Study of the breakdown failure mechanisms for power AlGaN/GaN HEMTs implemented using a RF compatible process

The breakdown failure mechanisms for a family of power AlGaN/GaN HEMTs were studied. These devices were fabricated using a commercially available MMIC/RF technology with a semi-insulating SiC substrate. After a 10 min thermal annealing at 425 K, the transistors were subjected to temperature dependent electrical characteristics measurement. Breakdown degradation with a negative temperature coefficient of ?0.113 V/K for the devices without field plate was found. The breakdown voltage is also found to be a decreasing function of the gate length. Gate current increases simultaneously with the drain current during the drain-voltage stress test. This suggests that the probability of a direct leakage current path from gate to the 2-DEG region. The leakage current is attributed by a combination of native and generated traps/defects dominated gate tunneling, and hot electrons injected from the gate to channel. Devices with field plate show an improvement in breakdown voltage from ～40 V (with no field plate) to 138 V and with lower negative temperature coefficient. A temperature coefficient of ?0.065 V/K was observed for devices with a field plate length of 1.6 μm.     

Polymer composite of poly(vinyl phenol)-reduced graphene oxide reduced by vitamin C in low energy consuming write-once–read-many times memory devices

We report the application of reduced graphene oxide, using vitamin C as reducing agent, to make a composite with poly(vinyl phenol) as the active layer of write-once–read-many times memory devices. These devices present a high ON/OFF current ratio of 10⁵ when read at 1 V, retain the information for a long time maintaining the ON/OFF current ratio constant, and require low energy for performing at 5 V the memory write (less than 10^?8 J cm^?2 device active area) and read operations.     

Solution-processed cathode-interlayer-free deep blue organic light-emitting diodes

Efficient non-doped deep blue organic light-emitting diodes (OLEDs) were fabricated by solution-processing method by using a series of small molecules consisting of various contents of triphenylamine and phosphonate-featured fluorene units as the emitting layer. Without any electron-injection layer, one of the optimal devices with a simple double-layer device configuration exhibits a maximal current efficiency of 2.59 cd A⁻¹ at 6.8 V (1.72 mA cm⁻²) with a CIE coordinates of (0.163, 0.097). These double-layer devices are demonstrated with excellent color-stability under a wide range of operating current density. The current work indicates that electron-rich triphenylamine moiety incorporated with phosphonate-featured fluorene units could be utilized as building blocks to construct a multi-functional platform combining good electron-injection property, carrier-transport property, and efficient electroluminescence. It also provides an approach to achieve a structure-simplified color-stable efficient blue OLED.     

Low and small resistance hole-injection barrier for NPB realized by wide-gap p-type degenerate semiconductor,LaCuOSe:Mg

LaCuOSe:Mg is a wide-gap p-type semiconductor with a high conductivity and a large work function. Potential of LaCuOSe:Mg as a transparent hole-injection electrode of organic light-emitting diodes (OLEDs) was examined by employing N,N′-diphenyl-N,N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB) for a hole transport layer. Photoemission spectroscopy revealed that an oxygen plasma treated surface of LaCuOSe:Mg formed a hole-injection barrier as low as 0.3 eV, which is approximately a half of a conventional ITO/NPB interface. Hole-only devices composed of a LaCuOSe:Mg/NPB/Al structure showed a low threshold voltage ∼0.2 V and high-density current drivability of 250 mA cm⁻² at 2 V, which is larger by two orders of magnitude than that of ITO/NPB/Al devices. These results demonstrate that LaCuOSe:Mg has great potential as an efficient transparent anode for OLEDs and other organic electronic devices.     

Field and hot electron-induced degradation in GaN-based power MIS-HEMTs

We investigate the degradation of AlGaN/GaN MIS-HEMTs submitted to gate step-stress experiments, and demonstrate the existence of field- and hot-electron induced processes. When the devices are submitted to gate-step stress with high V_DS > 50 V, four different regimes are identified: (i) for V_GS < − 10 V, no significant degradation is observed, since the devices are in the off-state; (ii) for − 10 V < V_GS < 0 V, hot electrons flow through the channel, as demonstrated by the (measurable) electroluminescence signal. These hot electrons can be trapped within device structure, inducing an increase in the threshold voltage. (iii) for V_GS > 0 V, the density of hot electrons is significantly reduced, due to the increased interface scattering and device temperature. As a consequence, EL signal drops to zero, and the electrons trapped during phase (ii) are de-trapped back to the channel, where they are attracted by the high 2DEG potential. (iv) Finally, for V_GS > 5 V, a significant increase in threshold voltage is detected. This effect is observed only for high positive voltages, i.e. when a significant leakage current flows through the gate. Such gradual degradation is ascribed to the injection of electrons from the 2DEG to the gate insulator, which is a field-driven effect. These results were obtained by combined electrical and optical characterization carried out at different voltages during the step stress.     

High-performance inverted top-emitting green electrophosphorescent organic light-emitting diodes with a modified top Ag anode

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated. A high current efficacy of 124.7 cd/A is achieved at a luminance of 100 cd/m² when an optical outcoupling layer of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0 V and exhibit low current efficacy roll-off through luminance values up to 10,000 cd/m². The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.     

Synthesis,electrochemical and electrochromic properties of novel 2,4,6-Tri(pyridine-4-yl)pyridilium derivatives

Four star-shaped like fused heterocyclic compounds multi-electrochromic materials 2,4,6-Tri(pyridine-4-yl)pyridilium derivatives (TPPDs) were successfully synthesized and characterized by NMR, Solid IR spectra, APCI-MS, and UV–vis spectroscopy. The electrochemical properties, electrochromic behavior, electro-optical properties, and electrochromic mechanism were investigated by thermogravimetric analysis, cyclic voltammetry and UV–vis absorption spectra. Electrochromic devices based on these compounds were fabricated with an active area of 3 cm × 4 cm and their electrochromic performances were further studied. It was found the ECDs presented a stable as well as multicolor electrochromic change from colorless to blue, then violet-blue and finally black-blue between 0 V and +4.0 V. In addition, the prepared electrochromic materials had high coloration efficiency, low switching time, and nice redox stability. This type of multi-electrochromic materials would thus be promising candidates for applications in electrochromic devices.     

Effects of annealing on the performance of 4H-SiC metal–semiconductor–metal ultraviolet photodetectors

4H-SiC-based metal–semiconductor–metal (MSM) photodetectors (PDs) have been designed, fabricated, and characterized, in which nickel Schottky contacts were needed. Current–voltage and spectral responsivity measurements were carried out at room temperature to character the effect of annealing on the performance of the MSM devices. It was found to improve both Schottky barrier height and ideality factors after annealing at appropriate temperature. The chemical component depth profiles of XPS measurement showed nickel silicides were produced at the interface, leading to improvement of the Ni/4H-SiC contacts and the performance of the PDs. The MSM PDs with RTA had a lower dark current (0.45 pA at 5 V bias voltage), a typical responsivity of 0.094 A/W at 20 V and displayed peak response wavelength at 290 nm.     

Growth enhancement and field emission characteristics of one-dimensional 3,4,9,10-perylenetetracarboxylic dianhydride nanostructures on pillared titanium substrate

We have utilized the π–π interactions between 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecules and temperature-induced morphology changes to synthesize one-dimensional (1D) nanostructures of PTCDA on a heated (ca. 100 °C) titanium substrate through vacuum sublimation. Because of the pillared Ti structures and the presence of reactive Ti–Cl sites, the titanium substrate played a crucial role in assisting the PTCDA molecules to form 1D nanostructures. The average diameter of the nanofibers deposited on the Ti-CVD substrate, a Ti substrate formed by chemical vapor deposition (CVD), at 100 °C was ca. 84 nm, with lengths ranging from 100 nm to 3 μm. When the PTCDA nanofibers were biased under vacuum, the emission current remained stable. The turn-on electric field for producing a current density of 10 μA/cm² was 8 V/μm. The maximum emission current density was 1.3 mA/cm², measured at 1100 V (E = 11 V/μm). From the slope of the straight line obtained after plotting ln(J/E²) versus 1/E, we calculated the field enhancement factor β to be ca. 989. These results demonstrate the PTCDA nanofibers have great potential for applicability in organic electron-emitting devices.     

An inverted,organic WORM device based on PEDOT:PSS with very low turn-on voltage

An organic Write-Once-Read-Many (WORM) device based on poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as the active layer was fabricated with an inverted architecture. Insertion of an ultrathin layer of poly(methylmethacrylate) (PMMA) between the bottom electrode and the PEDOT:PSS resulted in a systematic and substantial decrease in turn-on voltage, from 7.0 V to less than 1.0 V. An optimal thickness of the PMMA layer was found to yield the lowest consistent turn-on voltage of ∼0.8 V, with 0.5 V being the lowest value of all fabricated devices. The switching mechanism was attributed to filamentary doping of the PEDOT:PSS. Insertion of the PMMA acted to protect the underlying ZnO from being etched by the acidic PEDOT:PSS as well as to improve its wetting properties. Devices were demonstrated on both ITO and aluminum bottom electrodes, with aluminum yielding the highest ON/OFF ratios in the study. Owing to their inverted architecture, the devices demonstrated good stability, and the retention time of the ON-state was determined to be greater than twenty months while stored in air for devices with ITO bottom electrodes. In addition to deposition via spin-coating, blade-coating was demonstrated as a viable processing technique for applications requiring rapid or large-area manufacturing.     

Stacked zener trigger SCR for HV IC ESD protection

In HV BCD technology, SCR cannot been directly used as ESD protection owing to its high trigger voltage and low holding voltage. Stacked zener trigger SCRs for HV ESD protection is proposed and Compared to the traditional SCR. The proposed devices are fabricated with 0.35 um BCD process on P-type bulk wafers without buried layer. TLP testing results show that the single zener trigger SCR with 50 um width has a 6.7 V trigger voltage and 4.1 A failure current It2. Stacked 4 zener trigger SCRs is designed for 24 V I/O ESD protection, which has 8 V holding voltage, 27 V trigger voltage and pass7 KV HBM.     

Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE = 2.53 cd/A at 6.2 V, L = 9917 cd/m² at 8.3 V) than devices with PFN-OH (2.00 cd/A at 4.1 V, 3237 cd/m² at 7.2 V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400 mA/cm², indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC₇₀BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.     

Improving the efficiency of light-emitting diode based on a thiophene polymer containing a cyano group

We report on the overall improvement of a single layer organic light-emitting diode device based on poly{[3-hethylthiophene]-co-3-[2-(p-cyano-phenoxy)ethyl]thiophene} or namely PTOPhCN. This polymer was recently developed by adding a cyano group as a side-chain substituent of the thiophenic backbone onto the main polymer chain and showed promising results for light-emitting diode devices. Using an improved device layout, bright red electroluminescence was obtained at 4 V and showed a luminance of about 400 cd/m² at 8 V with current densities in the order of 6000 A/m².     

Multilevel characteristics and operating mechanisms of nonvolatile memory devices based on a floating gate of graphene oxide sheets sandwiched between two polystyrene layers

Nonvolatile organic memory devices were fabricated utilizing a graphene oxide (GO) layer embedded between two polystyrene (PS) layers. Scanning electron microscope images of GO sheets sandwiched between two PS layers showed that the GO sheets were clearly embedded in the PS layers. Capacitance–voltage (C–V) curves of the Al/PS/GO/PS/n-type Si devices clearly showed hysteresis behaviors with multilevel characteristics. The window margin of the nonvolatile memory devices increased from 1 to 7 V with increasing applied sweep voltages from 6 to 32 V. The cycling retention of the ON/OFF switching for the devices was measured by applying voltages between +15 and −15 V. While the capacitance of the memory devices at an ON state have retained as 230 pF up to 10⁴ cycles, that at an OFF state maintained as 16 pF during three times of repeated measurements. The extrapolation of the retention data for the devices maintained up to 10⁶ cycles. The operating mechanisms of the nonvolatile organic memory devices with a floating gate were described by the C–V results and the energy band diagrams.     

Highly efficient blue and all-phosphorescent white polymer light-emitting devices based on polyfluorene host

We report efficient blue electrophosphorescent polymer light emitting devices with polyfluorene (PFO) as the host and iridium bis[2-(4,6-difluorophenyl)-pyridinato-N,C²] picolinate (FIrpic) as the dopant. Despite the low-lying triplet energy level of the polyfluorene polymer host, phosphorescent quenching can be suppressed by using poly(N-vinylcarbazole) (PVK) as anode buffer layer, resulting in a high luminous efficiency of 26.4 cd A^?1, which is one of the best results in the literature based on conjugated polymer reported to date. The reduced phosphorescent quenching is found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/PFO interface due to the accumulation of holes. As compared with the devices based on non-conjugated host polymer PVK, the devices based on PFO showed a lower turn-on voltage (3.6 V vs. 4.4 V) and higher power efficiency (17 lm W^?1 vs. 8.3 lm W^?1) due to the higher mobility of PFO. When doubly doped with a newly synthesized yellow-emitting metallophosphor, white polymer light-emitting devices with superior device performance (a peak device efficiency of 40.9 cd A^?1, a CIE coordinates of (0.32, 0.48), and a power efficiency of 31.4 lm W^?1) was achieved. These findings can broaden our selection in polymer hosts for highly efficient phosphorescent blue emitting devices and can find potential applications in full color displays and solid-state lighting applications in the future.     

Synthesis,characterization and high-efficiency blue electroluminescence based on coumarin derivatives of 7-diethylamino-coumarin-3-carboxamide

A novel tripodal compound, tris[2-(7-diethylamino-coumarin-3-carboxamide)ethyl]amine (tren-C), and a model compound, N-butyl-7-(diethylamino)-coumarin-3-carboxamide, were synthesized and characterized by elemental analysis, infrared and ¹H NMR spectra. The structure of the model compound was characterized by single crystal X-ray crystallography. The electroluminescence devices of ITO/2-TNATA (5 nm)/NPB (40 nm)/CBP: tren-C or model compound (wt%, 30 nm)/Bu-PBD (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated and characterized. The EL spectra of the devices comprising vacuum vapour-deposited films using tren-C as a dopant are similar to the PL spectrum of tren-C in chloroform solutions. At the concentration of 0.5 wt% tren-C, a blue-emitting OLED with an emission peak at 464 nm, a maximum external quantum efficiency (EQE) of 1.39% and a maximum luminous efficiency of 2 cd/A at the current density of 20 mA/cm², and a maximum luminance of 1450 cd/m² at 12 V are achieved.     

Dependence of annealing on stability of transparent amorphous InGaZnO thin film transistor

Bottom-gate transparent IGZO–TFT had been successfully fabricated at relatively low temperature (200 °C). The devices annealing for 4 h at 200 °C exhibit good electrical properties with saturation mobility of 8.2 cm²V^?1s^?1, subthreshold swing of 1.0 V/dec and on/off current ratio of 5×10⁶. The results revealed that the stability of TFT devices can be improved remarkably by post-annealing treatment. After applying positive gate bias stress of 20 V for 5000 s, the device annealing for 1 h shows a larger positive V_th shift of 4.7 V. However, the device annealing for 4 h exhibits a much smaller V_th shift of 0.04 V and more stable.     

Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes

We demonstrate improved performances in polymer light-emitting diodes (PLEDs) using a composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and MoO₃ powder as a hole injection layer. The PLED with the composite film exhibits the current efficiency of 13.5 cd/A, driving voltage of 3.4 V, and half lifetime of 108.1 h, while those values of the PLED with a pristine PEDOT:PSS was 11.3 cd/A, 3.8 V, and 41.5 h, respectively. We also analyze the morphological, optical and electrical properties of the composite films by atomic force microscopy (AFM), UV–Vis-IR absorption, and ultraviolet photoemission spectroscopy (UPS). This work suggests that mixing MoO₃ into PEDOT:PSS is a simple and promising technique for use solution-based devices as an hole injection layer.     

Using metal/organic junction engineering to prepare an efficient organic base-modulation triode and its inverter

In this study, we investigated the influence of a buffer layer of molybdic oxide (MoO₃) at the metal/organic junction on the behavior of organic base-modulation triodes. The performance of devices featuring MoO₃/Al as the emitter electrode was enhanced relative to that of corresponding devices with Au and Ag, presumably because of the reduced in the contact barrier and the prevention of metal diffusion into the organic layer. The device exhibited an output current of ?16.1 μA at V_B = ?5 V and a current ON/OFF ratio of 10³. Using this architecture, we constructed resistance–load inverters that exhibited a calculated gain of 6.     

On the short-circuit and avalanche ruggedness reliability assessment of SiC MOSFET modules

This paper provides an insight into the operational robustness of commercially available SiC MOSFET power modules, during short-circuit (SC) and unclamped inductive switching (UIS) test environments. A set of five different power modules from three vendors rated from 1.2–1.7 kV and with various current ratings have been evaluated, where the possible failure mechanisms that cause the breakdown of the modules have been addressed. The SC pulse duration of the modules was gradually increased until the failure occurred. A critical short circuit energy in the order of 4.0–8.0 J was observed at a supply voltage of 800 V and a pulse duration of 4.0 μs. At lower supply voltage of 500 V, all modules survived until 10.0 μs. One of the modules, rated at 1.7 kV, survived SC tests at voltages up to 1000 V for a pulse duration of 4 μs, but failed when the supply voltage was increased to 1100 V. Prior to failure, a gate-source voltage drop has been recorded, which is associated with a high G-S leakage current. The main failure mechanism, however, is the thermal runaway which leads the devices into avalanche breakdown mode. During the UIS tests, multiple samples from the three vendors of the power modules failed. The failure of the modules was always caused by the external diode connected in parallel with the MOSFETs. One of the modules from the same vendor which does not have external diode and another module from a different vendor with external diode survived the UIS tests under nominal test conditions.