Comparative studies of Pt and Ir schottky contacts on undoped Al0.36Ga0.64N

Schottky contacts of Pt and Ir on undoped Al_0.36Ga_0.64N have been fabricated and the ideality factor, the built-in voltage and the reverse bias current were determined using current–voltage measurements to make a comparison.The smallest ideality factors, the lowest reverse bias current and the highest built-in voltages have been obtained for Ir Schottky contacts.We have studied the effect of an annealing for Pt and Ir Schottky contacts, on the ideality factor, the built-in voltage and the reverse bias current. A decrease of the ideality factor and the reverse bias current associated to an increase of the built-in voltage have been obtained except for high annealing temperature (T > 400 °C).Reductions of 37% and 43% of the ideality factor and improvements of 24% and 41% of the built-in voltage have been obtained for Pt and Ir Schottky contacts, respectively, after an annealing performed at 350 °C during 30 min.Two different electrical stresses have also been applied on the ohmic and Schottky contacts during 164 h to study the reliability of the employed technology. In a first time, the devices have been stressed with a drain-to-source voltage V_DS of 20 V and a gate-to-source voltage V_GS of −5 V to submit the devices to an electrical field only and not to a thermal effect induced by the electrical current. In a second time, the aging stress has been applied for a V_DS of 20 V and for a V_GS of 0 V in order to study the impact of the electrical field and the thermal effect induced by the drain current on the electrical behaviours of Al_0.36Ga_0.64N/GaN transistors. This study has also shown the existence of electrical traps in the device structure and proved the good reliability of the involved technology.These comparative studies demonstrate that Ir is a better candidate than Pt for the realisation of Schottky contacts on undoped Al_0.36Ga_0.64N.     

Shedding Light on the Operation of Polymer Light‐Emitting Electrochemical Cells Using Impedance Spectroscopy

A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light‐emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of ≈2–3 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer (V ≥ 2.5 V for superyellow), a light‐emitting p–n junction forms in situ, with a steady‐state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective p–n junction thickness (d_pn) for a device with an interelectrode gap of 90 nm decreases from ≈23 nm at 2.5 V to ≈6 nm at 3.9 V. The current increases with decreasing d_pn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when d_pn ≈ 10 nm. The existence of an optimum d_pn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing d_pn, due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the p–n junction into the surrounding non‐radiative doping regions.     

High breakdown voltage Schottky rectifier fabricated on bulk n-GaN substrate

Vertical Schottky rectifiers have been fabricated on a free-standing n-GaN substrate. Circular Pt Schottky contacts with different diameters (50 μm, 150 μm and 300 μm) were prepared on the Ga-face and full backside ohmic contact was prepared on the N-face by using Ti/Al. The electron concentration of the substrate was as low as 7 × 10¹⁵ cm⁻³. Without epitaxial layer and edge termination scheme, the reverse breakdown voltages (V_B) as high as 630 V and 600 V were achieved for 50 μm and 150 μm diameter rectifiers, respectively. For larger diameter (300 μm) rectifiers, V_B dropped to 260 V. The forward turn-on voltage (V_F) for the 50 μm diameter rectifiers was 1.2 V at the current density of 100 A/cm², and the on-state resistance (R_on) was 2.2 mΩ cm², producing a figure-of-merit (V_B)²/R_on of 180 MW cm⁻². At 10 V bias, forward currents of 0.5 A and 0.8 A were obtained for 150 μm and 300 μm diameter rectifiers, respectively. The devices exhibited an ultrafast reverse recovery characteristics, with the reverse recovery time shorter than 20 ns.     

A study on the improved programming characteristics of flash memory with Si3N4/SiO2 stacked tunneling dielectric

The programming characteristics of memories with different tunneling-layer structures (Si₃N₄, SiO₂ and Si₃N₄/SiO₂ stack) dielectrics are investigated using 2-D device simulator of MEDICI. It is theoretically confirmed that the memory with the SiO₂/Si₃N₄ stacked tunneling layer exhibits better programming characteristics than ones with single tunneling layer of SiO₂ or Si₃N₄ for programming by channel hot electron (CHE) injection. A 10-μs programming time with a threshold-voltage shift of 5 V can be obtained for the memory with SiO₂/Si₃N₄ stacked tunneling layer at V_cg = 10 V and V_ds = 3.3 V. This is attributed to the fact that the floating-gate voltage is close to drain voltage for the stacked tunneling dielectric (TD), and thus the CHE injection current is the largest. Furthermore, optimal substrate concentration is determined to be 5 × 10¹⁶–2 × 10¹⁷ cm⁻³, by considering a trade-off between the programming characteristics and power dissipation/lifetime of the devices. Lastly, the effects of interface states on the programming characteristics are investigated. Low interface-state density gives short programming time and small post-programming control-gate current.     

Effect of sweeping voltage and compliance current on bipolar resistive switching and white-light controlled Schottky behavior in epitaxial BaTiO3 (111) thin films

Bipolar resistive switching (RS) phenomenon without required electroforming has been observed in epitaxial (111)-oriented BaTiO₃ (BTO) thin films deposited by PLD technique on conducting Nb-doped substrate of SrTiO₃ (NSTO). Negative differential resistance (NDR) is observed at about −5 V when the maximum of positive voltage exceeds 7 V and the compliance current is more than 1.5 mA. And bipolar resistive switching has also been observed. In addition, the resistance of LRS decreases with increasing compliance current or the maximum of positive voltage while that of HRS barely changes, and the resistance of HRS increases with increasing the absolute of maximum of negative voltage while that of LRS scarcely changes. A typical rectifying behavior is observed when the maximum of positive voltage is less than 4 V (such as 2 V). In this case, the reverse biased current is strongly enhanced under illumination of white-light, and vice versa. The resistance of LRS and HRS can be controlled by the applied voltage or the compliance current. The rectifying behavior can be controlled by the white-light. The transition from rectifying behavior to bipolar resistive switching can be controlled by the applied voltage. The above results were discussed by considering the oxygen vacancies that can trap or release electrons as a trapping layer at the Pt/BTO interface.     

Radiation damage of Ge-on-Si devices

The radiation damage induced by 2-MeV electrons and 70-MeV protons in p⁺n diodes and p-channel MOS transistors, fabricated in epitaxial Ge-on-Si substrates is reported for the first time. For irradiation above 5×10¹⁵ e/cm², it is noted that both the reverse and forward current increase, and that the forward current is lower after irradiation for a forward voltage larger than about 0.5 V. The reason for this might be an increased resistivity of the Ge-on-Si substrate. For p-MOSFETs, for a 1×10¹⁶ e/cm² dose, a slight negative shift of the threshold voltage and a decrease of the drain current for input and output characteristics have been observed. In addition, g_m decreases after irradiation. The degradation of the transistor performance is thought to be due to irradiation-induced positive charges in the high-κ gate dielectric. The induced lattice defects are also mainly responsible for the leakage current increase of the irradiated diodes.     

Evaluation of a betavoltaic energy converter supporting scalable modular structure

Distinct from conventional energy‐harvesting (EH) technologies, such as the use of photovoltaic, piezoelectric, and thermoelectric effects, betavoltaic energy conversion can consistently generate uniform electric power, independent of environmental variations, and provide a constant output of high DC voltage, even under conditions of ultra‐low‐power EH. It can also dramatically reduce the energy loss incurred in the processes of voltage boosting and regulation. This study realized betavoltaic cells comprised of p‐i‐n junctions based on silicon carbide, fabricated through a customized semiconductor recipe, and a Ni foil plated with a Ni‐63 radioisotope. The betavoltaic energy converter (BEC) includes an array of 16 parallel‐connected betavoltaic cells. Experimental results demonstrate that the series and parallel connections of two BECs result in an open‐circuit voltage V_oc of 3.06 V with a short‐circuit current I_sc of 48.5 nA, and a V_oc of 1.50 V with an I_sc of 92.6 nA, respectively. The capacitor charging efficiency in terms of the current generated from the two series‐connected BECs was measured to be approximately 90.7%.     

Growth of 1‐eV GaNAsSb‐based photovoltaic cell on silicon substrate at different As/Ga beam equivalent pressure ratios

We report the performance of 1‐eV GaNAsSb‐based photovoltaic samples grown on a Si substrate using molecular beam epitaxy at different As/Ga beam equivalent pressure (BEP) ratios. The light current–voltage curve and spectral response of the samples were measured. The sample grown at an As/Ga BEP ratio of 10 showed the highest energy conversion efficiency with an open circuit voltage (V_OC) of 0.529 V and a short circuit current density of 17.0 mA/cm². This measured V_OC is the highest ever reported value in GaNAsSb 1‐eV photovoltaic cell, resulting in the lowest ever reported E_g/q‐V_OC of 0.50 eV. The increase in the As/Ga BEP ratio also resulted in an increase in the bandgap‐voltage offset value (E_g/q‐V_OC) and a decrease in quantum efficiency up to As/Ga BEP ratio of 18. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrical Characteristics and Photoresponse of ZnO/ZnTe Heterojunction Diodes

Heterojunction diodes consisting of n-type ZnO and p-type ZnTe were grown by pulsed laser deposition and molecular beam epitaxy, respectively, on GaAs (001) substrates. Strong diode rectifying behavior was observed in the current–voltage characteristics with a current on/off ratio of J _on /J _off = 1 × 10⁵ and a diode ideality factor of n = 1.5. A strong photoresponse in the energy range of 2.3 eV to 3.6 eV was observed, corresponding to the bandgap energies of ZnTe and ZnO, respectively. A photovoltaic response was observed with a relatively small fill factor with a short-circuit current J _sc ~ 0.8 mA/cm² and open-circuit voltage of V _oc ~ 60 mV subject to illumination by a tungsten lamp. The photovoltaic response and reverse saturation current are believed to be limited by defects related to the mismatch between the ZnTe and ZnO structures and defects in the ZnO layer deposited at low temperature. The spectral response of the diodes is modeled with a close match to experimental measurements.     

Effects of Arylene Diimide Thin Film Growth Conditions on n‐Channel OFET Performance

A series of eight perylene diimide (PDI)‐ and naphthalene diimide (NDI)‐based organic semiconductors was used to fabricate organic field‐effect transistors (OFETs) on bare SiO₂ substrates, with the substrate temperature during film deposition (T_d) varied from 70–130 °C. For the N,N′‐n‐octyl materials that form highly ordered films, the mobility (µ) and current on‐off ratio (I_on/I_off) increase slightly from 70 to 90 °C, and remain relatively constant between 90 and 130 °C. I_on/I_off and µ of dibromo‐PDI‐based OFETs decrease with increasing T_d, while films of N,N′‐1H,1H‐perfluorobutyl dicyanoperylenediimide (PDI‐FCN₂) exhibit dramatic I_on/I_off and µ enhancements with increasing T_d. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO₂ surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N′‐n‐octyl dicyanoperylenediimide (PDI‐8CN₂) and PDI‐FCN₂ films deposited at T_d = 130 °C. The SiO₂ surface treatments have modest effects on PDI‐8CN₂ OFET mobilities, but modulate the mobility and morphology of PDI‐FCN₂ films substantially. Most importantly, the surface treatments result in substantially increased V_th and decreased I_off values for the dicyanoperylenediimide films relative to those grown on SiO₂, resulting in V_th > 0.0 V and I_on/I_off ratios as high as 10⁸. Enhancements in current modulation for these high‐mobility, air‐stable, and solution‐processable n‐type semiconductors, should prove useful in noise‐margin enhancement and further improvements in organic electronics.     

Highly Efficient Rapid Annealing of Thin Polar Polymer Film Ferroelectric Devices at Sub‐Glass Transition Temperature

An unexpected rapid anneal of electrically active defects in an ultrathin (15.5 nm) polar polyimide film at and below glass transition temperature (T_g) is reported. The polar polymer is the gate dielectric of a thin‐film‐transistor. Gate leakage current density (J_g) through the polymer initially increases with temperature, as expected, but decreases rapidly at T_g ? 60 °C. After ≈2 min at T_g, the leakage is reduced by nearly three orders of magnitude. A concomitant observation is that the drain current (I_d)–gate voltage (V_g) hysteresis decreases with temperature, reaching zero at nearly the same temperature at which J_g collapses. As J_g drops further, the drain current hysteresis increases again but in the opposite direction. This combination strongly supports the interpretation of rapid defect annealing.     

High voltage applications and NBTI effects of DT-pMOSFETS with reverse Schottky substrate contacts

In this study, the characteristics of DT-pMOSFETs are discussed using the reverse Schottky substrate contacts. With this diode, the DTMOS can be operated at high voltage and temperature. In addition, it exhibited an improved driving current, DIBL, transconductance, and subthreshold slope. The driving current for DTMOS was 20% larger, and was 12 mV improved for DIBL under DTMOS operation. Furthermore, the NBTI effects of DTMOS were also reported for the first time. This is because DTMOS could operate just below 0.7 V of V_G due to the junction turn-on behavior. It is interesting to note that the shift of the ΔV_TH of pMOSFETs under NBTI measurement was significantly alleviated in the DT operating mode, about 30 mV improved after 10,000 s stressing, due to the alleviated electrical field across the gate oxide which was due to the substrate bias and the threshold voltage adjustment under DTMOS operation.     

Stress induced gate–drain leakage current in ultra-thin gate oxide

Ultra-thin gate oxide reliability, in large area MOSFETs, can be monitored by measuring the gate current when the substrate is depleted. When the channel length is scaled down, the tunneling current associated with the source/drain extension region (SDE) to the gate–overlap regions can dominate the gate current. In N-MOSFETs, as a function of the negative gate voltage two components of the gate–drain leakage current should be considered, the first for V_FB < V_G < 0 V and the second for V_G < V_FB. These components are studied in this work before and after voltage stresses. The aim of this work is to see whether this gate–drain current can be used to monitor the oxide degradation above or near the source and/or drain extension region in N-MOSFETs. It is important because the most serious circuit-killing breakdown occurs above or near the drain (or source) extension region. Finally, we show that it is necessary, before explaining the gate LVSILC curves obtained after stresses on short-channel devices, to verify which is the dominate current at low voltage.     

A 0.52 ppm/°C high-order temperature-compensated voltage reference

This paper proposed a new high-order curvature compensation technique for a new bandgap voltage reference structure using the temperature characteristics of current gain β and emitter bandgap narrowing factor ΔE _G of a lateral NPN bipolar transistor. The new structure can produce two voltage references, which are 1.209 and 2.418 V, respectively. The simulation results show that the temperature coefficients of the two output voltage are 0.52 ppm/°C, the PSRR is more than 60 dB for frequencies at 10 kHz, and the circuit dissipates 0.18 mW with 5-V supply.     

Fast soft recovery thyristors with axial lifetime profile fabricated using iridium diffusion

The paper presents experimental results on samples of fast soft reverse recovery thyristors with axial carrier lifetime gradient in the wide base, realised using combination of iridium diffusion with low dose of electron irradiation. Using this technique, fast thyristors of I_TAV=960 A, V_DRM=3 kV, turn-off time t_q≈110 μs and reverse recovery charge Q_rr≈700 μC, were fabricated. The softness factor t_f/t_s≈1 at −dI_T/dt=50 A/μs. Devices can be used in e.g. frequency converters based on the current source inverter.     

Performance of 1 eV GaNAsSb‐based photovoltaic cell on Si substrate at different growth temperatures

We report the performance of a 1 eV GaNAsSb photovoltaic cell grown on Si/Si–Ge substrate using molecular beam epitaxy at different growth temperatures. The sample grown at 420°C showed the highest energy conversion efficiency, with a short circuit current of 18 mA/cm² and open circuit voltage of 0.53 V. With different growth temperature, performance of the cells degrade, which is attributed to the increase of nitrogen‐related defects and the decrease of antimony incorporation at higher growth temperature. Copyright © 2017 John Wiley & Sons, Ltd.     

Charge‐Transporting Polymers based on Phenylbenzoimidazole Moieties

A series of novel styrene functionalized monomers with phenylbenzo[d]imidazole units and the corresponding homopolymers are prepared. These side‐chain polymers show high glass‐transition temperatures that even exceed the corresponding value for the common electron‐transporting material 1,3,5‐tris(1‐phenyl‐1H‐benzo[d]imidazol‐2‐yl)benzene (TPBI). Similar electronic behavior between the polymers and TPBI is shown. The polymers are used as matrices for phosphorescent dopants. The fabricated devices exhibit current efficiencies up to 38.5 cd A^?1 at 100 cd m^?2 and maximum luminances of 7400 cd m^?2 at 10 V with a minimum turn‐on voltage as low as 2.70 V in single‐layer devices with an ITO/PEDOT:PSS anode (ITO = indium tin oxide, PEDOT:PSS = poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate)) and a CsF/Ca/Ag cathode.     

Active Site Implantation for Ni(OH)2 Nanowire Network Achieves Superior Hybrid Seawater Electrolysis at 1 A cm−2 with Record-Low Cell Voltage

Direct seawater electrolysis provides a grand blueprint for green hydrogen (H₂) technology, while the high energy consumption has severely hindered its industrialization. Herein, a promising active site implantation strategy is reported for Ni(OH)₂ nanowire network electrode on nickel foam substrate by Ru doping (denoted as Ru Ni(OH)₂ NW²/NF), which can act as a dual-function catalyst for hydrazine oxidation and hydrogen evolution, achieving an ultralow working potential of 114.6 mV to reach 1000 mA cm⁻² and a small overpotential of 30 mV at 10 mA cm⁻², respectively. Importantly, using the two-electrode hydrazine oxidation assisted seawater electrolysis, it can drive a large current density of 500 mA cm⁻² at 0.736 V with over 200 h stability. To demonstrate the practicability, a home-made flow electrolyzer is constructed, which can realize the industry-level rate of 1 A cm⁻² with a record-low voltage of 1.051 V. Theoretical calculations reveal that the Ru doping activates Ni(OH)₂ by upgrading d-band centers, which raises anti-bonding energy states and thus strengthens the interaction between adsorbates and catalysts. This study not only provides a novel rationale for catalyst design, but also proposes a feasible strategy for direct alkaline seawater splitting toward sustainable, yet energy-saving H₂ production.     

Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

A series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W^?1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq₃.     

Analysis of a Parasitic‐Diode‐Triggered Electrostatic Discharge Protection Circuit for 12 V Applications

In this paper, an electrostatic discharge (ESD) protection circuit is designed for use as a 12 V power clamp by using a parasitic‐diode‐triggered silicon controlled rectifier. The breakdown voltage and trigger voltage (V_t) of the proposed ESD protection circuit are improved by varying the length between the n‐well and the p‐well, and by adding n+/p+ floating regions. Moreover, the holding voltage (V_h) is improved by using segmented technology. The proposed circuit was fabricated using a 0.18‐μm bipolar‐CMOS‐DMOS process with a width of 100 μm. The electrical characteristics and robustness of the proposed ESD circuit were analyzed using transmission line pulse measurements and an ESD pulse generator. The electrical characteristics of the proposed circuit were also analyzed at high temperature (300 K to 500 K) to verify thermal performance. After optimization, the V_t of the proposed circuit increased from 14 V to 27.8 V, and V_h increased from 5.3 V to 13.6 V. The proposed circuit exhibited good robustness characteristics, enduring human‐body‐model surges at 7.4 kV and machine‐model surges at 450 V.