The Use of Thermal Diffusion in Silicon Semiconductors.

            

             Doping of silicon wafers is an essential step in the processing of silicon wafers. Intrinsic (extremely pure) silicon wafers have a very high resistivity, and subsequently are poor conductors. With doping, chemical impurities are introduced to the intrinsic silicon wafers, changing their electrical properties. Silicon can be doped to be either p, or n type silicon. P-type silicon contains holes (the absence of electrons) as charge carriers. The most prevalent p-type dopant is boron. N-type silicon contains electrons as charge carriers. The most prevalent n-type dopant is phosphorus.1.

             When p-doped and n-doped regions on a silicon wafer are brought together, they form a p-n junction (Figure 1). When the two regions are brought together, a depleted zone forms between them, which is void of charge carriers. This depleted region can be made in such a way so as to form a diode, allowing electricity to flow one direction, but not the other.2.

             Figure 1: Illustration of a p-n junction. Electrons are the major charge carriers of the n-doped silicon, while holes are the major charge carrier of the p-doped silicon. The concentration of these charge carriers intersect in the space charge region, or depleted zone as illustrated.3 .

             By selectively doping a silicon wafer, p-n junctions can be formed, creating devices ranging from diodes and transistors to solar cells and LEDs. .

             To form a p-n junction, the p-type and n-type silicon must be brought together, which can be accomplished in two ways. First, by starting with a silicon wafer that is already p or n doped, a second dopant can be diffused through the surface to create the p-n junction. Second, by starting with an intrinsic silicon wafer, two dopants must be subsequently diffuses into the surface to create the p-n junction. Either of these paths can be accomplished through the use of thermal diffusion of the dopants.