When field effect devices such as insulated gate bipolar transistors (IGBTs) must turn off a high current with an inductive load, the device must support the high voltage induced by stopping the current in the inductive load while high charge concentrations are still present within the device because of the high current which was flowing while the device was on. The problem also exists for metal-oxide-semiconductor field effect transistors (MOSFETs), but is less severe because MOSFETs are majority carrier devices. The concurrent limits on safe values of current and voltage define a device operating region known as the safe operating area (SOA) of such a device. That is, as long as the current and voltage are both within the safe operating area, the device will not be damaged and will be successful in turning off an inductive load. If the current/voltage to which the device is subject exceeds the safe operating area, then the device often fails. The SOA of a device is expressed in terms of a maximum load current and an applied voltage. However, that specified load current is actually determined by the active area of the device and the maximum charge density which its semiconductor structure can support at the specified applied voltage without undergoing device breakdown. The current density which creates this charge is normally expressed in amperes per square centimeter (A/cm.sup.2).
It is known to increase the safe operating area of a device by increasing its physical area to reduce the current density to which the semiconductor material is subject at a given load current. However, this has the undesirable effect of decreasing processing yield and the maximum number of devices possible per processed wafer, both of which increase device cost. It is also known to increase the resistivity of the drift region of the device to increase the voltage at which the device breaks down. This has the undesirable effect of increasing the on-resistance of the device and its forward voltage drop when the device is on.
A known type of metal-oxide-semiconductor (MOS) power device having a main current path which includes a portion which is vertical through the thin dimension of the semiconductor includes a gate electrode grid having a rectangular array of rectangular windows therein through which the device active regions are double diffused. Minority carrier bypass sites are included in these windows as well as emitter regions. These minority carrier bypass sites and emitter regions are introduced by extending the high concentration base diffusion to the edge of the window to produce the bypass sites and spacing it from the edge of the window at the emitter sites. We have found that this configuration limits the safe operating area.
Accordingly, an object of this invention is to provide a technique which increases the safe operating area of a semiconductor field effect power device without requiring an increase in the device area or the resistance of its drift region.
Another object is to increase the safe operating area of a semiconductor field effect power device while retaining high gate grid conductivity.