−2 at the cathode–anode distance of 20 μm. Besides, dynamic on-resistance increases less than 15% under pulse voltage bias at −60 V which may account on the good interface between metal nitrides and AlGaN, which is beneficial to the high frequency and high power application.

Over 1.8 GW/cm2 beveled-mesa NiO/β-Ga2O3 heterojunction diode with 800 V/10 A nanosecond switching capability

In this Letter, we demonstrate a large-area (1-mm2) beveled-mesa p-NiO/β-Ga2O3 bipolar heterojunction diode (HJD) with a high Baliga's figure of merit of 1.84 (2.87) GW/cm2 from DC (pulsed) measurements. Benefiting from the suppression of electric field crowing at the designed mesa edge and bipolar current conductivity modulation, the resultant device exhibits advantageous characteristics, including a low subthreshold slope of 65 mV/decade, a low DC (pulsed) differential specific on-resistance of 2.26 (1.45) mΩ cm2, a high current density of 2 kA/cm2, and a high breakdown voltage of 2.04 kV. In particular, the Ga2O3 HJD exhibits an 800 V/10 A extreme switching capability with 16.4-ns reverse recovery characteristics, as well as high operation stability at a high temperature of 200 °C. This work, thus, makes a significant step toward reaching the promise of a high figure-of-merit in β-Ga2O3 power devices.

Surge Current and Avalanche Ruggedness of 1.2-kV Vertical GaN p-n Diodes

This letter reports the avalanche and surge current ruggedness of the industry's first 1.2-kV-class vertical GaN p-n diodes fabricated on 100-mm GaN substrates. The 1.2-kV vertical GaN p-n diodes with a 1.39-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device area and an avalanche breakdown voltage of 1589 V show a critical avalanche energy density of 7.6 J/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in unclamped inductive switching tests, as well as a critical surge current of 54 A and a critical surge energy density of 180 J/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in 10-ms surge current tests. All these values are the highest reported in vertical GaN devices and comparable to those of commercial SiC p-n diodes and merged p-n Schottky diodes. These GaN p-n diodes show significantly smaller reverse recovery compared to SiC p-n diodes, revealing less conductivity modulation in n-GaN. The negative temperature coefficient of differential on-resistance and the anticlockwise surge <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I–V</i> locus are believed to be due to the increased acceptor ionization in p-GaN and the decreased contact resistance at high temperatures. These results suggest a high ruggedness of GaN p-n junctions with small bipolar currents and fast switching capabilities. As the first electrothermal ruggedness data for industry's vertical GaN devices, these results provide key new insights for the development of vertical GaN devices as well as their application spaces.

High-k Oxide Field-Plated Vertical (001) β-Ga2O3Schottky Barrier Diode With Baliga’s Figure of Merit Over 1 GW/cm2

We report a vertical (001)β-Ga2O3 field-plated(FP) Schottky barrier diode (SBD) with a novel extreme permittivity dielectric field oxide. A thin drift layer of 1.7μm was used to enable a punch-through (PT) field profile and very low differential specific on-resistance (Ron-sp)of 0.32 mΩ-cm2. The extreme permittivity field plate oxide facilitated the lateral spread of the electric field profile beyond the field plate edge and enabled a breakdown voltage (Vbr) of 687 V. The edge termination efficiency increasesfrom 13.2%for non-field plated structure to 61%for high permittivity field plate structure. The surface breakdown electric field was extracted to be 5.45 MV/cm at the center of the anode region using TCAD simulations. The high permittivity field plated SBD demonstrated a record high Baliga's figure of merit (BFOM) of 1.47 GW/cm2 showing the potential of Ga2O3 power devices for multi-kilovolt class applications.

Recessed-anode AlGaN/GaN diode with a high Baliga's FOM by combining a p-GaN cap layer and an anode-connected p-GaN buried layer

In this article, a recessed-anode AlGaN/GaN Schottky barrier diode with a p-GaN cap layer and an anode-connected p-GaN buried layer was proposed and optimized by Silvaco TCAD. The impact of different structural parameters on the trade-off between breakdown voltage and differential specific on-resistance were systematically investigated. Compared with the conventional recessed-anode SBD, the introduction of a p-GaN buried layer with an optimum distance from 2DEG channel can improve the breakdown voltage mildly, reduce the on-resistance and increase the surge current capability obviously at relatively high forward bias. Besides, a p-GaN cap layers slightly decreases the current density but demonstrates an obvious improvement in the breakdown voltage. Therefore, a combining of the p-GaN buried layer and the p-GaN cap layer is promising to achieve the better trade-off between on-resistance and breakdown voltage, resulting in a higher Baliga's FOM.

Vertical β -Ga2O3 field plate Schottky barrier diode from metal-organic chemical vapor deposition

Vertical β-Ga2O3 Schottky diodes from metal-organic chemical vapor deposition (MOCVD)-grown epitaxial films are reported in this paper for high-power application devices. The Schottky diode, fabricated with a field termination structure, showed a low differential specific on-resistance of 0.67 mΩ cm2. Furthermore, the MOCVD-grown β-Ga2O3 vertical Schottky diodes exhibited a punch-through breakdown and a higher Baliga's figure-of-merit compared to those from other epitaxial growth methods of similar drift layer thickness. This suggests that the MOCVD growth, supporting high-quality epitaxy, can be promising for high-performance β-Ga2O3-based high-power devices.

Differential-drive CMOS rectifier with photovoltaic-assist and self-Vth-cancellation schemes for high power conversion efficiency under low input power conditions

In this study, a differential-drive CMOS RF rectifier equipped with photovoltaic assistance and self-Vth-cancellation (SVC) mechanism has been developed for efficient energy harvesting from ambient radio waves. In addition to the differential-drive dynamic biasing, in which by dynamically biasing the MOSFET gates with differential input signal, both on-resistance and reverse leakage current of the MOSFET are reduced and high power conversion efficiency (PCE) can be achieved, the combination of SVC technology and photovoltaic assist technology allows further improvement of the PCE under low input power conditions. As a result, the PCE of 38.5% was achieved at an input power level of −22 dBm, a frequency of 1 GHz, an output load resistance of 10 kΩ, and a light irradiance of 1 W m−2.

First Demonstration of L-Band High-Power Limiter with GaN Schottky Barrier Diodes (SBDs) Based on Steep-Mesa Technology

Gallium nitride (GaN) has attracted increased attention because of superior material properties, such as high electron saturation velocity and high electrical field strength, which are promising for high-power microwave applications. We report on a high-performance vertical GaN-based Schottky barrier diode (SBD) and its demonstration in a microwave power limiter for the first time. The fabricated SBD achieved a very low differential specific on-resistance (RON,sp) of 0.21 mΩ·cm2, attributed to the steep-mesa technology, which assists in reducing the spacing between the edge of the anode and cathode to 2 μm. Meanwhile, a low leakage current of ~10−9 A/cm2@−10 V, a high forward current density of 9.4 kA/cm2 at 3 V in DC, and an ideality factor of 1.04 were achieved. Scattering parameter measurements showed that the insertion loss (S21) was lower than −3 dB until 3 GHz. In addition, a microwave power limiter circuit with two anti-parallel diodes was built and measured on an alumina substrate. The input power level reached 40 dBm (10 watts) in continuous-wave mode at 2 GHz, with a corresponding leakage power of 27.2 dBm (0.5 watts) at the output port of the limiter, exhibiting the great potential of GaN SBD in microwave power limiters.

Field-Plated NiO/Ga2O3 p-n Heterojunction Power Diodes With High-Temperature Thermal Stability and Near Unity Ideality Factors

In this work, vertical NiO/Ga2O3 heterojunction diodes (HJDs) integrated with SiNx/Al2O3 double-layered insulating field plate (FP) structures have been demonstrated. With the additional post-annealing, the resultant diode exhibits a decreased differential specific on-resistance (Ron,sp) of 5.4 mωcm2 and an enhanced breakdown voltage (BV) of 1036 V. The improved performance is attributed by the combination of the FP-suppressed crowding electric field at the device edge and the reduced trap density at the NiO/Ga2O3 interface. In particular, the near-unity ideality factor has been achieved for this p-n HJD at an elevated temperature of 275∘C, indicating that the diffusion current is dominated. The high-temperature operation capability is owing to the quality improvement of NiO/Ga2O3 interface, where the Shockley-Read-Hall (SRH) recombination mediated by deep-level defects within the depletion region is thus suppressed.

SnO/β-Ga2O3 vertical pn heterojunction diodes

As a contribution to (transparent) bipolar oxide electronics, vertical pn heterojunction diodes were prepared by plasma-assisted molecular beam epitaxy of unintentionally doped p-type SnO layers with hole concentrations ranging from p=1018 to 1019 cm−3 on unintentionally doped n-type β-Ga2O3(−201) substrates with an electron concentration of n=2.0×1017 cm−3. The SnO layers consist of (001)-oriented grains without in-plane epitaxial relation to the substrate. After subsequent contact processing and mesa-etching (which drastically reduced the reverse current spreading in the SnO layer and associated high leakage), electrical characterization by current–voltage and capacitance–voltage measurement was performed. The results reveal a type-I band alignment and junction transport by thermionic emission in forward bias. A rectification of 2×108 at ±1 V, an ideality factor of 1.16, a differential specific on-resistance of 3.9 m Ω cm2, and a built-in voltage of 0.96 V were determined. The pn-junction isolation prevented parallel conduction in the highly conductive Ga2O3 substrate during van-der-Pauw Hall measurements of the SnO layer on top, highlighting the potential for decoupling the p-type functionality in lateral transport devices from that of the underlying n-type substrate. The measured maximum reverse breakdown voltage of the diodes of 66 V corresponds to a peak breakdown field of 2.2 MV/cm in the Ga2O3-depletion region and suggests the low bandgap of the SnO (≈0.7 eV) not to be the limiting factor for breakdown. Higher breakdown voltages that are required in high-voltage devices could be achieved by reducing the donor concentration in the β-Ga2O3 toward the interface to increase the depletion width, as well as improving the contact geometry to reduce field crowding.