Vertical Diodes Research Articles

Electrical Transport Characteristics of Vertical GaN Schottky-Barrier Diode in Reverse Bias and Its Numerical Simulation

We investigated the temperature-dependent reverse characteristics (JR-VR-T) of vertical GaN Schottky-barrier diodes with and without a fluorine-implanted edge termination (ET). To understand the device leakage mechanism, temperature-dependent characterizations were performed, and the observed reverse current was modeled through technology computer-aided design. Different levels of current were observed in both forward and reverse biases for the ET and non-ET devices, which suggested a change in the conduction mechanism for the observed leakages. The measured JR-VR-T characteristics of the non-edge-terminated device were successfully fitted in the entire temperature range with the phonon-assisted tunneling model, whereas for the edge-terminated device, the reverse characteristics were modeled by taking into account the emission of trapped electrons at a high temperature and field caused by Poole–Frenkel emission.

2.83-kV double-layered NiO/β-Ga2O3 vertical p-n heterojunction diode with a power figure-of-merit of 5.98 GW/cm2

This work demonstrates high-performance NiO/β-Ga2O3 vertical heterojunction diodes (HJDs) with double-layer junction termination extension (DL-JTE) consisting of two p-typed NiO layers with varied lengths. The bottom 60-nm p-NiO layer fully covers the β-Ga2O3 wafer, while the geometry of the upper 60-nm p-NiO layer is 10 μm larger than the square anode electrode. Compared with a single-layer JTE, the electric field concentration is inhibited by double-layer JTE structure effectively, resulting in the breakdown voltage being improved from 2020 to 2830 V. Moreover, double p-typed NiO layers allow more holes into the Ga2O3 drift layer to reduce drift resistance. The specific on-resistance is reduced from 1.93 to 1.34 mΩ·cm2. The device with DL-JTE shows a power figure-of-merit (PFOM) of 5.98 GW/cm2, which is 2.8 times larger than that of the conventional single-layer JTE structure. These results indicate that the double-layer JTE structure provides a viable way of fabricating high-performance Ga2O3 HJDs.

650V Vertical SiC MOSFETs and Diodes with Improved Terrestrial-Neutron Single-Event Burnout

Silicon carbide (SiC) power devices such as Schottky diodes and metal-oxide semiconductor field-effect transistors (MOSFETs) are susceptible to failure by terrestrial neutron single-event burnout (SEB) while in the high-voltage blocking state. In this study the effects of the drift layer design of 650V SiC vertical power diodes and MOSFETs have been studied. TCAD simulations of different device designs have been performed, and fabricated device single-event burnout (SEB) properties are compared between the devices fabricated. We find that the standard 650V devices have a low area-scaled failure-in-time (FIT/cm2) such that essentially no failures (0.01 FIT/cm2) are expected at 400V drain-source bias (VDS) operation and below. An improved design allows the SEB failure rate curve to be shifted downward in failure rate, such that at for a given VDS operation condition, the FIT/cm2 can be decreased by 10 - 100 times, depending on the VDS value. This allows operation at about 75V higher VDS value with a similar SEB failure rate, allowing these devices to be used in a wider range of applications.

Degradation of vertical GaN diodes during proton and xenon-ion irradiation

We investigated the material stability of a vertical GaN Schottky barrier diode (SBD) against proton irradiations by making real-time measurements. The reverse current gradually decreased with increasing proton fluence. The current of the GaN SBD was reduced by 18% after proton irradiations with a displacement-damage dose (D d) of 1012 MeV g−1. We also examined signal and current degradation occurring in a vertical GaN-on-GaN p–n diode (PND) during xenon-ion irradiations. The signal gradually decreased with increasing xenon-ion fluence. Xenon-ion irradiations of D d = 1012 MeV g−1 reduced the collected charge in the PND by 11%. This signal degradation was close to the current degradation in the GaN SBD caused by the proton irradiations. We found that irradiations with D d > ∼1012 MeV g−1 degraded the performance of the GaN devices.

General optimization of breakdown voltage and resistivity on power components in terms of doping level and thickness

The emergence of wide band-gap (WBG) materials promises improved performance in semiconductor devices, but also presents significant technical challenges. Developing this immature technology and adapting market-ready silicon-based technology to new materials requires a thorough understanding of the relationship between material properties and the final electrical performance of the component. In this work, we propose a model that relates generic material properties and device-specific parameters (such as doping level or thickness) to the final breakdown voltage and performance of unipolar vertical diodes. Our approach enables optimized device design and evaluation by connecting the electrical performance target with the optimal drift layer characteristics. This model can be applied universally to any material with known impact ionization coefficients, carrier mobility, and thermal activation energy. We focus on diamond as a paradigm for WBG materials, illustrating specific characteristics such as incomplete carrier ionization or elevated doping-dependent critical electric fields. Our model predicts that diamond-based vertical devices with 70 μm layers and doping levels of 1.1015 cm−3 can sustain 20 kV.

High breakdown electric field diamond Schottky barrier diode with HfO2 field plate

In this work, we fabricated a vertical diamond Schottky barrier diode (SBD) with a high breakdown electric field of 4.8 MV/cm and a forward current density of 2361 A/cm2. Compared with a regular diamond SBD, the breakdown electric field of SBD with a HfO2 field plate (FP) increased from 183 to 302 V, the current swing (ION/IOFF) was on the order of 1011. As the thickness of the HfO2 FP increased from 200 to 400 nm, the breakdown voltage of the SBD increased from 280 to 314 V, and the corresponding breakdown electric field increased from 4.5 to 5 MV/cm. We also measured the current–voltage characteristics at different temperatures to investigate the cause of the high on-resistance. As the measured temperature increased from 25 to 150 °C, the on-resistance of the device decreased from 4.7 to 1.7 mΩ·cm2. By studying the interface between HfO2 and the diamond, we found that HfO2 can reduce the interface state density of the Schottky contact. The interface state density of Zr/HfO2/diamond was lower than 1.5 × 1013 eV−1·cm−2. This work provides a simple strategy for realizing high-performance diamond SBDs.

Micro-Raman Spectroscopy Study of Vertical GaN Schottky Diode

In this work, the physical and the electrical properties of vertical GaN Schottky diodes were investigated. Cathodo-luminescence (CL), micro-Raman spectroscopy, SIMS, and current-voltage (I-V) measurements were performed to better understand the effects of physical parameters, for example structural defects and doping level inhomogeneity, on the diode electrical performances. Evidence of dislocations in the diode epilayer was spotted thanks to the CL measurements. Then, using 2D mappings of the E2h and A1 (LO) Raman modes, dislocations and other peculiar structural defects were observed. The I-V measurements of the diodes revealed a significant increase in the leakage current with applied reverse bias up to 200 V. The combination of physical and electrical characterization methods indicated that the electrical leakage in the reverse biased diodes seems more correlated with short range non-uniformities of the effective doping than with strain fluctuation induced by dislocations.

β-Ga2O3 junction barrier Schottky diode with NiO p-well floating field rings

Recently, β-Ga2O3, an ultra-wide bandgap semiconductor, has shown great potential to be used in power devices blessed with its unique material properties. For instance, the measured average critical field of the vertical Schottky barrier diode (SBD) based on β-Ga2O3 has reached 5.45 MV/cm, and no device in any material has measured a greater before. However, the high electric field of the β-Ga2O3 SBD makes it challenging to manage the electric field distribution and leakage current. Here, we show β-Ga2O3 junction barrier Schottky diode with NiO p-well floating field rings (FFRs). For the central anode, we filled a circular trench array with NiO to reduce the surface field under the Schottky contact between them to reduce the leakage current of the device. For the anode edge, experimental results have demonstrated that the produced NiO/β-Ga2O3 heterojunction FFRs enable the spreading of the depletion region, thereby mitigating the crowding effect of electric fields at the anode edge. Additionally, simulation results indicated that the p-NiO field plate structure designed at the edges of the rings and central anode can further reduce the electric field. This work verified the feasibility of the heterojunction FFRs in β-Ga2O3 devices based on the experimental findings and provided ideas for managing the electric field of β-Ga2O3 SBD.

Fast Response GaN Nanoscale Air Channel Diodes with Highly Stable 10 mA Output Current toward Wafer-Scale Fabrication.

Nanoscale air channel transistors (NACTs) have received significant attention due to their remarkable high-frequency performance and high switching speed, which is enabled by the ballistic transport of electrons in sub-100nm air channels. Despite these advantages, NACTs are still limited by low currents and instability compared to solid-state devices. GaN, with its low electron affinity, strong thermal and chemical stability, and high breakdown electric field, presents an appealing candidate as a field emission material. Here, a vertical GaN nanoscale air channel diode (NACD) with a 50nm air channel is reported, fabricated by low-cost IC-compatible manufacturing technologies on a 2-inch sapphire wafer. The device boasts a record field emission current of 11mA at 10V in the air and exhibits outstanding stability during cyclic, long-term, and pulsed voltage testing. Additionally, it displays fast switching characteristics and good repeatability with a response time of fewer than 10ns. Moreover, the temperature-dependent performance of the device can guide the design of GaN NACTs for applications in extreme conditions. The research holds great promise for large current NACTs and will speed up their practical implementation.

Current transport in Ni Schottky barrier on GaN epilayer grown on free standing substrates

In this paper, the Ni Schottky barrier on GaN epilayer grown on free standing substrates has been characterized. First, transmission electrical microscopy (TEM) images and nanoscale electrical analysis by conductive atomic force microscopy (C-AFM) of the bare material allowed visualizing structural defects in the crystal, as well as local inhomogeneities of the current conduction. The forward current-voltage (I-V) characteristics of Ni/GaN vertical Schottky diodes fabricated on the epilayer gave average values of the Schottky barrier height of 0.79 eV and ideality factor of 1.14. A statistical analysis over a set of diodes, combined with temperature dependence measurements, confirmed the formation of an inhomogeneous Schottky barrier in this material. From a plot of ΦB versus n, an ideal homogeneous barrier close to 0.9 eV was estimated, similar to that extrapolated by capacitance-voltage (CV) analysis. Local I-V curves, acquired by means of C-AFM, displayed the inhomogeneous distribution of the onset of current conduction, which in turn resembles the one observed in the macroscopic Schottky diodes. Finally, the reverse characteristic of the diodes fabricated in the defects-free region have been acquired at different temperature and its behaviour has been described by the thermionic field emission (TFE) model.

Simulation study of vertical diamond Schottky barrier diode with field plate assisted junction termination extension

In this paper, we designed vertical diamond Schottky barrier diodes with a field plate assisted junction termination extension (FP-JTE) structure by using Silvaco TCAD. It demonstrates that the JTE structure is more effective to suppress the electric field crowding around the electrode edges than the FP, but it also increases the on-resistance due to the decreasing conduction area. Then, the FP-JTE diamond SBDs are designed to realize the trade-off between the breakdown voltage and on-resistance. It is found that the JTE length should be shorter than the FP to enhance the current conduction, but it needs to cover the cathode edge to suppress the electric field crowding. We also confirm that the FP-JTE SBDs with the JTE length increase from the sidewall toward the center are recommended to take advantage of both FP and JTE structures.

2.1 kV (001)- β -Ga2O3 vertical Schottky barrier diode with high-k oxide field plate

We report a vertical β-Ga2O3 Schottky barrier diode (SBD) with BaTiO3 as field plate oxide on a low doped thick epitaxial layer exhibiting 2.1 kV breakdown voltage. A thick drift layer of 11 μm with a low effective doping concentration of 8 × 1015 cm–3 is used to achieve high breakdown voltage. Using the high-k dielectric with a dielectric constant of 248, the breakdown voltage increases from 816 V for the non-field-plated SBD to 2152 V (>2× improvement) for the field-plated SBD without compromising the on-state performance. The diode dimensions are varied to analyze the effect of edge high-field related leakage with reverse bias and also the effect of current spreading during forward operation. Very uniform distribution of breakdown voltages of 2152 ± 20 V are observed for the diode diameters from 50 to 300 μm for the field-plated SBDs. The on and off state power losses are also analyzed and compared with the non-field-plated devices and the switching losses are estimated analytically.

Breakdown voltage enhancement of vertical diamond Schottky barrier diodes by selective growth nitrogen-doped diamond field plate

Breakdown voltage enhancement of vertical diamond Schottky barrier diodes by selective growth nitrogen-doped diamond field plate

Transient Photocurrent From High-Voltage Vertical GaN Diodes Irradiated With Electrons: Experiments and Simulations

Radiation-hard high-voltage vertical GaN p-n diodes are being developed for use in power electronics subjected to ionizing radiation. We present a comparison of the measured and simulated photocurrent response of diodes exposed to ionizing irradiation with 70 keV and 20 MeV electrons at dose rates in the range of 1.4x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> - 5.0x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> rad(GaN)/s. The simulations correctly predict the trend in measured steady-state photocurrent and agree with experimental results within a factor of 2. Furthermore, simulations of the transient photocurrent response to dose rates with uniform and non-uniform ionization depth profiles uncover the physical processes involved that cannot be otherwise experimentally observed due to orders of magnitude larger RC time constant of the test circuit. The simulations were performed using an Exploratory Physics Development code developed at Sandia National Laboratories. The code offers the capability to include defect physics under more general conditions, not included in commercially available software packages, extending the applicability of the simulations to different types of radiation environments.

Correlating Structural and Electrical Characteristics of Threading Dislocations in GaN -on- Si Heterostructures and p - n Diodes by Multiple Microscopy Techniques

Structural and electrical properties of a- and a + c-type threading dislocations in metal-organic vapor-phase epitaxy-grown $\mathrm{Si}$-doped $\mathrm{Ga}\mathrm{N}$ (0001) are determined by combining multiple scanning probe microscopy approaches. The analysis examines the space-charge region (SCR) formed around dislocation cores and clarifies the role it plays in influencing the local recombination, the surface-potential characteristic, and even the conductivity. Direct evidence of the SCR is obtained from a differential capacitance (dC/dV) measurement at dislocation sites on the (0001) surface. Experimental dC/dV (V) measurement results supported by technology computer-aided design calculations of capacitance, accounting for the theoretical deep levels related to different atom-core structures of the identified dislocations, reveal quantitative differences in their respective trap densities. This study is extended to p-n $\mathrm{Ga}\mathrm{N}$ vertical diodes to analyze the doping distribution across active layers. Through a correlated cross-section investigation combining electron-channeling contrast-imaging microscopy, the underlying dislocations within the depth limited by the screening of their SCRs are found to severely impact the homogeneity of the cross-section dC/dV contrast in the low-doped n-type drift layer of the diodes. This raises serious concerns regarding any quantification efforts by differential capacitance measurements in $\mathrm{Ga}\mathrm{N}$ on $\mathrm{Si}$, where the dislocation density is in the order of ${10}^{9}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$.

Influence of fluorine implantation on the physical and electrical characteristics of GaN-on-GaN vertical Schottky diode

Fluorine implantation has been reported to be effective in suppressing electric fields at the edge of Schottky contacts in GaN due to the formation of fixed negative charged traps. In this study we show a detailed investigation into the behavior of fluorine implantation in GaN on material and electrical properties, and reproduce these electrical properties using TCAD simulations. A reduction in the free carrier concentration is observed by both μ-Raman, SSRM and C−V measurements, likely due to the trapping of free electrons by fluorine ions. The barrier height extracted from C-V measurements increases with increasing implant overlap area under the Schottky contact, whereas the barrier height extracted from I-V measurements remains constant. For the device without any implantation, the extracted barrier height remained the same for both extraction methods. This observation suggests the existence of a high and low double barriers originating from the implant and non-implant region respectively. TCAD simulations performed by including negative charge support the formation of a double barrier. By using the barrier height extracted from C-V measurements for the implanted region, we were able to use our TCAD model to estimate the profile of fixed negative charge, which is around 10% of the total implanted fluorine. This understanding shows that for effective design of edge terminations using fluorine implantation, consideration of the fixed negative charge as a percentage of the fluorine implantation is very important.

Impact of Substrate Morphology and Structural Defects in Freestanding Gallium Nitride on the Breakdown Characteristics of GaN-on-GaN Vertical Diodes

Impact of Substrate Morphology and Structural Defects in Freestanding Gallium Nitride on the Breakdown Characteristics of GaN-on-GaN Vertical Diodes

Theoretical Prediction of Semiconductor-Free Negative Differential Resistance Tunnel Diodes with High Peak-to-Valley Current Ratios Based on Two-Dimensional Cold Metals

The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS (complementary metal oxide semiconductor) computing because they offer several potential applications when integrated with transistors. We propose a semiconductor-free NDR tunnel diode concept that exhibits an ultrahigh peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of two cold metal electrodes separated by a thin insulating tunnel barrier. The NDR effect stems from the unique electronic band structure of the cold metal electrodes; i.e., the width of the isolated metallic bands around the Fermi level as well as the energy gaps separating higher- and lower-lying bands determine the current–voltage (I–V) characteristics and the PVCR value of the tunnel diode. By proper choice of the cold metal electrode materials, either a conventional N-type or Λ-type NDR effect can be obtained. Two-dimensional (2D) nanomaterials offer a unique platform for the realization of proposed NDR tunnel diodes. To demonstrate the proof of concept, we employ the nonequilibrium Green function method combined with density functional theory to calculate the I–V characteristic of the lateral (AlI2/MgI2/AlI2) and vertical (NbS2/h-BN/NbS2) heterojunction tunnel diodes based on 2D cold metals. For the lateral tunnel diode, we obtain a Λ-type NDR effect with an ultrahigh PVCR value of 1016 at room temperature, while the vertical tunnel diode exhibits a conventional N-type NDR effect with a smaller PVCR value of about 104. The proposed concept provides a semiconductor-free solution for NDR devices to achieve the desired I–V characteristics with ultrahigh PVCR values for memory and logic applications.

Electroluminescence of negatively charged single NV centers in diamond

The realization of electroluminescence (EL) of negatively charged nitrogen vacancy (NV−) centers is important toward all-electrical control of diamond quantum devices. In this study, we demonstrated electrical excitation and detection of EL of single NV− centers by using lateral diamond p+–i(n−)–n+ diodes. It had been grown by homoepitaxy using the plasma enhanced chemical vapor deposition technique. We introduced a lightly phosphorus doped i(n−) layer to stabilize the negative state of NV centers. It was estimated that the efficiency of the electrical excitation rate of the NV center was more than 30 times enhanced by introducing lateral diamond p+–i(n−)–n+ diodes structure compared with the previous vertical diode. Furthermore, the EL of a single NV− center embedded in the i(n−) layer region was characterized. The results show that the charge state of the single NV centers can be manipulated by the voltage applied to the p+–i(n−)–n+ diode, where the emission of EL is increasingly dominated by NV− in the range of 30 to 50 V.

Influence of swift heavy ion irradiation on electrical characteristics of β-Ga2O3 Schottky barrier diode

The radiation effect of swift heavy ions (16 MeV 181Ta) on the Au/Ni/β-Ga2O3 vertical Schottky barrier diodes (SBDs) were investigated at the fluence of 1 × 108, 3 × 108 and 3 × 109 cm−2. The SBDs were characterized by current density–voltage (J–V) and capacitance–voltage (C–V) measurements. It was found that Schottky barrier height φ decreased from 1.11 eV to 0.94 eV, the ideality factor n increased from 1.01 to 1.29, turn-on voltage V on increased from 0.52 V to 0.80 V after radiation of 3 × 109 cm−2. The reverse breakdown voltage was decreased from −405 V to −375 V, −350 V and −255 V after radiation of 1 × 108, 3 × 108 and 3 × 109 cm−2, respectively. In addition, the carrier concentration calculated from the capacitance–voltage curves was decreased significantly. Based on the G/ω–ω measurement results, the trap density at the Ni/β-Ga2O3 interface was extracted to be 2.89 × 1015–2.49 × 1016 cm−2 eV−1 and 2.18 × 1015–4.98 × 1016 cm−2 eV−1 with the energy level of 0.85–0.87 eV below the conduction band edge.