Forward Turn-on Voltage Research Articles

High dose Co-60 gamma irradiation of InGaN quantum well light-emitting diodes

InGaN multiquantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 410to510nm were irradiated with Co60 γ-rays with doses in the range 150–2000Mrad (Si). The forward turn-on voltage for all the irradiated LEDs was increased slightly (e.g., by only ∼0.1–0.15V for 500MRad dose irradiation) while the reverse breakdown voltage was unchanged within experimental error. The light output intensity for the 410nm diodes was decreased by 20% after a dose of 150MRad and 75% after ∼2GRad. The current transport in the LEDs was dominated by generation-recombination (ideality factor ∼2) both before and after irradiation. The morphology and appearance of the p and n-Ohmic metallization did not show any detectable change as a result of even the highest γ-ray dose.

Characterization of bulk GaN rectifiers for hydrogen gas sensing

Pd and Pt Schottky diodes were fabricated on free-standing 2-in.-diameter GaN substrates prepared by a combination of hydride vapor phase epitaxy of ∼350μm onto sapphire, substrate removal and subsequent growth of 3μm of epi GaN by metalorganic chemical vapor deposition. Vertical diodes with Ti∕Al∕Pt∕Au back contacts annealed at 850°C for 30s showed excellent rectification with an on/off ratio of ∼100 at 1.5V∕−10V. Both forward turn-on and reverse breakdown voltages showed negative temperature coefficients. Pd and Pt diodes showed detection of 10ppm H2 in N2 at 25°C, with fast (<10s) recovery times upon removal of hydrogen from the measurement ambient. The Pt showed higher detection sensitivity than Pd. Detection of C2H4 and C2H6 required much higher temperatures (∼450°C) and concentrations (10%) of the gases in N2 than hydrogen detection.

Role of Ion Energy and Flux on Inductively Coupled Plasma Etch Damage in InGaN/GaN Multi Quantum Well Light Emitting Diodes

The effect of inductive coupled plasma (ICP) etching of GaN light-emitting diodes (LEDs) on the device performance was investigated. InGaN/GaN multi-quantum well LEDs with emission wavelengths 420 nm were fabricated using Cl2/Ar ICP etching to form the mesa for the n-contact. Different rf chuck powers (controlling incident ion energy) and source powers (controlling ion flux) were used to examine their effect on junction leakage current. At high ion fluxes (source powers > 500 W), the junctions were very heavily damaged. The forward turn-on voltage was increased by using higher ion energies from 4.55 V for 120 eV ion energy to 5.1 V for 300 eV. The reverse bias current was much less sensitive to the ion energy during plasma etching and increased only above an average ion energy of ∼300 eV. The current transport in the LEDs is dominated by generation-recombination and surface leakage components (ideality factor ≥2) in all cases.

High efficiency n-ZnO/p-SiC heterostructure photodiodes grown by plasma-assisted molecular-beam epitaxy

Heteroepitaxial n-ZnO films have been grown on commercial p-type 6H-SiC substrates by plasma-assisted molecular-beam epitaxy, and n-ZnO/p-SiC heterojunction mesa structures have been fabricated and their photoresponse properties have been studied. Current–voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2×10 −4 A/cm 2 at −10 V, a breakdown voltage greater than 20 V, a forward turn-on voltage of ∼5 V, and a forward current of ∼2 A/cm 2 at 8 V. Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as 0.045 A/W at −7.5 V reverse bias was observed for photon energies higher than 3.0 eV.

Characteristics of unannealed ZnMgO∕ZnO p-n junctions on bulk (100) ZnO substrates

Zn 0.9 Mg 0.1 O ∕ Zn O p-n junctions were grown by pulsed laser deposition at ⩽500°C on bulk n-type, (100), nonpolar, a-plane ZnO substrates. No postgrowth annealing was performed, with the P-doped ZnMgO showing p-type conductivity (hole density ∼1016cm−3, mobility ∼6cm2V−1s−1) in the as-grown state. Front-to-back p-n junctions were fabricated with Ni∕Au used as the p-Ohmic contact and Ti∕Au as the backside n-Ohmic contact. The p contacts showed improved characteristics after annealing up to 400°C, but the n contacts were Ohmic as deposited. The junctions showed rectifying behavior up to 200°C. The forward turn-on voltage was ∼6.5V at 25°C. The simple, low-temperature growth and processing sequence show the promise of ZnO for applications in transparent electronics and UV light emitters.

Transconductance Linearity Improvement of Enhancement-Mode Pseudomorphic HEMT with High Gate Forward Turn-On Voltage

In this paper we discuss the sources of the transconductance reduction of enhancement-mode pseudomorphic high electron mobility transistor (E-pHEMT) at a high gate bias and methods of improving the transconductance at a high gate bias. E-pHEMT usually suffers from severe transconductance reduction at a high gate bias. Our simulation showed that this reduction is mainly due to the low channel carrier density and high access resistance of the ungated region rather than the parasitic MESFET phenomenon. An epi structure that facilitates electron transfer through the barrier layer of the ungated region and self align gate (SAG) process lead to an improvement in transconductance linearity.

Effect of inductively coupled plasma damage on performance of GaN–InGaN multiquantum-well light-emitting diodes

InGaN multiquantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 420to505nm were exposed to either Ar or H2 inductively coupled plasmas as a function of both rf chuck power (controlling incident ion energy) and source power (controlling ion flux). The forward turn-on voltage is increased by both types of plasma exposure and is a function of both the incident ion energy and flux. The reverse bias current in the LEDs is much larger in the case of H2 plasma exposure, indicating that preferential loss of nitrogen leads to increased surface leakage. The current transport in the LEDs is dominated by generation-recombination (ideality factor ∼2) both before and after the plasma exposures.

Fabrication and device characteristics of bulk GaN-based Schottky diodes

AbstractIn this investigation, Schottky diodes with different device sizes (150μm, 420μm and 700μm) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150μm diameter Schottky diodes. The best on-state resistance was 7.56 mΩcm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias.

Zn 0.9 Mg 0.1 O ∕ ZnO p – n junctions grown by pulsed-laser deposition

The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.

Temperature dependent characteristics of bulk GaN Schottky rectifiers on free-standing GaN substrates

The performance of Schottky rectifiers fabricated with dielectric overlap edge termination on epitaxial layers grown on a free-standing GaN template is reported. The power figure-of-merit (VB)2/RON where VB is the reverse breakdown voltage and RON is the on-state resistance was 11.5 MW cm−2. The forward turn-on voltage was ∼3.5 V at 25 °C, with an on-state resistance of ∼5×10−3 Ω cm2. The reverse recovery time was ⩽50 ns in switching from forward bias to reverse bias. The reverse breakdown showed a temperature coefficient of −0.45 V/C.

High electrostatic discharge protection of InGaN/GaN MQW LEDs by using GaN Schottky diodes

GaN Schottky diodes were built internally inside the GaN green LEDs by using etching and re-deposition techniques. By proper selecting the etching areas underneath the bonding pads, one can minimize the optical loss due to the etching process. Although the reverse current, the forward turn-on voltage and device lifetime degradation were all higher for the GaN LED with Schottky diode, it was found that the internal Schottky diode could significantly increase the ESD threshold from 450 V to 1300 V. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

160-A bulk GaN Schottky diode array

Pt Schottky rectifier arrays were fabricated on 200-μm-thick, freestanding GaN layers. Even with the reduced dislocation density in these layers (∼105 cm−2) relative to conventional GaN on sapphire (>108 cm−2), rectifiers fabricated on the freestanding GaN show a strong dependence of reverse breakdown on contact diameter. We show that by interconnecting the output of many (∼130) smaller (500 μm×500 μm) rectifiers, we can achieve high total forward output current (161 A at 7.12 V), low forward turn-on voltage of ∼3 V, and maintain the reverse breakdown voltage. The on/off ratio of the rectifier array was ∼8×107 at 5 V/−100 V.

Current–voltage and reverse recovery characteristics of bulk GaN p-i-n rectifiers

p-i-n rectifiers were fabricated on epitaxial layers grown on free-standing GaN substrates. The forward turn-on voltage, VF was ∼5 V at 300 K and displayed a positive temperature coefficient. The specific on-state resistance (RON) was ∼5 mΩ cm2 at 300 K, with an ideality factor of ∼2 and activation energy for low forward current density of ∼1.6 eV. This is consistent with carrier recombination in the space charge region via a midgap deep level. The figure-of-merit, VB2/RON, where VB is the reverse breakdown voltage, was 0.32 MW cm−2. The reverse recovery time was ⩽600 ns at 300 K. The improved forward characteristics relative to previous heteroepitaxial p-i-n GaN rectifiers show the advantages of employing a GaN substrate to make a true vertical transport geometry device.

2.6 A, 0.69 MW cm −2 single-chip bulk GaN p-i-n rectifier

The performance of a 9 device array of 500 × 500 μm 2 GaN p-i-n rectifiers fabricated on epitaxial layers grown on a free-standing GaN template is reported. The forward turn-on voltage was ∼5.5 V at 25 °C, with an on-state resistance of ∼5 × 10 −3 Ω cm 2. The total forward current was 1 A at ∼8.8 V and 2.6 A at 18 V. The power figure-of-merit for the array, V B 2/ R ON, was 0.69 MW cm −2, with a reverse recovery time of ⩽300 ns. The individual p-i-n rectifiers were interconnected using electroplated Au and clamped in a Cu pressure pack for thermal management.

Stability of SiC Schottky Rectifiers to Rapid Thermal Annealing

4H-SiC Schottky rectifiers with dielectric overlap edge termination were rapid thermal annealed at temperatures from 700 to 1100°C for periods of 60-240 s under ambient. The forward turn-on voltage ideality factor and on-state resistance were all improved by anneals <1000°C. At higher temperatures and longer duration, the Ni Schottky contact showed ohmic behavior and degraded surface morphology. Since the reverse characteristics were less affected by the lower temperature anneals than the forward characteristics, the diodes exhibited an increase in on/off ratio of up to ∼20%. © 2003 The Electrochemical Society. All rights reserved.

Temperature dependence of forward current characteristics of GaN junction and Schottky rectifiers

A comparison was made of the forward current–voltage characteristics of bulk GaN Schottky and p–n junction rectifiers using a quasi-three-dimensional simulator. The model includes incomplete ionization of the deep Mg acceptor (175 meV) in the p +-GaN side of the junction and the temperature dependence of mobility and GaN band gap. The forward turn-on voltages, V F, decrease with increasing temperature for both types of rectifier and are ∼2.5 V at 100 A cm −2 at 573 K for the junction diodes and ⩽1 V under similar conditions for the Schottky diodes. The effect of p-layer thickness and doping in the p–n junction was also investigated.

Improved ESD protection by combining InGaN-GaN MQW LEDs with GaN Schottky diodes

GaN Schottky diodes were built internally inside the GaN green LEDs by using etching and redeposition techniques. By properly selecting the etching areas underneath the bonding pads, one can minimize the optical loss due to the etching process. Although the reverse current and the forward turn-on voltage were both higher for the GaN LED with a Schottky diode, it was found that the internal Schottky diode could significantly increase the electrostatic discharge threshold from 450 to 1300 V.

Thermal simulations of high power, bulk GaN rectifiers

A finite element simulation was used to quantitatively estimate the effectiveness of flip-chip bonding in the temperature rise of bulk GaN Schottky rectifiers under various conditions of current density, duty cycle, forward turn-on voltage and on-state resistance. The temperature difference between flip-chip bonded devices and bottom bonded devices was 20 °C even at modest current densities. The maximum temperature in the bulk cases occurred in the center of the GaN substrate thickness. The transit time of the temperature reaching the steady state for the flip-chip bonding device is in the range of millisecond, which is faster than that of most power switch applications. Flip-chip bonding is suggested to improve the heat dissipation of high power, bulk GaN rectifiers.

Influence of PECVD of SiO[sub 2] Passivation Layers on 4H-SiC Schottky Rectifiers

The effect of the deposition conditions of plasma enhanced chemical vapor deposition (PECVD) SiO 2 layers on the electrical properties of 4H-SiC Schottky rectifiers is reported. In a SiH 4 /N 2 O plasma chemistry, the reverse breakdown voltage, forward turn-on voltage, and on-state resistance of the rectifiers all increase with increasing plasma power and SiH 4 content and decreasing deposition pressure. The maximum changes in all these parameters were ≤20% over a broad range of plasma conditions and show that 4H-SiC is relatively resistant to changes induced by the ion bombardment and high atomic hydrogen flux present during PECVD of dielectrics for surface passivation.

Simulation and Experimental Characteristics of 4H-SiC Schottky Power Rectifiers

The dc performance of dielectric overlap-terminated 4H-SiC Schottky rectifiers was simulated for different drift layer doping levels and dielectric thicknesses. The devices were then fabricated with different Schottky contact diameters and the dc results compared to the simulations. There was a strong dependence of reverse breakdown voltage on contact diameter (ϕ) ranging from for 100 μm ϕ to for 1000 μm. The reverse leakage current scaled with diode diameter, indicating that the surface contributions are dominant. The specific on-state resistance was close to the theoretical minimum, with a forward turn-on voltage of 2.14 V at The rectifiers were tested to maximum forward currents of 2 A. The contact resistivity of back-side Ni contacts annealed at 970°C was The yield of diodes with breakdown was over a quarter of a 2 in. diam wafer. © 2002 The Electrochemical Society. All rights reserved.