Current Spreading Research Articles

Structure Design of UVA VCSEL for High Wall Plug Efficiency and Low Threshold Current

Vertical-cavity surface emitting lasers in UVA band (UVA VCSELs) operating at a central wavelength of 395 nm are designed by employing PICS3D(2021) software. The simulation results indicate that the thickness of the InGaN quantum well and GaN barrier layers affect the emission efficiency of UVA VCSELs greatly, suggesting an optimal thicknesses of 2.2 nm for the well layer and 2.7 nm for the barrier layer. Additionally, an overall consideration of threshold current, series resistance, photoelectric conversion efficiency, and optical output power results in the optimized thickness of the ITO current spreading layer, ~20 nm. Furthermore, by employing a five-pair Al0.15Ga0.85N/GaN multi-quantum barrier electron blocking layer (EBL) instead of a single Al0.2Ga0.8N EBL, the device shows a ~51% enhancement in the optical output power and a ~48% reduction in the threshold current. The number of distributed Bragg reflector (DBR) pairs also plays crucial roles in the device’s photoelectric performance. The device designed in this study demonstrates a minimum lasing threshold of 1.16 mA and achieves a maximum wall plug efficiency of approximately 5%, outperforming other similar studies.

An improved 4H-SiC trench MOS barrier Schottky diode with current spreading layer and low resistance layer

This paper investigates an improved trench MOS barrier Schottky (TMBS) structure with extra double epitaxial layers (DE-TMBS) based on the conventional TMBS (C-TMBS) featuring a high-doped N-type current spreading layer (CSL) and a high-doped low resistance layer (LRL). Compared to the C-TMBS, the CSL in the improved structure is grown on the N-type drift region, and the LRL is extended on the CSL. According to the numerical simulations and analytical models, the specific on-resistance (Ron,sp) of DE-TMBS can be significantly reduced compared to the conventional one. This is primarily due to the high doping concentration in CSL, which effectively lowers both the JFET resistance and spreading resistance. Additionally, the increased doping concentration in LRL reduces the channel resistance and JFET resistance. Moreover, the doping concentration and thickness of CSL and LRL are optimized to maximize the figure of merit (FOM), that Ron,sp is reduced by 40.9 % and the FOM (BV2/Ron,sp) is improved by 67.9 % compared to the C-TMBS structure.

Dimensional downscaling and quantum engineering: A path to high-performance micro-LEDs

The investigation of the optoelectronic performance of AlGaN-based ultraviolet-C (UV-C) micro light-emitting diodes (μLEDs) emitting at 273 nm is carried out numerically by reducing the chip area from large LED (300 × 300 μm2) to μLED (25 × 25 μm2). However, due to the high surface to volume ratio of μLED, surface recombination becomes dominant that is generated due to robust sidewall defects. The enhanced current spreading in μLED further affects the carrier injection in the active region as the electrons and holes are captured by sidewall defects. These effects are more dominant at low current density in μLED while at high current density, the sidewall defects get saturated, and the surface recombination weakens. Various optimization strategies, such as quantum wells (QWs) width, quantum barriers (QBs) width, and QW number are carried out to study the effect on the performance of 25 × 25 μm2 UV-C μLED. These optimization strategies at low current density (0.1 A/cm2) further improved the electrical/optical properties of AlGaN-based UV-C μLEDs.

Advancing high-volume GaN manufacturing: precision simulation of electrical and geometrical deviations through current spreading

Abstract Manufacturing process deviations pose significant challenges in GaN manufacturing especially when modern technologies demand extreme chip densities. More than a thousand of each of three distinct GaN-based flip-chips were manufactured where the standard deviations of the measured voltages ranged from 13 to 23 mV. By integrating Monte Carlo and finite element methods in the simulations which relies on the theoretical models, the results were validated by comparing the voltage measurements of the three thousand manufactured chips. Validation was even successful considering the voltage deviations of the three distinct designs equivalently, i.e., affected the geometrical and electrical properties in each wafer. In addition, comparing the three designs, Chip A emerged as the optimal choice for low current resistivity. Looking ahead, our theoretical modeling and simulation hold promise for high-accuracy predictions in high-volume GaN-based chip manufacturing, enhancing reliability and performance.

Asymmetric trench SiC MOSFET with integrated channel accumulation diode for enhanced reverse conduction and switching characteristics

A novel asymmetric trench Silicon Carbide Metal Oxide Semiconductor Field Effect Transistor (SiC MOSFET), featuring an integrated channel accumulation diode (CAD-MOS), has been proposed and investigated through numerical simulation. This innovative design aims to mitigate switching losses and eliminate the bipolar degradation of the body diode. The current spreading layer (CSL) channel, strategically positioned in the centre of the dummy gate, offers a low-barrier reverse conduction path. This represents a substantial advancement over the traditional PN body diode, significantly reducing the reverse conduction voltage drop from 2.84 V in the PN body diode to a mere 1.39 V in the CAD-MOS. Meanwhile, the reverse recovery charge of the CAD-MOS is reduced to 0.95 μC/cm2, and the peak reverse recovery current stands at 45 A/cm2. Compared to conventional asymmetrical trench SiC MOSFET (CON-MOS), the CAD-MOS exhibits a 68.1 % reduction in reverse recovery charge and a 63.4 % decrease in peak reverse recovery current. The split-gate design also reduces the device gate to source capacitance (CGS), resulting in a 17.4 % reduction in total switching losses to 3.64 mJ/cm2. CAD-MOS also exhibits a reduced gate turn-on charge and demonstrates an enhancement in high-frequency figure of merit (HF-FOM) by 8.1 %.

Effect of current spreading layer on Internal Quantum efficiency and optical power of flip chip gallium nitride LEDs with circular contacts

The unique properties of the Indium Tin Oxide (ITO) make it an excellent choice as a current spreading layer in Flip Chip Light Emitting Diodes (FCLEDs) and other optoelectronic devices. Herein, the performance of FCLEDs is analyzed by a precise mathematical model for the current spreading length (Ls) produced by the ITO layer under circular-shaped contacts. The expressions are formulated without approximations using ABC-model for extracting the Internal Quantum Efficiency (IQE, ηint), optical power (Pint) and Emission Intensity (EI). The thickness (tITO) and resistivity (ρITO) of the ITO layer are varied for different current densities, and their adverse effects on IQE are determined. At lower current densities, IQE increases with thickness and decreases for high resistivity of the ITO layer. At higher current densities, there is a gradual decrease in IQE irrespective of the ITO layer presence due to “Efficiency Droop”. The IQE in the proposed work is 82 % at a thickness of 50–200 nm and current density of 8 A/cm2, and the optical power is around 40 mW, showing good agreement with the experimental data, making it feasible for future high-performance FCLEDs.

Design strategies and systematic analysis of GaN vertical MPS diodes with T-shaped shielding rings

Abstract We report gallium nitride (GaN) vertical merged pn-Schottky (MPS) diodes with T-shaped p-GaN shielding rings (SRs). The embedded T-shaped SR features an overlapped depletion region by the lateral and vertical p n junctions under reverse bias condition, which can effectively enhance the charge coupling effect and homogenize the 2-D electric field distribution. In the meantime, the incorporation of the T-shaped SRs can minimize unnecessary depletion of the vertical conduction channel and guarantee a decent current spreading through the drift region. The impact of the key design parameters on the reverse breakdown and forward conduction performance are investigated systematically. We found that the doping concentration and the geomatical shape of the T-shaped p-GaN SRs are closed related to the electric field distribution and forward current conduction behavior, which determines the reverse breakdown and forward conduction performance. With optimum design parameters of the T-shaped SRs, the breakdown voltage of the MPS diodes features a dramatic improvement to 2749 V from 1123 V in conventional MPS diodes. The results can provide a design strategy for high performance vertical GaN power diodes towards high-power and high-frequency applications.

Modelling and Analysis of Inhomogeneous Back-to-Back Schottky Junction Diode

Back-to-back Schottky diode (BBSD) is a simple device structure made from Schottky junctions that can be applied in various applications. In modelling and simulating the BBSD, the inhomogeneity of the barrier height at the Schottky interface should be considered. This paper presents device models and simulation result of an inhomogeneous BBSD. Two device models with different current spreading condition are considered. The distribution of the inhomogeneous barrier height is represented by Gaussian distribution. All the computation was done using Python programming software. When the standard deviation of the barrier height increases, the apparent barrier height that influence the electrical characteristic decreased. This is consistent with the reported experimental data. Device model that considers full current spreading showed 37.8% reduction of apparent barrier height at standard deviation of 100 meV. This is greater that the model with no current spreading. The ideality factor showed correlation with the standard deviation of barrier height, particularly when no current spreading is considered. At standard deviation of 100 meV, the ideality factor increased 2.6 %. The obtained non-unity ideality factor is associated to the significant fluctuation in the barrier height. The computed temperature-dependent current curves also showed good agreement with the reported experimental results. The device model can be a convenient tool to investigate the device operation and to validate the method used for extraction of Schottky barrier parameters.

Design of 1.2 kV SiC Double Trench MOSFETs for a Low On-Resistance Through Optimized Current Spreading Layer Concentration

Design of 1.2 kV SiC Double Trench MOSFETs for a Low On-Resistance Through Optimized Current Spreading Layer Concentration

Estimation of Electron Drift Mobility along the <i>c</i>-Axis in 4H-SiC by Using Vertical Schottky Barrier Diodes

Electron mobility along the c-axis is the most important in SiC because the current flows along this direction in vertical SiC devices. However, previous reports on the drift mobility along the c-axis are still limited because of the difficulty of sample preparation or analysis. In this study, the authors presented the method to estimate the electron drift mobility of a lightly-doped epitaxial layer by using SiC(0001) vertical Schottky barrier diodes (SBDs). For the analyses, the effects of current spreading and series resistance were carefully considered based on experimental results obtained from SBDs with various device parameters, leading to a more accurate estimation. The mobility along the c-axis was obtained as 1070 ± 290 cm2/Vs for a donor density of 1 × 1015 cm-3, and it was compared with the results by Hall effect measurement.

Conductivity-Modulated GaN-Based Ultraviolet Light-Emitting Diodes With Broadening Current Spreading Capability

Conductivity-Modulated GaN-Based Ultraviolet Light-Emitting Diodes With Broadening Current Spreading Capability

Ultraviolet-B Resonant-Cavity Light-Emitting Diodes with Tunnel Junctions and Dielectric Mirrors.

We demonstrate the first electrically injected AlGaN-based ultraviolet-B resonant-cavity light-emitting diode (RCLED). The devices feature dielectric SiO2/HfO2 distributed Bragg reflectors enabled by tunnel junctions (TJs) for lateral current spreading. A highly doped n++-AlGaN/n++-GaN/p++-AlGaN TJ and a top n-AlGaN current spreading layer are used as transparent contacts, resulting in a good current spreading up to an active region mesa diameter of 120 μm. To access the N-face side of the device, the substrate is removed by electrochemically etching a sacrificial n-AlGaN layer, leading to a smooth underetched surface without evident parasitic etching in the n- and n++-doped layers of the device. The RCLEDs show a narrow emission spectrum with a full width at half-maximum (FWHM) of 4.3 nm compared to 9.4 nm for an ordinary LED and a more directional emission pattern with an angular FWHM of 52° for the resonance at 310 nm in comparison to ∼126° for an LED. Additionally, the RCLEDs show a much more stable emission spectrum with temperature with a red-shift of the electroluminescence peak of about ∼18 pm/K and a negligible change of the FWHM compared to LEDs, which shift ∼30 pm/K and show spectrum broadening with temperature. The demonstration of those devices, where a highly reflective mirror is spatially separated from an ohmic metal contact, opens up a new design space to potentially increase the poor light extraction efficiency in UV LEDs and is an important step toward electrically injected UV vertical-cavity surface-emitting lasers.

Schottky-contact intrinsic current blocking layer for high efficiency AlGaInP-based red mini-LEDs.

AlGaInP-based red light emitting diodes (LEDs) are considered as promising light sources in future full-color displays. At present, vertical chip configuration is still the mainstream device structure of AlGaInP-based red LEDs. However, current crowding around p-electrode severely hinders an efficient improvement. Here, we propose a Schottky-contact current blocking layer (SCBL) to enhance current spreading and to improve light extraction efficiency of AlGaInP-based red vertical miniaturized LEDs (mini-LEDs). By utilizing the Schottky contact between ITO and p-GaP, the SCBL can hinder current crowding around the p-electrode. The current is forced to inject into an active region through a p-GaP+ ohmic contact layer, avoiding light absorption by p-electrode. Through the transfer length method, the Schottky contact characteristics between the ITO and p-GaP as well as the ohmic contact characteristics between ITO and p-GaP+ are demonstrated. Benefiting from superior current spreading and improved light extraction, a mini-LED with SCBL realizes an enhancement of 31.8% in external quantum efficiency (EQE) at 20 mA in comparison with a mini-LED without SCBL.

Graphene-Enhanced UV-C LEDs.

Light-emitting diodes in the UV-C spectral range (UV-C LEDs) can potentially replace bulky and toxic mercury lamps in a wide range of applications including sterilization and water purification. Several obstacles still limit the efficiencies of UV-C LEDs. Devices in flip-chip geometry suffer from a huge difference in the work functions between the p-AlGaN and high-reflective Al mirrors, whereas the absence of UV-C transparent current spreading layers limits the development of UV-C LEDs in standard geometry. Here it is demonstrated that transfer-free graphene implemented directly onto the p-AlGaN top layer by a plasma enhanced chemical vapor deposition approach enables highly efficient 275nm UV-C LEDs in both, flip-chip and standard geometry. In flip-chip geometry, the graphene acts as a contact interlayer between the Al-mirror and the p-AlGaN enabling an external quantum efficiency (EQE) of 9.5% and a wall-plug efficiency (WPE) of 5.5% at 8V. Graphene combined with a ≈1nm NiOx support layer allows a turn-on voltage <5V. In standard geometry graphene acts as a current spreading layer on a length scale up to 1mm. These top-emitting devices exhibit a EQE of 2.1% at 8.7V and a WPE of 1.1%.

High Color Conversion Efficiency Realized in Graphene‐Connected Nanorod Micro‐LEDs Using Hybrid Ag Nanoparticles and Quantum Dots

AbstractIn this paper, a uniform nanorod (NR) array is etched onto the surface of Micro‐Light‐Emitting‐Diodes (µLEDs) and mix Ag nanoparticles (NPs) with QDs to fill the gaps between the nanorods. Simultaneously, the study utilizes graphene to connect individual nanorods and enhance current spreading. The nanorod array's structure significantly reduces the distance between the QDs and the quantum well (QW), reducing energy loss from the excitation light source through a non‐radiative energy transfer (NRET) mechanism. Additionally, the Ag NPs function as localized surface plasmons (LSPs), further enhancing the CCE of QDs via the absorption resonance. In this study, the effects of two types of Ag NPs are compared with different absorption resonance peaks on device performance. The results demonstrate that Ag NPs with absorption resonance peaks matching the emission wavelength of QDs play a more crucial role in the system. This configuration achieves a CCE of 77.78% for µLEDs with nanorod arrays, operating at a current of 10 mA. Compared to the conventional µLED structure with QDs only on the surface, the proposed method improves the CCE of µLEDs by an impressive 86.5%. This outcome underscores the significant contribution of the NR structure and LSPs in enhancing the CCE of QD‐µLEDs.

Solar-blind ultraviolet emission-detection monolithic integration of AlGaN multiple-quantum-well diodes via concentric ring-circle configuration

AlGaN multiple-quantum-well diode-based solar-blind ultraviolet emission-detection monolithic integration system shows great application value due to its advantages of multifunctionality, secure communication, and anti-interference ability. To reduce the lateral optical propagation loss and improve the emitting light detection efficiency, we have proposed a concentric ring-circle configuration for the system, where the out-ring light-emitting diode is the emitter at 253 and 267 nm, and the inner-circle detector is the receiver. The out-ring light-emitting diode exhibits about twice the injection current at the same bias and slightly higher light output power at the same current due to better current spreading and sidewall light extraction compared to the conventional square–square configuration. Simultaneously, the concentric inner-circle detector maximizes the collection of the emitted light flux. Under the emission-detection mode for the monolithic integration system, compared to the conventional square–square configuration, the concentric ring-circle design presents an improvement in the ratio of emitter injection current to detector output photocurrent and higher output signal amplitude under the same transmission work mode, demonstrating the improved system energy and coupling efficiency. This design provides a potential approach to achieve low power consumption and high bandwidth in the monolithic integrated optoelectronic chips.

High-efficiency and high-brightness broad area laser diodes with buried implantation current blocking

Buried-regrown-implant-structure (BRIS) technology combines two-step epitaxial regrowth with an intermediate ion implantation step in order to realise a buried current aperture close to the active region of a laser diode. In this paper we carry out a systematic performance comparison demonstrating the benefit of BRIS technology in single emitter broad-area lasers (BALs). We investigate stripe width W = 100 μ m and resonator length L = 4 mm single emitter lasers emitting at wavelength λ = 915 nm, comparing the performance of BRIS devices with different implantation depths with reference devices with only contact layer implantation. We show that using BRIS technology we achieve a continuous wave output power of 20 W at 57% efficiency, with a peak efficiency of 68%, and maintain a lateral brightness of 3.4 mm · mrad up to 19 W, improved over the reference devices due to reduced lateral current spreading in the BRIS devices. Further, we show results of ongoing aging experiments, which has shown no device degradation up to 5000 hours from BRIS devices.

High-temperature performance of InGaN-based amber micro-light-emitting diodes using an epitaxial tunnel junction contact

This study investigated the temperature-dependent electroluminescent (EL) performance of InGaN-based amber micro-light-emitting diodes (μLEDs) with a diameter of 40 μm using an epitaxial tunnel junction (TJ) contact for current spreading. The TJ-μLEDs could achieve a high electrical efficiency of 0.935 and a remarkable wall-plug efficiency of 4.3% at 1 A/cm2 at room temperature, indicating an excellent current injection efficiency of the TJ layers regrown by molecular beam epitaxy. Moreover, the current injection of the amber TJ-μLEDs at the forward bias could be further improved at elevated temperatures. The improvement can be explained by the enhanced tunneling probability and acceptor ionization in p-GaN based on the theoretical simulation. The redshift coefficient, which describes the temperature-dependent peak wavelength shift, is obtained as small as 0.05 nm/K, and the high-temperature-to-room-temperature EL intensity ratio is calculated as &amp;gt;0.56 even at a low current density of 0.5 A/cm2 at the temperatures up to 80 °C. This thermal droop behavior was attributed to the enhanced non-radiative recombination, which was confirmed by the shorter carrier lifetime measured at high temperatures.

A 4H-SiC p-channel insulated gate bipolar transistor with higher breakdown voltage and superior V F·C res figure of merit

A silicon carbide p-channel insulated gate bipolar transistor (IGBT) with higher breakdown voltage (BV) and low V F·C res figure of merit (FOM) has been simulated, fabricated, and characterized successfully. The proposed IGBT adds two n-type implant regions in the junction FET (JFET) area and increases the gate oxide thickness above the JFET area to reduce the reverse transfer capacitance (C res) and gate oxide electric field (E ox). The proposed structure notably lowers E ox below 3 MV cm−1 while elevating the BV to 16.6 kV. A new FOM of V F·C res is defined to evaluate the trade-off between the on-state and the C res characteristics. The experimental results demonstrate that a lower V F·C res FOM of 0.369 V·pF is achieved from the proposed IGBT with a reduction of 66.4%, compared to the conventional current spreading layer IGBT. Meanwhile, the simulated turn-on and turn-off times of the proposed IGBT are reduced by 29.4% and 20%, respectively.

240 nm AlGaN-based deep ultraviolet micro-LEDs: size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated. Then, the external quantum efficiency (EQE) and light extraction efficiency (LEE) are systematically investigated by comparing size and edge effects. Here, it is revealed that the peak optical output power increases by 81.83% with the size shrinking from 50.0 to 25.0 μm. Thereinto, the LEE increases by 26.21% and the LEE enhancement mainly comes from the sidewall light extraction. Most notably, transverse-magnetic (TM) mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design. However, when it turns to 12.5 μm sized micro-LEDs, the output power is lower than 25.0 μm sized ones. The underlying mechanism is that even though protected by SiO2 passivation, the edge effect which leads to current leakage and Shockley-Read-Hall (SRH) recombination deteriorates rapidly with the size further shrinking. Moreover, the ratio of the p-contact area to mesa area is much lower, which deteriorates the p-type current spreading at the mesa edge. These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm, which will pave the way for wide applications of deep ultraviolet (DUV) micro-LEDs.