Forward Bias Operation Research Articles

Origin of Limiting and Overlimiting Currents in Bipolar Membranes.

Bipolar membranes (BPMs), a special class of ion exchange membranes with the unique ability to electrochemically induce either water dissociation or recombination, are of growing interest for environmental applications including eliminating chemical dosage for pH adjustment, resource recovery, valorization of brines, and carbon capture. However, ion transport within BPMs, and particularly at its junction, has remained poorly understood. This work aims to theoretically and experimentally investigate ion transport in BPMs under both reverse and forward bias operation modes, taking into account the production or recombination of H+ and OH-, as well as the transport of salt ions (e.g., Na+, Cl-) inside the membrane. We adopt a model based on the Nernst-Planck theory, that requires only three input parameters─membrane thickness, its charge density, and pK of proton adsorption─to predict the concentration profiles of four ions (H+, OH-, Na+, and Cl-) inside the membrane and the resulting current-voltage curve. The model can predict most of the experimental results measured with a commercial BPM, including the observation of limiting and overlimiting currents, which emerge due to particular concentration profiles that develop inside the BPM. This work provides new insights into the physical phenomena in BPMs and helps identify optimal operating conditions for future environmental applications.

Effects of surface roughness and single Shockley stacking fault expansion on the electroluminescence of 4H-SiC

The electroluminescence of a 4H silicon carbide (SiC) bipolar junction transistor was studied using the base-collector junction after a side-wall facet was exposed. This sidewall was ground and polished in sequential stages with increasing grit numbers. After each stage, an electrical stress test under forward bias was performed. Electroluminescence spectra with peaks at 390 nm, 445 nm and 500 nm were initially observed. These peaks were seen to evolve under operation and after changes to the surface condition. Expansion of single Shockley stacking faults (1SSFs) in the device was observed during forward biased operation as evidenced by the growth of the 420nm emission peak, while the broad 500 nm peak was seen to diminish with increasing surface smoothness. Defect-enabled radiative recombination in SiC is a useful pathway for SiC defect characterization and it offers a new opportunity for light emission from SiC.

P‐11.1: The interface engineering with quaternary ammonium ionic liquid enabling efficient blue perovskite light‐emitting diode

Metal halide perovskite light-emitting diodes (PeLEDs) have become a rising star in recent years. Interface plays an important role in the development of the PeLEDs performance. In this work, we demonstrate that the interface engineering with quaternary ammonium ionic liquid that could be used to improve the efficiency of the blue PeLEDs. We find the interface engineering can balance hole and electron injection. In addition, quaternary ammonium cation promotes the growth and nucleation of the blue perovskite emitter, improving its film morphology and reducing the occurrence of defects. The use of the ionic liquid interfacial layer is also helpful to inhibit the ion migration in the perovskite under forward bias operation. In conclusion, by exploring the quaternary ammonium ionic liquid interfacial layer, the luminous efficiency can be greatly improved.

Metal-organic decomposed cerium oxide thin film in mixed ambient at different temperatures for MOS capacitor

Post-deposition annealing of CeO2 thin films in nitrogen/oxygen/nitrogen ambient from 400 until 1000 °C successfully transformed the films from oxygen poor to oxygen-rich state, in which an excess of oxygen was detected at higher temperatures as the oxygen interstitials. The adsorbed nitrogen attaching to oxygen vacancies in the CeO2 thin films would inhibit further oxidation of the films, and thus controlling the film thickness in addition to expanding the lattice. The increase of temperature has enhanced adsorption and diffusion of nitrogen and oxygen, decreasing the attachment of nitrogen to the oxygen vacancies. The dissociated nitrogen would reside as dangling bonds that caused a difficulty in controlling the oxidation and hence interfacial layer growth was expected. An exaggerated film growth dominated by interfacial layer happened at the highest temperature, surpassing the CeO2 layer thickness. The lowest interface trap density was attained but a degradation in the current-voltage characteristic happened due to the presence of excess oxygen and nitrogen as the scattering center for the incoming electrons during forward bias operation. A better performance was achieved by the film annealed at 800 °C via the demonstration of the lowest leakage current level, surpassing other films. Detailed explanation was discussed in this work.

Occurrence of nonohmic trend in the ballistic transport mode of a modelled low dimensional device capable of performing electronic functions

Abstract Electronic transport mechanism has been investigated through a modelled one-dimensional carbon chain consisting of four atoms, connected to two identical planar zigzag graphene nanoribbons (ZGNR) electrodes. Non equilibrium green's function (NGEF) formalism, coupled with density functional theory (DFT) is employed in stabilizing the geometry of the constructed device and in calculating the electronic transmission (conduction) at room temperature. The primitive pristine carbon chain device shows three distinct behaviours in the context of negative differential resistance (NDR) within a small voltage range 0.0–2.0 V. The first nonlinear trend shows a sharp NDR feature. It appears within an extremely low operating voltage range 0.0–0.5 V. Doping with nitrogen and boron atoms in different parts of the device (scattering and electrode region) tends to alter the transport behaviour differently. It is exciting to observe the functioning of the device upon doping in two different ways. Doping in scattering region of the device keeps the shape of I–V curve same in forward and reverse bias operation. However, doping in the electrode region results in altering the shape of I–V curve on biasing differently. Eventually in former case doping in scattering region enhances (amplify) the magnitude of current and in later, doping in electrode region makes it suitable for rectification. Analysis of the computational data generated by the, extremely small device gives exceptionally high values of PVR (peak to value ratio) and rectification ratio. Devices demonstrating high values of these two parameters are of outmost importance and often desired, as they are suitable in performing useful electronic functions. The advantage of the present device over enormous number of similar devices reported in past, being small consisting of only four atoms long carbon chain and short electrodes, its performance potential is comparatively high. The proposed carbon chain device reported here can be utilized as a nano transistor in miniaturized electronic circuitry. Uniqueness of the reported device is its small size coupled with multiple NDR, owing to smallness of dimensions the packing density of such transistors on a single chip operational capability can be increased manifold thereby enhancing its functional performance.

Recessed AlGaN/GaN Heterojunction-Based Hydrogen Sensor Operated by Reverse Bias Mode

A Pt-functionalized hydrogen sensor was fabricated on a recessed AlGaN/GaN heterojunction platform where the thickness of the recessed AlGaN barrier layer under the Pt catalyst was 10 nm. Sensing characteristics were investigated under two different bias modes: forward and reverse bias operations. Unlike conventional diode type sensors, the recessed heterojunction sensor exhibited superior sensing characteristics under the reverse bias operation mode in comparison to the forward bias operation mode. The thin AlGaN barrier layer enhanced the field emission process under reverse bias operation, thus enabling a high response, which had another benefit of significantly lowering standby power consumption compared to the forward bias operation mode. A response of ~870 % and a response time of ~11 sec were achieved at 200°C with a standby power consumption of 0.03 W/cm2.

Surface-Potential-Based Compact Model for the Gate Current of p-GaN Gate HEMTs

The gate leakage current of p-GaN gate HEMTs is modeled based on surface potential calculations. The model accurately describes the bias and temperature dependence of the gate leakage. Thermionic emission is the main mechanism of the gate current in forward bias operation while hopping transport component is the main mechanism of gate current in reverse bias operation. This newly developed gate current model was implemented in Verilog-A. A good agreement between the simulations and experimental data demonstrates the accuracy of the model.

Three-Port Optical Phase-Shifters and Modulators With Ultra-High Modulation Efficiency, Positive RF-Linking Gain, and Low Residual Amplitude Modulation

A three-port optical phase-shifter and Mach-Zehnder modulator (MZM) based on PNP-type bipolar junction transistor (BJT) is demonstrated. Significant plasma (injected carrier) induced changes of the refractive index for the optical waveguide become possible with an extremely small driving-voltage and a compact device size during operation of this BJT between the saturation and forward active modes. Devices with a standard MZM structure and a small foot-print (0.5 mm) exhibit a moderate optical insertion loss (2 dB), extremely small V π (0.18V) and P π (0.21mW), fast rise/fall time (~1ns), and a residue-amplitudemodulation (RAM) as small as 0.18 dB. Furthermore, thanks to the ultra-high modulation efficiency characteristic of our device, a +4.0 dB net RF-linking gain can be obtained under dynamic operation. Compared to 2-port (base-collector) forward bias operation, under three-port operation, the extra bias current from the base-emitter junction provides a lower V π (0.18 vs. 0.22 V), a smaller RAM (0.18 vs. 0.6 dB), and a larger RF-linking gain (+4 vs. -3.2 dB). The superior performances of the three-port to two-port operations can be attributed to the additional forward bias B-E junction being able to provide more injected carriers to induce stronger plasma effects for optical phase-shifting.

(Invited) Unified Understanding of the Shockley Stacking Fault Formation in 4H-SiC Crystals under Thermal Equilibrium and Non-Thermal Equilibrium Conditions Based on the Quantum Well Action Concept

It is well-known that SiC crystal deficiencies are delaying the market penetration of outstandingly superior SiC power devices. Efforts to date have centred on eradicating extended defects from 4H-SiC bulk crystals and epitaxial thin films. Among the extended defects existing in these materials, basal plane stacking faults (SFs) are one of the most serious concerns in SiC power devices because they are formed during the forward-bias operation of 4H-SiC PiN diodes and detrimental to SiC power devices, such as SiC MOSFETs as well as SiC BJTs and PiN diodes. In this phenomenon, i.e., the so-called “bipolar degradation”, single Shockley-type SFs (SSSFs) expand via the recombination of minority carriers (holes) at the partial dislocations in the Si core, which encompass the SSSFs. Recombination-enhanced dislocation glide (REDG) is widely accepted as the most probable kinematic mechanism of this phenomenon, and several other kinematic and energetic effects have been discussed in conjunction with the REDG mechanism.Another negative effect of SF formation in SiC crystals is the spontaneous formation of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals, which largely increases the resistivity of the crystals. When the nitrogen concentration exceeds the critical value (2–3 × 1019 cm−3), DSSFs are spontaneously formed in the crystals during high-temperature annealing (> 1100°C). DSSFs have been shown to heal (shrink) upon annealing at even higher temperatures (1800°C), and the co-doping of aluminum acceptors tends to reduce the density of DSSFs in heavily nitrogen-doped 4H-SiC crystals.The above-mentioned two phenomena can be explained based on the concept of quantum well action (QWA). In the QWA, electrons in the conduction band of 4H-SiC enter the SF-induced quantum well, consequently decreasing the electronic energy of the system. However, from the viewpoint of thermodynamics, the two phenomena differ from each other. Whereas DSSF formation is a thermodynamically equilibrium process, SSSF expansion in SiC PiN diodes occurs under non-equilibrium conditions, such as the forward biasing of the PiN diode or ultraviolet light illumination of the materials. In this respect, it is worthy to note that SSSF expansion only occurs under non-equilibrium conditions and has never been observed in thermal equilibrium even when the nitrogen concentration in 4H-SiC crystals exceeds 1 × 1020 cm−3.In this paper, I will try to clarify the physical origin of the driving force of the SSSF expansion during the forward-biasing of 4H-SiC PiN diodes via theoretical investigation of the energetics of the SF formation in 4H-SiC crystals under non-equilibrium conditions. Unified understanding of the SF expansion under thermal equilibrium and non-thermal equilibrium conditions will also be attempted based on the QWA concept. The similarity and dissimilarity in underlying physics between the SF formation in forward-biased 4H-SiC PiN diodes and that in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing are discussed, taking into account the electron trapping into the SF and the resulting positive charge formation in the materials. The difference in the dimensionality of the electron systems in SFs (two-dimensions) and bulk 4H-SiC (three-dimensions) provides different energy dependencies of the density of states for electrons in the two systems and plays an important role in the temperature dependence of the SF expansion in 4H-SiC crystals.

Silicon carbide schottky diodes forward and reverse current properties upon fast electron radiation

This paper investigates on the reaction of 10 and 15MGy, 3MeV electron irradiation upon off-the-shelves (commercial) Silicon Carbide Schottky diodes from Infineon Technologies (model: IDH08SG60C) and STMicroelectronics (model: STPSC806). Such irradiation reduces the forward-bias current. The reduction is mainly due to the significant increase of the series resistance (i.e. Infineon: 1.45Ω at before irradiation → 121×103 Ω at 15MGy); STMicroelectronics: 1.44Ω at before irradiation → 2.1×109 Ω at 15MGy). This increase in series resistance gives 4.6 and 8.2 orders of magnitude reduction for the forward-bias current density of Infineon and STMicroelectronics respectively. It is also observed that the ideality factor and the saturation current of the diodes increases with increasing dose (i.e. ideality factor- Infineon: 1.01 at before irradiation → 1.05 at 15MGy; STMicroelectronics: 1.02 at before irradiation → 1.3 at 15MGy | saturation current- Infineon: 1.6×10-17A at before irradiation → 2.5×10-17A at 15MGy; STMicroelectronics: 2.4×10-15A at before irradiation → 8×10-15A at 15MGy). Reverse-bias leakage current density in model by Infineon increases by one order of magnitude after 15MGy irradiation, however, in model by STMicroelectronics decreases by one order of magnitude. Overall, for these particular samples studied, Infineon devices have shown to be better in quality and more radiation resistance toward electron irradiation in forward-bias operation while STMicroelectronics exhibit better characteristics in reverse-bias operation.

Increasing the Operation Voltage of Integrated Solid-State Diodes through Nanostructured Porous Silicon Technology

Here, we report on the use of nanostructured porous-silicon (PS) technology as an alternative to standard field-plates technology to increase the operation voltage of silicon-based integrated solid-state devices. As a case study, we show that integration of nanostructured porous silicon in the high-field regions of a solid-state diode allows a significant increase of the breakdown voltage (VBR> 65V) to be achieved with respect to a reference diode (VBR=25V) under reverse-bias operation. On the other hand, the electrical characteristics of the diode under forward-bias operation are not affected from the presence of nanostructured PS. We argue that the nanocrystalline nature of PS reduces the mean-free path of charge carriers accelerated in high-field regions of the diode reducing, in turn, carrier velocity developed for a given applied voltage under reverse-bias operation. A standard n+/p diode with size varying between 1 and 10 um is used as a reference in this work. Under reverse-bias operation, a depletion zone establishes across the np junction, with an intense electric field developing across the depletion zone at high reverse bias. The electric field has higher intensity in correspondence of the curved edges of the junction with respect flat central regions, due to the thinner depletion zone of the former. Therefore, charge carriers are highly accelerated in the curved regions of the np junctions giving rise to breakdown effects. Electrical measurements of the current-voltage curves of the diodes shows that: i) Ron in forward biasing is of a few tens to hundreds Ohm, depending on the size of the diodes; ii) the breakdown voltage VBD is of about 25 V. The reference diode was modified through electrochemical conversion of p-type silicon to nanostructured porous-silicon in correspondence of the curved regions of the np junction. This allows the mean-free path of charge carriers in correspondence of the curved regions of the diode to be shorted and, in turn, carrier velocity developed at high electric field to be reduced under reverse-bias operation. Electrical measurements of the I-V curves of diodes modified with nanostructured PS highlight that: i) electrical characteristics of the diode in forward biasing are not affected by the presence of the PS layer (e.g. Ron changes of a few percent with respect to that of the reference diode); ii) the breakdown voltage of the diode in reverse biasing is increased to more than 65 V (i.e. a factor about 3). TO further verify our experimental findings, we carried out failure analysis through emission microscopy (EMMI). EMMI analysis of both reference and PS-modified diodes clearly shows that in the reference diode avalanche breakdown starts around 25V in correspondence of the curved regions of the n+/p junction; conversely, no breakdown effects are evident in the same regions for the diode modified with nanostructured porous silicon, which actually occurs above 65 V in other regions of the diode where PS was not produced, thus corroborating our hypothesis the presence of PS reduce the charge carrier velocity, increasing, in turn, the breakdown voltage.

Improving the Figure-of-Merit of Integrated Solid-State Diodes through the Use of Nanostructured Porous Silicon

Here, we report on the use of nanostructured porous-silicon (PS) technology as an alternative to standard technologies to increase the breakdown voltage VBR of silicon-based integrated solid-state diodes, while leaving the on-state resistance Ron unaltered. Specifically, we show that integration of nanostructured porous silicon in the high-field regions of a standard diode allows a significant increase of the breakdown voltage (VBR > 65 V) to be achieved with respect to a reference diode (VBR = 25 V) under reverse-bias operation. On the other hand, the electrical characteristics of the diode under forward-bias operation are not affected by the presence of nanostructured PS, with Ron changing of only a few percent. If we define as figure-of-merit (FoM) the ratio VBR /Ron , the presence of PS allows the FoM to be improved of a factor of 3. We argue that the nanostructured nature of PS reduces the mean-free path of charge carriers accelerated in high-field regions of the diode reducing, in turn, carrier velocity developed for a given applied voltage under reverse-bias operation.

Amplifier-Free Bias-Free Receiver Based on Low-Capacitance Nanophotodetector

The substantial energy dissipation of photoreceivers will obstruct dense photonic-electronic integration in keeping the development of processors and communications on a chip. Nanophotodetectors offer the opportunity to overcome this problem by allowing us to realize an amplifier-free bias-free receiver. Low device capacitance and forward-biased operation are the keys to achieving such a receiver, which can operate with signal light alone, namely without extra electrical energy. A photonic-crystal waveguide combined with an embedded absorber is a good candidate for this purpose, since our simulation reveals that light confinement and carrier depletion in a narrow dimension lead to fast and efficient operation. We also carry out circuit analyses to investigate the receiver performance and compare it with that of a conventional receiver with amplifiers. We show that an amplifier-free bias-free receiver can be expected to offer advantages in terms of energy consumption, signal-to-noise ratio, footprint, and delay.

Forward bias operation of silicon photonic Mach Zehnder modulators for RF applications.

In this paper, we demonstrate that forward bias (+0.9V) of a high-speed silicon (Si) optical Mach-Zehnder modulator (MZM) increases the radio-frequency (RF) link gain by 30 dB when compared to reverse bias operation (-8V). RF applications require tunable, narrowband electro-optic conversion with high gain to mitigate noise of the optical receiver and realize high RF spur-free dynamic range. Compared to reverse bias, the forward bias gain rolls off more rapidly but offers higher RF link gain improvement of more than 13.2 dB at 20 GHz. Furthermore, forward bias is shown to result in comparable spurious-free dynamic range (SFDR: 104.5 dB.Hz2/3). We demonstrate through an analytical dc transfer curve the existence of simultaneous high gain and OIP3 and verify the theoretical results with measurement under forward bias at a bias point of around +0.9 V.

Exponential ADE Solution Based Compact Model of Planar Injection Enhanced IGBT Dedicated to Robust Power Converter Design

The compact model of an injection enhanced insulated gate bipolar transistors based on the exponential solution of the ambipolar diffusion equation is presented in this paper. To model plasma carrier distribution, an exponential shape function is used, and in steady-state forward bias operation, the plasma carrier concentration has a distribution of catenary form with just two exponential basis functions, while in transient operation, more complex profiles can be approximated using a number of exponential basis functions with a range of decay length parameters, shorter than the steady state ones. The device model developed has been implemented in Saber circuit simulator and successfully tested against complete set of high current, high voltage experimental results.

High-speed compact silicon photonic Michelson interferometric modulator.

We present the detailed analysis and characterization of a silicon Michelson modulator with short 500 μm phase shifters and a low VπLπ of 0.72 V-cm under reverse bias. We investigate optical modulation of reverse biased p-n and forward biased p-i-n junctions. We demonstrate for the first time that error-free operation up to 40 Gbps is possible with lumped silicon interferometric modulators. For reverse bias operation, we show that even greater bandwidth can be obtained with lower impedance drivers. Forward bias operation with pre-emphasized signals is shown to have clean eye diagrams up to 40 Gbps, however, error counting reveals a strong dependence on test patterns and that error-free operation is achievable for short pattern lengths.

Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Sandwich polymer light-emitting electrochemical cells (LECs) with low electrolyte loading have been investigated. The LECs can be turned on to emit light at low voltages by applying either a forward- or reverse-bias current. The reverse operation, however, displays significantly higher luminance, frozen-junction behavior, and voltage instability. Moreover, the LECs display de-coupled luminance decay and voltage drift. Under forward-current operation, the luminance decreases for most of the test duration, while the operating voltage remains nearly unchanged. Under reverse-current operation, the luminance remains constant for over 200 h while the operating voltage more than doubles. The highly asymmetric device characteristics between forward and reverse operation can be explained by the different doping profiles created when the initial charge injection barriers are different. Under forward-current operation, the LEC film is predominantly p-doped. This leads to heavy luminescence quenching but a stable operating voltage. Under reverse operation, a predominately n-doped film is responsible for the higher luminance, frozen-junction behavior, and voltage drift. The anomalous device characteristics offer additional insights into the complex operating mechanism of LECs and the behaviours of a doped luminescent polymer.

Enhanced Forward Bias Operation of 4H-SiC PiN Diodes Using High Temperature Oxidation

ABSTRACTIn this paper, high temperature (>1400°C) thermal oxidation has been applied, for the first time, to 4H-SiC PiN diodes with thick (110 μm) drift regions, for the purpose of increasing the carrier lifetime in the semiconductor. PiN diodes were fabricated using 4H-SiC material that had undergone thermal oxidation performed at 1400°C, 1500°C and 1600°C, then were electrically characterized. Forward current-voltage (I-V) measurements showed that thermally oxidized PiN diodes exhibited considerably improved electrical characteristics, with devices oxidized at 1500°C having a forward voltage drop (VF) of 4.15 V and a differential on-resistance (Ron,diff) of 8.9 mΩ-cm2 at 100 A/cm2 and 25°C. Compared to typical control sample PiN diode characteristics, this equated to an improvement of 8% and 23% for VF and Ron,diff, respectively. From analysis of the reverse recovery characteristics, the carrier lifetime of the PiN diodes oxidized at 1500°C was found to be 1.05 μs, which was an improvement of around 30% compared to the control sample PiN diodes.

Compact PIN-Diode-Based Silicon Modulator Using Side-Wall-Grating Waveguide

We developed PIN-diode-based silicon Mach-Zehnder modulators, which have side-wall-gratings in the phase-shifter sections. Such passive waveguides with gratings were fabricated using ArF immersion lithography, which showed a small scattering loss of 0.4 dB/mm. We extensively investigated the forward-biased operation of the modulators by using equivalent circuit analysis and the measurement of the fabricated devices. We argue carrier recombination time only plays a minor role for the overall performance of the modulator. Dependences of the modulation efficiency on other various critical parameters are discussed. In particular, if we use relatively short phase shifter, the forward-biased operation provides smaller VπL than reversed one even at high frequency of 20 GHz, at the expense of the narrow bandwidth. Our approach enables high-speed operation up to 50 Gb/s, by using phase shifter as short as 250 μm and preemphasis signals. For 12.5-Gb/s operation, the modulator cell size was only 300 μm × 50 μm, which was suitable for the applications of high-density optical interconnects.

Experimental evidence for mobile luminescence center mobility on partial dislocations in 4H-SiC using hyperspectral electroluminescence imaging

We report on the formation, motion, and concentration of localized green emission centers along partial dislocations (PDs) bounding recombination-induced stacking faults (RISFs) in 4H-SiC pin diodes. Electroluminescence imaging depicted the motion of these green emitting point defects during forward bias operation along carbon-core PDs that bound the RISFs. Following high temperature annealing, these green emitting point defects did not contract with the PDs, but remained in the final location during the expansion. This implies that the motion of these green emitting point dislocations is enabled through a recombination-enhanced motion, similar to the process for RISF expansion and contraction within SiC.