This invention relates to rectifiers and, more particularly, to gold doped fast turnoff rectifiers.
Since their introduction, power semiconductors have become more popular as circuit designers have continued to employ them in new and different ways. As new applications for these devices are developed, greater emphasis is placed on improving their performance under demanding conditions of operation. Different applications require that the emphasis be placed on different characteristics of the devices. For example, some potential applications will become commercially feasible only if the cost of the components can be reduced. Other applications require that the devices be able to withstand higher reverse voltages or that they switch from a conductive to a non-conductive state more rapidly. Thus, the device designer is faced with a multi-faceted problem.
One of the principal problems facing the device designer is the interrelationship among the various device characteristics. For example, it is known that if gold is diffused into the interior n-doped region of a power semiconductor, the minority carried lifetime in that region is decreased, and thus the turnoff time is reduced. However, the affect of the gold diffusion step on the cost of the device must be considered. Also, it is known that glass passivation of semiconductor pellets has many beneficial effects. For example, the reverse voltage characteristics are improved as is device reliability. Furthermore, glass passivation improves the manufacturing yield and thus reduces cost. Consequently, a device exhibiting desirable properties could seemingly be made by utilizing both gold diffusion and glass passivation techniques. However, this has not heretofore been possible. This is so for the following reason. Glasses used with semiconductors melt at a lower temperature than that required to perform the gold diffusion. Thus, the glass passivation step must be performed following the gold diffusion step. But, the glass passivation process includes an oxide growing step which is performed at a high enough temperature to cause uncontrollable migration and redistribution of the gold. Thus, heretofore, selectively localized gold doping and glass passivation have been considered incompatible operations. Consequently, gold doped power semiconductors are generally manufactured by a process including blanket gold diffusion of an entire wafer followed by pelletization and individual passivation of silicone rubber.
It is an object of this invention, therefore, to provide a power semiconductor with gold doping and glass passivation, and, to disclose a method for making such a device.