60 November/December 2023 | E-Mobility Engineering trench-type power semiconductor device, a strong electric field can be concentrated in the gate and easily break the insulating film. To correct that, Mitsubishi has developed the electric field-limiting structure that protects the gate insulating film by implanting aluminium and nitrogen to change the electrical properties of the semiconductor layer, taking advantage of the trench structure. First, aluminium is implanted vertically and a film is laid down on the bottom of the trench to provide more isolation. The electric field applied to the film is reduced to the level of a conventional planar power semiconductor device, improving reliability while maintaining the breakdown voltage of over 1500 V. The source electrode is formed by using a newly developed technique to implant aluminium in a particular pattern to enable high-speed switching and reduced switching losses. Locally formed high-impurity doped layers achieve the low on-resistance, as the trench SiC MOSFET has transistor cells that are smaller than those of planar types. This allows more cells on a single chip. To achieve this, Mitsubishi developed a new method for implanting nitrogen to form a local layer of SiC with a high concentration of nitrogen, which allows electricity to be conducted easily in the current path. As a result, even when cells are arrayed densely, resistivity can be reduced by about 25% compared to the case of no high-concentration layer. The new manufacturing method also allows intervals of the side grounding to be optimised. The result is a specific on-resistance of 1.84 mΩ/cm2 at room temperature – about half that of planar designs – while maintaining the 1500 V breakdown voltage. Infineon also uses a trench design for its 1200 V MOSFETs. This gives 25% lower switching losses compared to Infineon’s first-generation planar devices and higher switching frequencies, which in turn leads to higher power density and smaller system sizes. With a gate-source threshold voltage of over 4 V and a very low ratio between the feedback capacitance and input capacitance, the trench design provides reliable turn-off at a threshold voltage of 0 V without risking parasitic turn-on. This allows a simpler control system, significantly reducing system cost. The devices also have a lower on-resistance, which reduces conduction losses over the entire -55 to +175 oC temperature range. The package for the devices is also an important consideration, as this increases the overall resistance and can increase the losses. However, many of the packages are standardised by the international JEDEC organisation to allow devices from different manufacturers to be easily used in power electronics designs. This standardisation in size and footprint can limit the innovation to reduce the resistance though, so there is a focus on using different materials and structures inside the package to reduce the overall resistance without reducing overall reliability. So-called Kelvin packages use a fourth lead alongside the emitter, base and collector of the transistor to spread out the current and reduce the overall resistance. There are also leadless packages that allow more space for a connection to the heat sink for cooling, allowing higher currents to be used. The improved connection between the die and the cooling system reduces the junction temperature of this SiC MOSFET by 25% compared to the first generation. The MOSFET also features a creepage distance of 5.89 mm, meeting package-level requirements for 800 V systems. This leads to the only 9 mΩ variant currently available in a TO263-7 package. Onsemi is also developing SiC MOSFETs using a trench structure rather than a planar one, with samples expected next year. The company has an SiC wafer plant near Brno in Czechia (the Czech Republic) with device manufacturing at a fab in Korea. The move to a trench structure allows more devices to be built on a 150 mm wafer, driving down the cost of the devices. The company Deep insight | Improving power electronics An IGBT power module (Courtesy of Infineon Technologies)
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