In the past it was common for the source material for solar cell production to be off cuts from the integrated circuit industry. The source material has to be checked for impurities and to make sure that it has the right dopant type and resistivity. Almost all cells are presently made on p-type substrates. Notable exceptions are the rear contact cells made by Sunpower and amorphous/crystalline silicon heterojunction cells manufactured by Sanyo. The solar cell industry has grown very rapidly in the past few years and now uses more silicon than the entire integrated circuit industry. The rapid expansion has pushed up the price of raw crystalline silicon feed stock from under $20/kg to over $200/kg.