Mitsubishi Develops new Trench-type SiC-MOSFET
Electric-field-limiting structure will help to shrink power electronic equipment
Mitsubishi Electric has developed a trench-type SiC MOSFET with a new electric-field-limiting structure for a power semiconductor device that achieves what it believes is a world-leading specific on-resistance of 1.84 mΩ cm2 and a breakdown voltage of over 1,500 V.
Mitsubishi Electric announced its new trench-type SiC-MOSFET today at the International Conference on Silicon Carbide and Related Materials (ICSCRM) 2019, which is being held at the Kyoto International Conference Centre in Japan from September 29 to October 4.
The picture above shows the cross sectional view of a conventional planar MOSFET (left) with the company's new trench MOSFET.
Unique electric-field-limiting structure ensures device reliability
SiC-MOSFETs control current flowing through the semiconductor layer between the drain and source electrodes by applying a voltage to the gate electrode. To achieve control with a small voltage, a thin gate-insulating film is required. If high voltage is applied in a trench-type power semiconductor device, a strong electric field can concentrate in the gate and can easily break the insulating film.
To correct this, Mitsubishi Electric developed a unique 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 (Fig. 2 above).
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.
Next, the side grounding connecting the electric-field-limiting layer and the source electrode is formed by using a newly developed technique to implant aluminium in an oblique direction to enable high-speed switching and reduced switching loss.
Locally formed high-impurity doped layers achieve lowest level of on-resistance
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. If transistor intervals between the gate electrodes are too narrow, however, current flow becomes difficult and device resistivity increases. Mitsubishi Electric developed a new method for implanting nitrogen in an oblique direction to locally form a 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 approximately 25 percent compared to the case of no high-concentration layer.
The new manufacturing method also allows intervals of the side grounding to be optimised (Fig. 3). The result is a specific on-resistance of 1.84 mΩ (milliohms) cm2 at room temperature, about half that of planar types, while maintaining a breakdown voltage of over 1,500 V.
The company says that mounting the transistor in power semiconductor modules for power electronic equipment will lead to energy savings and equipment downsizing. After improving the performance and confirming the long-term reliability of its new power semiconductor devices, Mitsubishi Electric expects to put its new trench-type SiC-MOSFET into practical use sometime after the fiscal year beginning in 2021.
