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Conventional vertical power MOSFETs are limited at high voltages (>500V) by the appreciable resistance of their epitaxial drain region. In a new MOS-gate controlled device called a COMFET (or an IGR), this limitation is overcome by modulating the conductivity of the resistive drain region, thereby reducing the on-resistance of the device by a factor of at least 10. However, the device previously described is slow in turn-off, having a fall time in the range 8 to 40 µs. The purpose of our present work has been to reduce the fall time significantly and to increase the latching current level of the COMFET, while retaining its desirable features. By modification of the epitaxial structure and addition of recombination centers, we have achieved fall times as low as 0.1 µs and latching currents as high as 50 A, while retaining on-resistance values < 0.2 ohms for a 0.09 cm2chip area. The techniques used for the introduction of recombination centers include electron, gamma-ray, and neutron irradiation, as well as heavy metal doping. For a series of COMFETs (with forward blocking voltage capabilities of 400-600V), the fall time can be reduced by more than one order of magnitude with a penalty of less than a 20% increase in on-resistance.