Reverse Bias Voltage Research Articles

Fabrication of Photodiodes Based on Solution-Processed Perovskite Single Crystals

Solution-processed organic-inorganic pero-vskites MAPbX3 (MA = CH3NH3 and X = Cl, Br, and I) have been emerged as potential material for photodetection. Especially photodiodes based on perovskites have shown low dark current and high gain. However, intrinsic perovskites require functional layers to establish photodiodes and these functional layers’ small charges mobility and high traps density make photodiodes based on perovskites low response speed. Here, we directly grow homojunctions based on perovskite single crystals (PSCs) during the crystallization process. The pristine PSCs act as p-type donor and epitaxial growth layer with Se doping act as n-type acceptor. With simple structure Au- homojunctions- Au, our photodiodes could work under a large reverse voltage bias range from 0 to 200 V with low noise and result wide dynamic range and high response speed. Using applied voltage to control the width of depletion layer, we achieve enhanced photodetection in near-band photo detection. Our work provides useful information in solution-processed devices design.

An improved power loss model of full-bridge converter under light load condition.

When the full-bridge converter works under the light load condition, the power efficiency obtained by the theoretical model is much different from that of the actual converter. Facing with this situation, an improved power loss model based on the typical power loss model is proposed. In this paper, the typical power loss model is called typical model for short and the improved power loss model is called proposed model for short. Firstly, the antiparallel freewheeling diodes at the arms of full-bridge circuit are taken into account. Every barrier junction capacitance of Schottky diode in the rectifier circuit is neglected. Then, the turning-off loss of full-bridge and the core loss of inductive components (the transformer and the filter inductor) in the typical model are compensated and modified by combining the theoretical values with the measured input current under the minimum and the maximum output current. In addition, it also corrects the equivalent resistance related to the conduction loss of converter. Eventually, the proposed model is established. The rise time and fall time of the midpoint voltage of two arms, and the fluctuation degree of the reverse bias voltage related to the Schottky rectifier diodes are regarded as the local indexes. The conduction time of the metal oxide semiconductor field effect transistor (MOSFET) in each switching period, and the power efficiency of converter are regarded as the global indexes. Based on the analyses, the local indexes are compared qualitatively, while the global indexes are compared quantitatively. It is found that the differences of the local indexes between the proposed model and the experiment are smaller than those between the typical model and the experiment. Meanwhile, the global indexes of the proposed model are closer to the experimental results. Therefore, it can be further demonstrated that the proposed model is more approximate to the actual converter than the typical model.

Experimental research on the relationship between bypass diode configuration of photovoltaic module and hot spot generation * * Project supported by the National Natural Science Foundation of China (No. 61504139, 61275040) and Chinese Academy of Sciences (No. Y072051002).

A hot spot is a reliability problem in photovoltaic (PV) modules where a mismatched or shaded cell heats up significantly and degrades the PV module output power performance. High PV cell temperature due to a hot spot can damage the cell encapsulate and lead to second breakdown, which both cause permanent damage to the PV module. In present systems, bypass diodes are used to mitigate the hot spot problem. In this work, five commercial polysilicon PV modules configured with different numbers of bypass diodes are used to study the influence of bypass diodes on the reverse bias voltage of a shaded cell and the resulting hot spot phenomenon. The reverse bias voltage of the shaded cell, and the hot spot probability and severity decrease as the number of bypass diodes increases. Negative terminal voltage of a shaded cell accompanied by a switched-off bypass diode are the necessary condition for hot spot generation. In an extreme case where each cell has an individual bypass diode in a PV module, it still cannot avoid the hazards of a hot spot under the shading areas of 5–7 cm2, but the probability of a hot spot is reduced to a minimum of 0.41%.

Operating Conditions Dependence of the Junction Characteristics of Optocouplers

The present paper is a trial to shed further light on the dependence of the junction characteristics of the proposed optocoupler type 4N25 on the operating conditions; the applied voltage and signal frequency. For the input light emitting diode and output phototransistor, their forward and reverse (C–V) characteristics were investigated and plotted at different applied bias voltage- and frequency- levels. In this concern, the diffusion- and transition- capacitances, impedance, quality- and dissipation- factors, and the phase angle were investigated. Concerning the light emitting diode, at frequency value of 200 kHz, as an example, the diffusion capacitance value increases exponentially from 0.166 nF, up to 4.71 nF, measured at forward bias voltages of 0.01 Volt and 1.20 Volts, respectively. Also, at an applied forward bias voltage of 1.0 Volts, the diffusion capacitance increases from 1.76 nF up to 2.84 nF, measured at frequency levels of 50 kHz and 300 kHz, respectively. Considering the transition capacitance, and at signal frequency of 200 kHz, its value was shown to be decreased rapidly from 178.36 pF down to 30.42 pF whenever measured at 0.01 Volt and 0.30 Volt, respectively. But for higher bias voltages, the decreasing rate was shown to be negligible. On the other hand, at applied bias voltage of 1.0 Volt, its value was shown to be decreased from 28.76 pF down to 17.59 pF, measured at frequency levels of 50 kHz and 300 kHz. For the phototransistor, its emitter junction capacitance increased exponentially as a function of the emitter-base bias voltage, where at frequency of 200 kHz, as an example, its value increased from 0.581 nF up to 3.048 nF, measured at 0.01 Volt and 0.80 Volt, respectively. On the other hand, the collector junction capacitance was shown to be decayed rapidly as a function of the applied reverse bias voltage. Where its value decreased from 1.29 nF down to 0.012 nF, measured at 0.01 Volt and 0.90 Volt, at frequency of 200 kHz, as an example. Finally, it is proved that for both capacitances, their values are a direct decreasing functions of the operating frequency, where at applied bias voltage of 0.8 Volt, a values of 7.6878 nF and 0.05338 nF were decreased down to 1.811 nF and 0.0254 nF, respectively, with increasing the frequency from 50 kHz up to 300 kHz.

Back Migration Based Long Lifetime Approach for Organic Light‐Emitting Diode

Lifetime is one critical reliability issue in electronics, especially in organic devices including organic light emitting diodes (OLEDs), solar cells, thin film batteries, thin film transistors, and memories, etc. Many causes that contribute to device failures have been found out. So far, several methods for improvement in the device lifetime have been proposed, albeit effective approaches are still under developing and the ideal will be the one that enables cost‐effective fabrication with ease of processing. Here an easy‐to‐apply method based on the back migration to improve the lifetime of OLED as well as LED devices are presented. That is done by simply applying a reverse bias voltage with an appropriate magnitude. For example, lifetime of a studied white OLED device at 9500 cd m−2 can be increased from 288 to 685 h, an increment of 137%, as a reverse bias of 10 V is applied to the pristine device for 2 h. It can be further increased to 857 h, an increment of 197%, as the reverse bias is kept for 4 h. The lifetime can still be improved for non‐pristine devices.

Large-Area 4H-SiC Ultraviolet Avalanche Photodiodes Based on Variable-Temperature Reflow Technique

In this letter, we report high-performance 4H-SiC separated absorption charge multiplication ultraviolet (UV) avalanche photodiodes (APDs) with a large active area (800- $\mu \text{m}$ diameter). For the first time, a variable-temperature photoresist reflow technique is presented to obtain very smooth sidewalls for positive beveled mesa, which is very useful for suppressing the reverse leakage and premature edge breakdown. At room temperature, the dark current of our fabricated large-area APD remains at ~1-pA level (0.2 nA/cm2) at low reverse bias voltage, and a high multiplication gain of over 106 is achieved. The peak responsivity at the wavelength of 274 nm reaches 0.18 A/W, corresponding to a maximum external quantum efficiency of 81.5% at unity gain. Moreover, an excellent UV/visible rejection ratio of more than 103 is obtained. To the best of our knowledge, this is the best result reported for visible-blind UV detectors based on the large-area 4H-SiC APDs.

A study on the voltage-dependent response of a GaInNAs-based pin photodetector with a quasi-cavity

We present a characterisation of a GaInNAs/GaNAs quantum well-based photodetector with a bottom distributed Bragg reflector (quasi-cavity). The detector is designed to be used at the 1.3 μm optical fibre communication window. The quantum efficiency of the photodetector is measured as 24% at 1286 nm under −2 V applied reverse bias. As the reverse bias voltage is increased, a carrier multiplication-related increase and oscillations are observed in the voltage-dependent responsivity curve. The observed carrier multiplication is explained by the high electrical field-induced impact ionisation mechanism in the pin junction region, while the observed voltage-dependent oscillations are explained by the Franz–Keldysh effect (FKE). At the wavelength of 1286 nm, which is close to the absorption wavelength of the active region of the photodetector, FKE-related oscillations (FKOs), start at very low reverse bias values and the responsivity of the photodetector is dominated by FKOs. On the other hand, FKOs quench at higher wavelengths and an impact ionisation-related increase at the voltage-dependent responsivity curve dominates. At λ = 1310 nm, only impact ionisation mechanisms have an effect over the R (V) curve. The multiplication factor for 1310 nm is calculated as M = 12 at room temperature. The applied electric field and excitation wavelength dependence of the absorption coefficient is calculated and a good match with the experimental results at the applied voltages is achieved.

Dual-band ultraviolet photodetectors comprising nanostructured MgZnO on ZnO films

To extend the wavelength sensing limitation, dual-band ultraviolet photodetectors (UV-PDs) were studied. The ZnO/MgZnO dual-band UV-PDs were constructed by stacking MgZnO nanorods on ZnO films. The wide ultraviolet wavelength from 250 to 360 nm could be detected by the proposed dual-band UV-PDs. When a reverse bias voltage of −5 V was applied on the dual-band UV-PDs, the photoresponsivity at 310 and 360 nm was 196.0 and 0.70 A/W, respectively. The noise equivalent power at 310 and 360 nm was 9.81 × 10−15 W and 2.78 × 10−12 W, respectively. Furthermore, the specific detectivity at 310 and 360 nm was 9.67 × 1013 and 3.41 × 1011 cmHz0.5W−1, respectively.

A multichannel gated neutron detector with reduced afterpulse for low-yield neutron measurements in intense hard X-ray backgrounds.

A design of multichannel gated photomultiplier tube (PMT) is presented for the 960-channel neutron time-of-flight detector at the Institute of Laser Engineering of Osaka University. This is important for the fusion science and the nuclear photonics where intense hard X-rays are generated from the interaction of ultra-short laser pulse of petawatt power density with matter. The hard X-rays often overload PMTs and cause signal-induced background noises called afterpulses, making the detection of subsequent neutrons impossible. For this reason, the PMTs are coupled with an electrical time-gating (ETG) system to avoid overloading. The ETG system disables the PMT by modulating the dynode potential during the primary X-ray flash. An after-pulsing suppression technique is demonstrated by applying a reverse bias voltage between the photocathode and the first dynode. The presented multichannel scheme provides a gate response time of 80 ns, a signal cutoff ratio of 2.5 × 102, and requires reasonably low power consumption.

Production of p‐Type Si/n‐Type β‐FeSi2 Heterojunctions Using Facing‐Targets Direct‐Current Sputtering and Evaluation of Their Resistance and Interface State Density

Without a post‐annealing procedure, the β‐FeSi2 thin films are epitaxially grown on Si(111) wafer substrates via facing‐targets direct‐current sputtering. During epitaxial growth, the temperature for heating of substrates is maintained at 600 °C. The resultant p‐type Si/n‐type β‐FeSi2 heterojunctions are produced. At room temperature, a large leakage current under an applied reverse bias voltage together with a small photo‐detective performance is observed from the measured dark and irradiated current density–voltage curves of the created heterojunctions. Both of the conductance–voltage (G/ω–V) and capacitance–voltage (C–V) measurements at different frequencies (f) in the range of 5 kHz–1 MHz are performed in the dark at room temperature. The interface state density (Nss) and series resistance (Rs) in the created p‐type Si/n‐type β‐FeSi2 heterojunctions are computed and analyzed from the measured C–V–f and G/ω–V–f curves. Nss is found to be 3.48 × 1012 eV−1 cm−2 at 5 kHz and decreased to 4.68 × 1011 eV−1 cm−2 at 1 MHz. Moreover, the values of Rs at zero bias are 2.21 kΩ at 5 kHz and 13.66 Ω at 1 MHz. These results review the presence of Nss and Rs in the created heterojunctions, and they can be the cause to degrade the heterojunction performance.

A reconfigurable patch antenna with independent frequency and polarization agility

ABSTRACTA frequency and polarization reconfigurable patch antenna is proposed in this letter. The antenna consists of a circular patch surrounded by four sector-shaped patches. Four pairs of varactor diodes are introduced to bridge the gaps between the circular patch and the sector-shaped patches; the capacitance of the varactors is controlled by two DC signals. By adjusting the reverse bias voltage of the varactors, the proposed antenna can switch between linear polarization (LP), right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). Furthermore, the operating frequency of each polarization state can be independently tuned. A prototype has been designed and fabricated. The results indicate the proposed antenna with a continuously tunable frequency range from 1.82 to 2.28 GHz in LP states, and from 1.88 to 2.25 GHz in either RHCP or LHCP states. Meanwhile, stable radiation patterns with low cross-polarization and back radiation levels (<−15 dB) are achieved for all states.

Resistive switching device with highly asymmetric current–voltage characteristics: a solution to backward sneak current in passive crossbar arrays

With the view towards future non-volatile random access memories that can be integrated at a large scale, extensive study on resistive switching (RS) devices arranged in a crossbar array is currently underway. Although the crossbar array architecture offers relatively simple and acceptable scalability, the presence of sneak current is recognized as a critical issue that needs to be resolved at device level. In addressing this issue, we demonstrate a new type of RS device fabricated by combining graphene oxide (G–O) and zinc oxide (ZnO) with highly asymmetric current–voltage (I–V) characteristics depending on the polarity of bias voltage. The distinctive highly asymmetric I–V characteristics result from the presence of a hetero-junction interface formed between the G–O and ZnO layers. This hetero-junction manifests resistance in the range of GΩ under both forward and reverse bias voltage when the device is in the OFF state, in contrast, when the device is in the ON state, it exhibits resistance in the range of MΩ or kΩ under forward bias and GΩ under reverse bias. We propose to employ demonstrated RS devices with highly asymmetric I–V characteristics to mitigate adverse effects of the sneak current.

Using Photoelectric-Diode Volt-Ampere Characteristic to Detect Visibility

Based on the linear relationship between light illumination and output current of photoelectric-diode under reverse bias voltage, several photoelectric circuit was design. According to Bouguer-Lambert's law and Koschmic principle, the relationship between output voltage and illumination is changed into a quantitative relationship between output voltage and visibility. Using the relation, We can get the relationship between light transmittance and output voltage, and use the photoelectric circuit to detect visibility. Through the experiment, we find the results matched our theory. With the aid of these relationships, We can measure instant visibility.

An improved SOI trench LDMOST with double vertical high-k insulator pillars * * Project supported by the National Natural Science Foundation of China (Nos. 51237001, 51607026) and the Fundamental Research Funds for the Central Universities (No. ZYGX2016J048).

An SOI trench LDMOST (TLDMOST) with ultra-low specific on-resistance (Ron,sp) is proposed. It features double vertical high-k insulator pillars (Hk1 and Hk2) in the oxide trench, which are connected to the source electrode and drain electrode, respectively. Firstly, under reverse bias voltage, most electric displacement lines produced by the charges of the depleted drift region in the source side go through the Hk1, and thus the average electric field strength under the source can be enhanced. Secondly, two additional electric field peaks are induced by the Hk1, which further modulate the electric field in the drift region under the source. Thirdly, most electric displacement lines produced by the charges of the depleted drift region in the drain side enter into the Hk2. This not only introduces one more electric field peak at the corner of the oxide trench around the Hk2, but also forms the enhanced vertical reduced surface field effect, which modulates the electric field in the drift region under the drain. With the effects of the two Hk insulator pillars, the breakdown voltage (BV) and the drift region doping concentration are significantly improved. The simulation results indicate that compared with the oxide trench LDMOST (previous TLDMOST) with the same geometry, the proposed double Hk TLDMOST enhances the BV by 86% and reduces the Ron,sp by 88%.

Development of surface barrier detectors of low leakage current employing argon cold plasma assisted silicon surface cleaning prior to detector fabrication

The performance of silicon surface barrier detectors was improved by atmospheric pressure argon cold plasma assisted surface cleaning compared to routine cleaning using organic solvents, prior to edge protection using epoxy, then followed by metallization. The silicon wafers' surfaces were exposed to plasma etching, after lapping and chemical etching. The plasma cleaning conditions were optimized with respect to feed gas composition, flow rates and exposure time. Various Au/n-Si surface barrier detectors fabricated thereof were characterized by measuring forward and reverse Current-voltage (I-V) characteristics and alpha energy spectra obtained on exposure to α-particles. The variation of alpha energy resolution with applied reverse bias voltage for each detector was also studied. The results reveal that the leakage current of the new detectors treated with cold plasma formed by a mixture of Argon and Air (90% Argon and 10% Air) was substantially lower than that of surface barrier detectors fabricated following conventional methods.

Phonon-assisted optical absorption due to Franz–Keldysh effect in 4H-SiC p–n junction diode under high reverse bias voltage

Photocurrent in a 4H-SiC p–n junction diode under illumination with sub-bandgap light was investigated. Under a high reverse bias condition, the photocurrent significantly increased with an increase in the reverse bias voltage. We calculated the photocurrent taking into consideration the phonon-assisted optical absorption due to the Franz–Keldysh effect. The calculated photocurrent showed good agreement with the experimental results. The photocurrent also increased at elevated temperatures, which could be quantitatively explained by the redshift of the 4H-SiC absorption edge (the shrinkage of the bandgap) and the increase in the phonon occupation number with rising temperature.

Frequency-tunable metamaterial absorber with three bands

We propose a three-band metamaterial absorber composed of two opposite-trapezoid rings with only one varactor diode. By changing the reverse bias voltage on the varactor diode from 0 V to −15 V, the frequencies at the first and second absorption peaks can be dynamically tuned from 2.77 GHz to 4.36 GHz and 8.11 GHz to 8.41 GHz, respectively. With the increase of the reverse bias voltage, the frequency shift of the first absorption peak may reach 1.59 GHz with the absorptivity from 68.4% to 99.8%; for the second absorption peak, the frequency shift reaches 0.30 GHz with the absorptivity of above 97.4%; while the third absorption peak hardly shifts keeping nearly perfect absorptivity of above 99.9%. The absorption mechanism is analyzed by surface current distributions and the simulation results are verified by the experimental measurement. The proposed metamaterial absorber may be useful in the fields of multi-band electromagnetic stealth, biosensors, reflectors and so on.

Reliability of Silicon Avalanche Photodetector in Geiger Mode

APDs are high-gain semiconductor diodes that produce an avalanche effect at reverse bias voltage, also known as impact ionization. When APDs work in the Geiger mode, due to the theoretically infinite magnification factor, APD can detect single-photon signal. However, compared with the design progress of APDs, life and reliability tests are rarely reported. In this paper, a method for assessing the reliability of silicon APDs is proposed, including the rapid estimation of APD fault activation energy (Ea), the analysis of APD failure mechanisms and assessment of lifetime. The operating life of the APDs is expected to be tens of thousands of hours. For such long-life devices, accelerated life testing is suitable for APDs. In a temperature stress test, in order to maintain the unity of the failure mechanism, temperature limit should be determined by monitoring dark current through a progressive stress test at first. Then another set of samples of the same structure was selected for step stress experiments.The test time and temperature interval were optimized using Arrhenius formula according to the stress limit and failure activation energy. Compared to 15,000h, the expected lifetime of our samples, the test time is greatly shortened, proving the high efficiency of our method. Fail occurs when extending the test time. The main cause of fail is the increase of surface leakage current, which will be verified in subsequent tests. Failure rate increases with time in wear out phase, and strongly depends on the structure of the device. Therefore, in addition to the assessment of lifetime, it is of great significance to improve the device design.

Electrical Characteristics of Solder-Free SiC Die/Metal Foil/AlN Plate Junctions Fabricated Using Surface Activated Bonding

In conventional power device modules, Si-based dies are attached to metal layers on ceramic plates by using solders. Given that power devices made of widegap materials such as SiC and GaN are going to be launched into market, packaging technologies that exploit their superiority are strongly sought for. Solder-free modules, i.e., modules with dies directly bonded to metal layers on ceramic plates are likely to be ideal since fracture in the solder layer is one of the essential factors that limit the thermal tolerance of conventional power modules. In this work, we fabricated an SiC die/metal/ceramic plate junction by using surface activated bonding (SAB) and examined its current-voltage (I-V) characteristics at elevated temperatures. As ceramics AlN was used since the thermal expansion coefficient of AlN is close to that of SiC. The metal layer on AlN plate was made of an Al foil.We prepared a 635-μm-thick AlN ceramic plate and a 30-μm-thick Al foil. The average roughness (Ra) of the surface in AlN plate was ∼0.5 μm. Ra of the Al foil was 0.06 and 0.19 μm in its mirror and matte surfaces, respectively. The matte surface of Al foil was directly bonded to the AlN plate through a surface activation process using an Ar fast atom beam. In a preparatory study we fabricated an AlN/Al foil/AlN junction by bonding AlN plates to the both sides of an Al foil. We observed the cross section of AlN/Al foil/AlN junction by using scanning electron microscopy. We found that the AlN plate and Al foil were bonded to each other without gaps at the both AlN/Al interfaces although Ra of AlN plates and Al foils was much larger than typical Ra of semiconductor substrates that were bonded using SAB (≤~1 nm). We prepared an SiC epi substrate by growing an n+-SiC buffer layer and an n--SiC epi layer on an n+-4H-SiC (0001) substrate. We formed an ohmic contact on the backside of the epi substrate by evaporating an Al/Ni/Au and annealing at 1000 °C. Then we made a 300-μm-ϕ Ni/Au Schottky contact on the top by evaporation and lift-off. An Al layer was evaporated on the top of ohmic contact so that an SiC Schottky diode die was fabricated. Then the Al surface of ohmic contact of SiC die was bonded to the mirror surface of Al foil on AlN plate using SAB. Note that no intermediate layers such as solder were inserted at bonding interfaces.We measured the I-V characteristics between the Schottky contact of SiC die and the surface of Al foil at temperatures between room temperature and 300 °C. A reverse-bias current was ~10-7 A/cm2 at a reverse-bias voltage up to 6 V for temperatures lower than 250 °C. A larger reverse-bias current (~10-6 A/cm2) was observed in measurement at 300 °C. In the forward characteristics, lower turn-on voltages and larger series resistances were observed for higher temperatures. It is notable that these features are typically observed for Schottky diodes. In addition, no change was observed in appearance of the SiC die/Al foil/AlN junction even after the measurement at 300 ℃. Although the bonding strength in Al foil/AlN interfaces is still unclarified, these results suggest that SAB technologies could be useful for realizing solder-free modules of power devices.Acknowledgment—The Al foil employed in the work was supplied from Toyo Aluminium K.K. Figure 1

Investigation of an n− layer in a silicon fast recovery diode under applied bias voltages using Kelvin probe force microscopy

We have imaged an n− layer of a commercial silicon fast recovery diode (Si-FRD) under applied bias voltages using frequency modulation atomic force microscopy and Kelvin probe force microscopy. It was possible to image the potential contrast of the exposed active region surface of the Si-FRD owing to the dominant behavior of carriers induced by applying the bias voltages. Under applied reverse bias voltages, potential contrasts at a pn− junction were recognized in potential images. At the applied forward bias voltage of 1.5 V, a potential drop at an n−n interface under conductivity modulation was clearly observed in a potential image. From the distance between the pn− junction and the n−n interface, the n− layer width of a power device was determined to be 36.6 µm.