Trench Type SiC-MOSFETS With Electric Field Limit For Saving Energy

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Unique electric field limiting feature prevents breakage due to strong electric field concentration at gate and improves current flow

SiC-MOSFETs are widely used in power electronic devices such as electric appliances, industrial equipment, automobiles and railcars. These devices increasingly require energy savings and high efficiency while controlling and converting electric power. To reduce overall device resistivity, the trench type is being used increasingly in place of the conventional planar type as it allows cells to be arrayed densely in substrate trenches rather than mounting gate electrodes on the substrate. This however has posed problems with its gate-insulating film breaking at high voltage.

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In order to rectify this, Mitsubishi Electric has developed a trench-type silicon-carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) with unique electric-field-limiting capabilities. This allows a power semiconductor device to achieve a specific on-resistance of 1.84 mΩ (milliohms) and a breakdown voltage of over 1,500 V.

The method of mounting transistors in power semiconductor modules leads to energy savings and equipment downsizing.

Electric field limit

The unique electric-field-limiting structure ensures SiC-MOSFETs to control current flowing through the semiconductor layer between the drain and source electrodes by applying a voltage to the gate electrode. Mitsubishi’s unique electric-field-limiting structure protects the gate-insulating film from breakage due to strong electric field concentration at the gate.

Figure 1: Cross-sectional view of conventional planar SiC-MOSFET (left) and new trench SiC MOSFET (right).

This is done so by implanting aluminium and nitrogen to change the electrical properties of the semiconductor layer. First, aluminium is implanted vertically and an electric-field-limiting layer is formed on the bottom surface of the trench. The electric field applied to the gate insulating film is reduced to the level of a conventional planar power semiconductor device, thereby improving reliability while maintaining the breakdown voltage of over 1,500V.

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Next, the side grounding connecting the electric-field-limiting layer and the source electrode is formed by implanting aluminium in an oblique direction to enable high-speed switching and reduced switching loss.

Figure 2: Manufacturing process for trench-type SiC MOSFET.

Placing more cells in dense way

The trench SiC-MOSFET has transistor cells that are smaller than those of planar types, allowing more cells to be arrayed on a single chip. To avoid transistor intervals between the gate electrodes to become narrow which can make current flow difficult, the SiC-MOSFET has nitrogen implanted in an oblique direction that locally forms a layer of SiC with a high concentration of nitrogen. This allows easy flow of electricity in the current path. As a result, even when cells are arrayed densely, resistivity can be reduced by approximately 25 percent and side grounding be optimized. The result is a specific on-resistance of 1.84 mΩ (milliohms) at room temperature, about half that of planar types, while maintaining a breakdown voltage of over 1,500 V.

Figure 3: Three-dimensional schematic of new trench-type SiC MOSFET.

Mitsubishi Electric expects to put its new trench-type SiC-MOSFET into practical use s in 2021.

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