The disclosure relates to a method of upgrading metallurgical grade silicon to semiconductor grade for making low cost silicon devices and particularly solar cells. This is accomplished by passing conductive fibers such as graphite or the like which are compatible with the later processing steps through an area which is cooled below 700.degree. C and which contains silicon difluoride and a proper N-type dopant. At these temperatures, the silicon difluoride gas will break down into pure silicon which will deposit onto the fiber with the formation of silicon tetrafluoride gas which is then recycled into a further chamber. In the further chamber, the gaseous silicon tetrafluoride is mixed with the impure metallurgical grade silicon at temperatures above 700.degree. C to form the silicon difluoride gas which is then fed into the former chamber for deposition of pure silicon onto the continuously moving fibers of graphite or the like. A p-type layer can then be formed over the n-type layer in any standard manner, such as by then passing the coated fibers through a further reaction chamber wherein p-type dopant is diffused into the top surface of the n-type layer that has been formed. The dopants alternatively could be added in the gas stream of SiF.sub.4 or the p-layer formed by ion implantation. In this way, relatively inexpensive p-n junction devices are formed without the requirement of purifying, cutting and polishing a silicon slice in the standard manner.