Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

Silicon carbide (SiC) semiconductor devices for high power applications are now commercially available as discrete devices. Recently Schottky diodes are offered by both USA and Europe based companies. Active switching devices such as bipolar junction transistors (BJTs), field effect transistors (JFETs and MOSFETs) are now available on the commercial market. The interest is rapidly growing for these devices in high power and high temperature applications. The main advantages of wide bandgap semiconductors are their very high critical electric field capability. From a power device perspective the high critical field strength can be used to design switching devices with much lower losses than conventional silicon based devices both for on-state losses and reduced switching losses. This paper reviews the current state of the art in active switching device performance for both SiC and GaN. SiC material quality and epitaxy processes have greatly improved and degradation free 100 mm wafers are readily available. The SiC wafer roadmap looks very favorable as volume production takes off. For GaN materials the main application area is geared towards the lower power rating level up to 1 kV on mostly lateral FET designs. Power module demonstrations are beginning to appear in scientific reports and real applications. A short review is therefore given. Other advantages of SiC is the possibility of high temperature operation (> 300 °C) and in radiation hard environments, which could offer considerable system advantages.

SiC power devices — Present status, applications and future perspective

Silicon as a semiconductor material is well established and first choice for the vast majority of devices. However, due to continuous device optimisation and improvements in the production process, the material properties are more and more the limiting factor. Workarounds like the super junction stretch the limits but usually at substantial cost. So a lot of effort is spent into the more straight forward approach, i.e. changing the semiconductor material. For power devices, wide band-gap semiconductors are most attractive because of low conduction and switching losses, high temperature capability, and high thermal conductivity. Despite some material and process issues back then, the first wide band-gap device, a silicon carbide Schottky-diode, was commercialised eight years ago and found a reasonable market niche. In the meantime significant progress has been made in terms of material quality and cost. However, the silicon carbide Schottky-diode is still the only wide band-gap device on the market and, in particular, there is no wide band-gap switch commercially available yet. Of course, the material cost is still two orders of magnitude higher than for silicon and there are still some material defects that lead to degradation of bipolar devices, but in general the material quality and wafer size is no longer a road block on the way to commercialisation of further devices and the device concepts are there also. So it became rather an economical than a technological question — and silicon is a strong competitor, as the case of the super junction MOSFET shows. On the other hand silicon is not just a competitor but also a strong ally, when it comes to the development of packaging suited for higher operation temperatures, frequencies, and switching speeds. So at the end the question remains: Which additional wide band-gap devices will be able to find and sustain their respective market positions?

State of the art and the future of wide band-gap devices

Wide bandgap power devices have emerged as an often superior alternative power switch technology for many power electronic applications. These devices theoretically have excellent material properties enabling power device operation at higher switching frequencies and higher temperatures compared with conventional silicon devices. However, material defects can dominate device behavior, particularly over time, and this should be strongly considered when trying to model actual characteristics of currently available devices. Compact models of wide bandgap power devices are necessary to analyze and evaluate their impact on circuit and system performance. Available compact models, i.e., models compatible with circuit-level simulators, are reviewed. In particular, this paper presents a review of compact models for silicon carbide power diodes and MOSFETs.

https://works.bepress.com/kang-peng/1/download/

Modeling of Wide-Bandgap Power Semiconductor Devices—Part II

With the commercial availability of SiC power devices, their acceptance is expected to grow in consideration of the excellent low switching loss, high-temperature operation, and high-voltage rating capabilities of these devices. This paper presents the comparative performance evaluation of different SiC power devices in the matrix converter at various temperatures and switching frequencies. To this end, first, gate or base drive circuits for normally-off SiC JFET, SiC MOSFET, and SiC BJT by taking into account the special demands for these devices are presented. Then, four two-phase to one-phase matrix converters are built with different Si and SiC power devices to measure the switching waveforms and power losses for them at different temperatures and switching frequencies. Based on the measured data, four different SiC and Si power devices are compared in terms of switching times, conduction and switching losses, and efficiency at different temperatures and switching frequencies. Furthermore, a theoretical investigation of the power losses of the three-phase matrix converter with normally-off SiC JFET, SiC MOSFET, SiC BJT, and Si IGBT is described. The power losses estimation indicates that a 7-kW matrix converter would potentially have an efficiency of approximately 94% in high switching frequency if equipped with SiC devices.

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Experimental and Analytical Performance Evaluation of SiC Power Devices in the Matrix Converter

In this brief, the electrical performance in terms of maximum current gain and breakdown voltage is compared experimentally and by device simulation for 4H-SiC BJTs passivated with different surface-passivation layers. Variation in bipolar junction transistor (BJT) performance has been correlated to densities of interface traps and fixed oxide charge, as evaluated through MOS capacitors. Six different methods were used to fabricate SiO2 surface passivation on BJT samples from the same wafer. The highest current gain was obtained for plasma-deposited SiO2 which was annealed in N2O ambient at 1100°C for 3 h. Variations in breakdown voltage for different surface passivations were also found, and this was attributed to differences in fixed oxide charge that can affect the optimum dose of the high-voltage junction-termination extension (JTE). The dependence of breakdown voltage on the dose was also evaluated through nonimplanted BJTs with etched JTE.

Surface-Passivation Effects on the Performance of 4H-SiC BJTs

Implantation-free mesa-etched 4H-SiC PiN diodes with a near-ideal breakdown voltage of 4.3 kV (about 80% of the theoretical value) were fabricated, measured, and analyzed by device simulation and optical imaging measurements at breakdown. The key step in achieving a high breakdown voltage is a controlled etching into the epitaxially grown p-doped anode layer to reach an optimum dopant dose of ~ 1.2 times 1013 cm-2 in the junction termination extension (JTE). Electroluminescence revealed a localized avalanche breakdown that is in good agreement with device simulation. A comparison of diodes with single- and double-zone etched JTEs shows a higher breakdown voltage and a less sensitivity to varying processing conditions for diodes with a two-zone JTE.

High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension

Accurate physical modeling has been developed to describe the current gain of silicon carbide (SiC) power bipolar junction transistors (BJTs), and the results have been compared with measurements. Interface traps between SiC and SiO2 have been used to model the surface recombination by changing the trap profile, capture cross section, and concentration. The best agreement with measurement is obtained using one single energy level at 1 eV above the valence band, a capture cross section of 1 × 10-5 cm2, and a trap concentration of 2 × 1012 cm-2. Simulations have been performed at different temperatures to validate the model and characterize the temperature behavior of SiC BJTs. An analysis of the carrier concentration at different collector currents has been performed in order to describe the mechanisms of the current gain fall-off at a high collector current both at room temperature and high temperatures. At room temperature, high injection in the base (which has a doping concentration of 3 × 1017 cm-3) and forward biasing of the base-collector junction occur simultaneously, causing an abrupt drop of the current gain. At higher temperatures, high injection in the base is alleviated by the higher ionization degree of the aluminum dopants, and then forward biasing of the base-collector junction is the acting mechanism for the current gain fall-off. Forward biasing of the base-collector junction can also explain the reduction of the knee current with increasing temperature by means of the negative temperature dependence of the mobility.

Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs

High-voltage blocking (2.7-kV) implantation-free SiC bipolar junction transistors with low ON-state resistance (12 mOmegaldrcm2) and high common-emitter current gain of 50 have been fabricated. A graded-base doping was implemented to provide a low-resistive ohmic contact to the epitaxial base. This design features a fully depleted base layer close to the breakdown voltage providing an efficient epitaxial JTE without ion implantation. Eliminating all ion implantation steps in this approach is beneficial for avoiding high-temperature dopant activation annealing and for avoiding generation of lifetime-killing defects that reduce the current gain.

Fabrication of 2700-V 12- $\hbox{m}\Omega \cdot \hbox{cm}^{2}$ Non Ion-Implanted 4H- SiC BJTs With Common-Emitter Current Gain of 50

In this paper, implantation-free 4H-SiC bipolar junction transistors (BJTs) with a high breakdown voltage of 2800 V have been fabricated by utilizing a controlled two-step etched junction-termination extension in the epitaxial base layer. The small-area device shows a maximum direct-current (dc) gain of 55 at JC = 0.33 A (JC = 825 A/cm2) and VCESAT = 1.05 V at Ic = 0.107 A that corresponds to a low specific ON-state resistance of 4 mΩ · cm2. The large-area device has a maximum dc gain of 52 at JC = 9.36 A (JC = 289 A/cm2) and VCESAT = 1-14 V at Ic = 5 A that corresponds to a specific ON-state resistance of 6.8 mΩ · cm2. In addition, these devices demonstrate a negative temperature coefficient of the current gain (β = 26 at 200 °C) and a positive temperature coefficient of the specific ON-state resistance (RON = 10.2 mΩ · cm2 at 200 °C). The small-area BJT shows no bipolar degradation and a low-current-gain degradation after a 150-h stress of the base-emitter diode with a current level of 0.2 A (JE = 500 A/cm2). Furthermore, the large-area BJT shows a VCE fall time of 18 ns during turn-on and a VCE rise time of 10 ns during turn-off for 400-V switching characteristics.

Benedetto Buono

Papers

Surface-Passivation Effects on the Performance of 4H-SiC BJTs

High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension

Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs

Fabrication of 2700-V 12- $\hbox{m}\Omega \cdot \hbox{cm}^{2}$ Non Ion-Implanted 4H- SiC BJTs With Common-Emitter Current Gain of 50

High-Voltage (2.8 kV) Implantation-Free 4H-SiC BJTs With Long-Term Stability of the Current Gain