While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

Organic light detectors: photodiodes and phototransistors.

Silicon carbide (SiC) power devices have been investigated extensively in the past two decades, and there are many devices commercially available now. Owing to the intrinsic material advantages of SiC over silicon (Si), SiC power devices can operate at higher voltage, higher switching frequency, and higher temperature. This paper reviews the technology progress of SiC power devices and their emerging applications. The design challenges and future trends are summarized at the end of the paper.

https://ieeexplore.ieee.org/ielaam/41/8031005/7815312-aam.pdf

Review of Silicon Carbide Power Devices and Their Applications

Power semiconductor devices are key components in power conversion systems. Silicon carbide (SiC) has received increasing attention as a wide-bandgap semiconductor suitable for high-voltage and low-loss power devices. Through recent progress in the crystal growth and process technology of SiC, the production of medium-voltage (600?1700 V) SiC Schottky barrier diodes (SBDs) and power metal?oxide?semiconductor field-effect transistors (MOSFETs) has started. However, basic understanding of the material properties, defect electronics, and the reliability of SiC devices is still poor. In this review paper, the features and present status of SiC power devices are briefly described. Then, several important aspects of the material science and device physics of SiC, such as impurity doping, extended and point defects, and the impact of such defects on device performance and reliability, are reviewed. Fundamental issues regarding SiC SBDs and power MOSFETs are also discussed.

https://iopscience.iop.org/article/10.7567/JJAP.54.040103/pdf

Material science and device physics in SiC technology for high-voltage power devices

Silicon carbide (SiC) switching power devices (MOSFETs, JFETs) of 1200 V rating are now commercially available, and in conjunction with SiC diodes, they offer substantially reduced switching losses relative to silicon (Si) insulated gate bipolar transistors (IGBTs) paired with fast-recovery diodes. Low-voltage industrial variable-speed drives are a key application for 1200 V devices, and there is great interest in the replacement of the Si IGBTs and diodes that presently dominate in this application with SiC-based devices. However, much of the performance benefit of SiC-based devices is due to their increased switching speeds ( di/dt, dv/ dt), which raises the issues of increased electromagnetic interference (EMI) generation and detrimental effects on the reliability of inverter-fed electrical machines. In this paper, the tradeoff between switching losses and the high-frequency spectral amplitude of the device switching waveforms is quantified experimentally for all-Si, Si-SiC, and all-SiC device combinations. While exploiting the full switching-speed capability of SiC-based devices results in significantly increased EMI generation, the all-SiC combination provides a 70% reduction in switching losses relative to all-Si when operated at comparable dv/dt. It is also shown that the loss-EMI tradeoff obtained with the Si-SiC device combination can be significantly improved by driving the IGBT with a modified gate voltage profile.

An Experimental Investigation of the Tradeoff between Switching Losses and EMI Generation With Hard-Switched All-Si, Si-SiC, and All-SiC Device Combinations

A review of the basic mechanisms affecting the stability of the threshold voltage in response to a bias-temperature stress is presented in terms of the charging and activation of near-interfacial oxide traps. An activation energy of approximately 1.1 eV was calculated based on new experimental results. Implications of these factors, including the recovery of some bias-temperature stress-activated defects, for improved device reliability testing are discussed.

Basic Mechanisms of Threshold-Voltage Instability and Implications for Reliability Testing of SiC MOSFETs

The wide-bandgap nature of silicon carbide (SiC) makes it an excellent candidate for applications where high temperature is required. The metal-oxide-semiconductor (MOS)-controlled power devices are the most favorable structure; however, it is widely believed that silicon oxide on SiC is physically limited, particularly at high temperatures. Therefore, experimental measurements of long-term reliability of oxide at high temperatures are necessary. In this paper, time-dependent dielectric-breakdown measurements are performed on state-of-the-art 4H-SiC MOS capacitors and double-implanted MOS field-effect transistors (DMOSFET) with stress temperatures between 225°C and 375°C and stress electric fields between 6 and 10 MV/cm. The field-acceleration factor is around 1.5 dec/(MV/cm) for all of the temperatures. The thermal activation energy is found to be ~ 0.9 eV, independent of the electric field. The area dependence of Weibull slope is discussed and shown to be a possible indication that the oxide quality has not reached the intrinsic regime and further oxide-reliability improvements are possible. Since our reliability data contradict the widely accepted belief that silicon oxide on SiC is fundamentally limited by its smaller conduction-band offset compared with Si, a detailed discussion is provided to examine the arguments of the early predictions.

Reliability Issues of SiC MOSFETs: A Technology for High-Temperature Environments

RECENT progress in wide bandgap power (WBG) switches shows great potential. Silicon carbide (SiC) is a promising material for power devices with breakdown voltages of several hundred volts up to 10 kV. SiC Schottky power diodes have achieved widespread commercial acceptance. Recently, much progress has been made on active SiC switches, including JFETs, thyristors, BJTs, IGBTs, and MOSFETs. Many a great promise has been made, and wondrous claims abound, but the question remains: will they live up to the hype? We explore this question for the class of high-frequency, hard-switched converters with input voltages of up to 600 VDC and power throughputs in the kilowatt range. Experimental evidence shows that both superior efficiency and higher power density may be obtained via the use of SiC MOSFETs. A direct comparison is made using silicon power devices (IGBTs and MOSFETs) and SiC MOSFETs in a 200 kHz, 6 kW, 600 V hard-switched converter. The losses are measured and conduction and switching losses of the active devices are estimated. Total losses can be reduced by a factor of 2–5 by substitution of SiC MOSFETs for Si active power devices.

Direct comparison of silicon and silicon carbide power transistors in high-frequency hard-switched applications

Emerging silicon carbide (SiC) MOSFET power devices promise to displace silicon IGBTs from the majority of challenging power electronics applications by enabling superior efficiency and power density, as well as capability to operate at higher temperatures. This paper reports on the recent progress in development of 1200V SiC power MOSFETs. Two different chip sizes were fabricated and tested: 15A (0.225cm×0.45cm) and 30A (0.45cm×0.45cm) devices. First, the 30A MOSFETs were packaged as discrete components and static and switching measurements were performed. The device blocking voltage was 1200V and typical on-resistance was less than 50 mΩ with gate-source voltages of 0V and 20V, respectively. The total switching losses were 0.6 mJ, over five times lower than the competing devices. Next, a buck converter was built for evaluating long-term stability of the MOSFETs and typical switching waveforms are presented. Finally, the 15A MOSFETs were used for fabrication of 150A all-SiC modules. The module on-resistance values were in the range of 10 mQ, resulting in the best-in-class on-state voltage values of 1.5V at nominal current. The module switching losses were 2.3 mJ during turn-on and 1 mJ during turn-off, also significantly better than competing designs. The results validate performance advantages of the SiC MOSFETs, moving them a step closer to power electronics applications.

Recent advances in silicon carbide MOSFET power devices

Hall-effect measurements of n-channel MOS devices were used to determine the main scattering mechanisms limiting mobility in SiC MOSFETs. MOS-gated Hall characterization, which was performed as a function of gate bias and body bias, indicates that surface-roughness scattering and Coulomb scattering are the main scattering mechanisms limiting electron mobility in SiC MOSFETs at room temperature. A charge-sheet model, including incomplete ionization and Fermi-Dirac statistics, is used to calculate the surface electric fields in order to develop an expression for surface-roughness scattering. In the samples used for this paper, at electron sheet densities less than 1.8times1012 cm-2, Coulomb scattering dominates, while surface roughness is dominant at higher sheet densities.

Electron-Scattering Mechanisms in Heavily Doped Silicon Carbide MOSFET Inversion Layers

A MOSFET device and a method for fabricating MOSFET devices are disclosed. The method includes providing a semiconductor device structure including a semiconductor device layer of a first conductivity type, and ion implanting a well structure of a second conductivity type in the semiconductor device layer, where the ion implanting includes providing a dopant concentration profile in a single mask implant sequence.

Kevin Matocha

Papers

Reliability Issues of SiC MOSFETs: A Technology for High-Temperature Environments

Direct comparison of silicon and silicon carbide power transistors in high-frequency hard-switched applications

Recent advances in silicon carbide MOSFET power devices

Electron-Scattering Mechanisms in Heavily Doped Silicon Carbide MOSFET Inversion Layers

Mosfet devices and methods of making