Power MOSFETs are well known and processes for the manufacture of MOSFETs which are operable in high radiation [megarad ionizing radiation] environments are also known. Thus, it is known that a “late gate”, which is a non-self aligned gate relative to an invertible channel region, should be used in the manufacture of such devices, known as Rad Hard (radiation hardened) devices. By using a late gate process, the gate oxide is not subjected to the high diffusion temperatures used for the diffusion of base and source regions. The exposure of the gate oxide to such high temperatures reduces the ability of the device to operate in a high radiation environment such as that experienced at very high altitudes or by orbiting space vehicles. Such processes are described in U.S. Pat. No. 5,338,693 in the name of Kyle Spring et al.; U.S. Pat. No. 5,475,252 in the name of Kyle Spring et al.; U.S. Pat. No. 5,831,318 in the name of Perry Merrill; and U.S. patent application Ser. No. 09/263,916 filed Mar. 5, 1999 in the name of Milton Boden et al. Those references describe the radiation problem and propose respective solutions.
In many applications of Rad Hard devices, a P channel MOSFET is desired. U.S. Pat. No. 6,165,821 describes a P channel Rad Hard device which is not only resistant to high ionizing radiation, but is also resistant to SEE (single or plural high energy particle) problems.
The topology employed for Rad Hard devices may be cellular, but in some cases, such as for MOSFETs with a low gate capacitance it is also known that a stripe geometry is preferred, as described in U.S. Ser. No. 09/263,916.
Many MOSFET applications have the required condition of high voltage (in excess of about 50 volts), and low RDSON. It is known that the RDSON of a cellular MOSFET can be reduced by an increasing (often termed enhancing) the conductivity in the JFET region between spaced bases. This reduces the resistance of the inherent JFET region and reduces the pinch-off of the region by reducing the expansion of the depletion regions from the spaced bases forming the JFET. This type of enhanced concentration region and the process for its formation are described in U.S. Pat. Nos. 4,376,286; 4,593,302 and 4,680,853, both in the names of Lidow and Herman. These patents do not disclose or suggest how such an enhancement can be applied to a Rad Hard type device.