1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background Art
In recent years, an IGBT (Insulated Gate Bipolar Transistor) is widely used as a power semiconductor device having a breakdown voltage of 600 V or more. Since the IGBT is normally used as a switch, it is desired to be low in ON resistance and high in switching speed.
The widely used IGBT having a trench structure includes a p-type base and an n-type base vertically adjacent to each other, an n-type emitter provided on a surface of the p-type base, and a p-type collector provided on a rear surface of the n-type base. If a positive bias is applied to a gate electrode with a potential of the emitter set as a reference potential, then an inversion layer is formed on the p-type base, and electrons are injected into the n-type base. As a result, the p-type emitter is positively biased relative to the n-type base, and holes are injected from the p-type collector into the n-type base. As a consequence, the IGBT becomes an ON state.
The holes injected at this time drift in the n-type base and flow into the p-type base. When the holes flow into the p-type base entirely, accumulated holes are reduced and an ON resistance is thereby increased near the p-type base. To solve this disadvantage, an IEGT (Injection Enhanced Bipolar Transistor) is proposed. The IEGT makes use of an IE (Injection Enhancement) effect of increasing a quantity of accumulated holes by disconnecting a part of the p-type base layer to an emitter electrode. An ON resistance of the IEGT can be reduced by the IE effect.
The IGBT and the IEGT is large in capacity COX generated by a gate oxide film, so that there is a limit to acceleration of a switching speed. In order to turn ON and OFF the IGBT or IEGT, a gate-collector capacity CGC should be charged and discharged during a switching operation. This is because, as the capacity COX is larger, the gate-collector capacity CGC is larger. Therefore, a frequency of the IGBT or IEGT at a breakdown voltage of, for example, 600 V is limited to about 100 kHz, and that of the IGBT or IEGT at a breakdown voltage of, for example, 4.5 kV is limited to about 500 Hz. It is noted that the IGBT and the IEGT are disclosed in Japanese Patent No. 3325424.
Therefore, a semiconductor device having a low ON resistance and an accelerated switching speed is desired.