1. Field of the Invention
The present invention relates generally to semiconductor devices and more particularly to a voltage-controlled self turn-off power semiconductor device with insulated-gate structure.
2. Description of the Related Art
Power semiconductor devices having higher withstanding or breakdown voltage characteristic are increasingly in demand in the recent manufacture of industrial-use equipment. For example, in the field of motor-controllers for railroad vehicles and industrial-use inverters, high-voltage strain switching devices with the withstanding voltage of more than several thousands volts in magnitude are required. Conventionally, self turn-off thyristors such as gate turn-off (GTO) thyristors are known as the semiconductor devices that fulfill such demand.
The self turn-off thyristor is principally one of the current-controlled switch devices. The thyristor will latch up when it turns on, and has an advantage of attaining a decreased "on-resistance" thereof. Decrease in the on-resistance potentially decreases the on-voltage of the thyristor, causing its on-characteristic to be improved. On the other hand, this type of thyristor remains inherently low in the maximum cut-off current density. In particular, a certain type of GTO thyristor that turns off using an insulated gate or metal-oxide-semiconductor (MOS) structure, such as a MOS-controlled thyristor (MCT), suffers from the poor current cut-off ability. This means that such type of thyristor should have a limit in its high-voltage strain characteristic. The breakdown voltage limit is a serious bar to the wide applicability of the GTO thyristor as power controller devices.
In the recent past, there has been proposed and developed a bipolar-type semiconductor device that includes a bipolar transistor section, which is driven to turn on and off by an insulated gate structure. This type of device is generally known as the "insulated-gate bipolar transistor (IGBT)". With a presently available IGBT, a MOS transistor having an insulated gate electrode is added to the base of the transistor section, thereby to enable switching operations to be performed by potentially changing the insulated gate electrode. This means that the IGBT is principally a voltage-controlled switch device. In this respect, the IGBT may be more suitable than the GTO thyristor in applicability as an industrial-use high-voltage strain switch device. Unfortunately, while the IGBT is high in the maximum cut-off current density achievable, it suffers from a serious disadvantage that the on-resistance stays high. This is due to the technical limitation that the IGBT device must be so designed that a latch-up will not occur in an internal thyristor structure, which is constituted by an alternate lamination (PNPN) structure of P- and N-type layers inherent within the IGBT.
The above analysis teaches that the presently available GTO thyristors or insulated-gate GTO thyristors are low in the maximum cut-off current density, whereas IGBTs are high in the on-resistance, and that each of the devices is disadvantageous in that they fail to meet the essential requirements for the high demand self turn-off power semiconductor devices. To date, a self turn-off power semiconductor device has not been achieved yet which can attain both a decreased on-resistance and an increased maximum cut-off current density (enhanced withstanding voltage characteristic).