Reverse Bias Stress Test Research Articles

On the insignificance of dislocations in reverse bias degradation of lateral GaN-on-Si devices

The role of threading dislocations in the intrinsic degradation of lateral GaN devices during high reverse bias stress tests (RBSTs) is largely unknown. We now present the results on lateral p-GaN/AlGaN/2DEG heterojunctions with a width of 200 μm in GaN-on-Si. A time-dependent permanent degradation of the heterojunction under high reverse bias and elevated temperatures can be observed, ultimately leading to a hard breakdown and device destruction. By using an integrated series p-GaN resistor, the device is protected from destruction and, consequently, the influence of dislocations on the degradation mechanism could be studied. Localization by emission microscopy could show that the transient current increase during a RBST is the result of the creation of a limited amount of highly localized leakage paths along the whole device width. We could establish a 1:1 correlation of leakage sites with a structural material degradation within the AlGaN barrier for nine individual positions on two different devices by planar transmission electron microscopy analysis. To unambiguously show whether dislocations in GaN-on-Si even should be considered a potential trigger for the RBST degradation in lateral heterojunctions, a combined planar and cross-sectional lamella approach was used for the first time for larger devices. This enabled the visualization of the three-dimensional propagation path of the dislocations close to the degradation sites. It was found that there is no statistically significant link between the material degradation and pre-existing dislocations. Our findings offer new insights into the GaN-on-Si material system, upon which upcoming power technologies are built upon.

Test concept for a direct correlation between dislocations and the intrinsic degradation of lateral PIN diodes in GaN-on-Si under reverse bias

Although often discussed, the role of threading dislocations (TDs) in the degradation of lateral GaN-on-Si p-GaN HEMTs under reverse bias stress test (RBST) has never been clearly shown in experiment until today. Here we present a novel test concept to establish a direct correlation between dislocations and degradation sites in RBST and demonstrate it on lateral p-GaN/AlGaN/GaN PIN diodes. By scaling down structures down to ~500 nm width, the point of degradation is always forced to be located at a defined position, independent of the local position of TDs. Furthermore, an integrated serial p-GaN resistor has been implemented with the PIN diode, which effectively limits the current and protects the device from a large meltdown due to parasitic capacitances and enables TEM analysis of structural degradation. A large quantity of lateral PIN devices exhibit a similar time-to-breakdown under RBST, indicating the intrinsic nature of this failure mode. Planar TEM analysis of a degraded and failed device clearly reveals structural damages and a breakdown of the AlGaN barrier as the failure signature. A combined planar and cross-sectional TEM lamella analysis approach of the expected failure site was introduced and applied. The results unambiguously show, that the observed degradation in the AlGaN barrier, which is attributed to the intrinsic failure mode, is not directly linked to the position of a dislocation.

Study of the reverse I–V in component subcells of III–V multijunction space solar cells

AbstractThe reverse bias operation of triple‐junction solar cells (GaInP/Ga(In)As/Ge), typically used for space photovoltaics, is poorly understood. In this work, we conduct reverse bias stress tests on both isotype subcells (GaInP, Ga(In)As and Ge) as well as in the complete triple‐junction solar cell. After each reverse bias step, forward dark and lighted I–Vs are measured and modelled using Shockley and Spirito and Albergamo models to fit the characteristic parameters of each individual subcell and quantify the variations evolution caused by the stress. The changes in these parameters are thoroughly analysed in order to comprehend the basic physical processes behind the degradation observed after the reverse bias is applied. Finally, we demonstrate that the individual parameters obtained from each subcell can be combined to simulate the final I–V curve of the complete triple junction and understand how it is affected when reverse bias is applied, linking the changes observed to a sudden degradation of the GaInP top subcell at low reverse voltages.

A robust and area-efficient guard ring edge termination technique for 4H-SiC power MOSFETs

A robust and area efficient adjusted multi-section guard rings (AMS) edge termination structure is employed to a 4H-SiC power MOSFET device rated at 1.7kV and fabricated without extra process steps or masks in this paper. The lightly doped p-type guard ring with adjusted multi-section spacing, which is similar with varied lateral doping technology widely used in silicon edge termination, creates a smoother and more uniform surface electric field distribution across all rings, resulting in a higher reverse blocking voltage with minimal edge termination area. The fabricated AMS-MOSFET devices with an active area of 2.5mm2 have reverse blocking voltage up to 2.5kV at room temperature. Compared to the conventional SiC MOSFETs with the uniform ring spacing, the AMS-MOSFET with the same edge termination area shows more than a 25% increase in the reverse blocking voltage. Furthermore, high-temperature reverse bias stress test is performed to verify junction and termination robustness of the AMS-MOSFET device. A small variation in the reverse blocking voltage and drain-source leakage current are achieved under 168h high-temperature reverse bias stress with the drain bias of 1360V at ambient temperature of 175°C, which suggests an expected robustness in high power application for the fabricated SiC MOSFET with the AMS structure.

Degradation of AlGaN/GaN Light Emitting Diodes caused by Carbon Contamination with Reverse-bias Stress Test in Water Vapor

Resolving failure origins of AlGaN/GaN light emitting diodes (LED) has received intensive study recently. In this study, formation of GaCO3 caused by carbon contamination may result in deformation of the electrode near the surface and degrade the device. The electrochemical reactions may cause device damages. Degradation in electrical properties is observed in I-V characteristics. Forward-bias and reverse-bias EL images are used to trace the damaged areas. Furthermore, focus ion beam (FIB), scanning electron microscope (SEM), energy dispersive X-ray diffraction (EDX) are applied to examine the damaged areas. Results indicate that formation of GaCO3 may deform the electrode, generate the reverse-bias EL and cause the degradation.

Degradation of AlGaInP light emitting diodes under reverse-bias operations in salty water vapor

GaP-based light emitting diodes (LEDs) reliability tests are crucial to further development for high-performance LED lighting technology. In this study, we perform reverse-bias stress tests for LEDs in water vapor and salty water vapor ambient, respectively. The results indicate that salty water vapor can quickly degrade LEDs. To investigate the failure mechanisms, electrical measurements, forward-bias and reverse-bias electroluminescence images, and multiple material analyses have been taken to study the failure mechanism. Results indicate that NaCl clustering solids and atom diffusion may cause deformation of the electrode and enhance the degradation of the LEDs.

Trapping and Reliability Assessment in D-Mode GaN-Based MIS-HEMTs for Power Applications

This paper reports on an extensive analysis of the trapping processes and of the reliability of experimental AlGaN/GaN MIS-HEMTs, grown on silicon substrate. The study is based on combined pulsed characterization, transient investigation, breakdown, and reverse-bias stress tests, and provides the following, relevant, information: 1) the exposure to high gate-drain reverse-bias may result in a recoverable increase in the on-resistance (RON), and in a slight shift in threshold voltage; 2) devices with a longer gate-drain distance show a stronger increase in RON, compared to smaller devices; 3)current transient measurements indicate the existence of one trap level, with activation energy of 1.03 ± 0.09 eV; and 4) we demonstrate that through the improvement of the fabrication process, it is possible to design devices with negligible trapping. Furthermore, the degradation of the samples was studied by means of step-stress experiments in off-state. Results indicate that exposure to moderate-high reverse bias (<; 250 V for LGD = 2 μm) does not induce any measurable degradation, thus confirming the high reliability of the analyzed samples. A permanent degradation is detected only for very high reverse voltages (typically, VDS = 260-265 V, on a device with LGD = 2 μm stressed with VGS = - 8 V) and consists of a rapid increase in gate leakage current, followed by a catastrophic failure. EL measurements and microscopy investigation revealed that degradation occurs close to the gate, in proximity of the sharp edges of the drain contacts, i.e., in a region where the electric field is maximum.

Fabrication and reliability of dye solar cells: A resonance Raman scattering study

In this work, the effect of reverse bias stress tests on Dye Solar Cells (DSCs) based on N719 dye was investigated in detail using resonant micro-Raman spectroscopy. First the Raman lines were assigned to vibrations from the different constituents in a fresh solar cell. Then the mechanism of thiocyanato (SCN−) loss under stress conditions was reported.

A Review on the Physical Mechanisms That Limit the Reliability of GaN-Based LEDs

We review the failure modes and mechanisms of gallium nitride (GaN)-based light-emitting diodes (LEDs). A number of reliability tests are presented, and specific degradation mechanisms of state-of-the-art LED structures are analyzed. In particular, we report recent results concerning the following issues: 1) the degradation of the active layer induced by direct current stress due to the increase in nonradiative recombination; 2) the degradation of LEDs submitted to reverse-bias stress tests; 3) the catastrophic failure of advanced LED structures related to electrostatic discharge events; 4) the degradation of the ohmic contacts of GaN-based LEDs; and 5) the degradation of the optical properties of the package/phosphors system of white LEDs. The presented results provide important information on the weaknesses of LED technology and on the design of procedures for reliability evaluation. Results are compared with literature data throughout the text.

Reliability of High Voltage 4H-SiC MOSFET Devices

AbstractThe commercialization of 4H-SiC MOSFETs will greatly depend on the reliability of gate oxide. Long-term gate oxide reliability and device stability of 1200 V 4H-SiC MOSFETs are being studied, both under the on- and off-states. Device reliability is studied by stressing the device under three conditions: (a) Gate stress - a constant gate voltage of +15 V is applied to the gate at a temperature of 175°C. The forward I-V characteristics and threshold voltage are monitored for device stability, (b) Forward current stress – devices are stressed under a constant drain current of Id = 4 A and Vg = 20 V. The devices were allowed to self-heat to a temperature of Tsink = 125°C and the I-V curves are monitored with time, and (c) High temperature reverse bias testing at 1200 V and 175°C to study the reliability of the devices in the off-state. Our very first measurements on (a) and (b) show very little variation between the pre-stress and post-stress I-V characteristics and threshold voltage up to 1000 hrs of operation at 175°C indicating excellent stability of the MOSFETs in the on-state. In addition, high temperature reverse bias stress test looks very promising with the devices showing very little variation in the reverse leakage current with time.