Example 7: Phosphorous Diffusion to form a PN Junction: a case study

Suppose we do a relatively shallow implant of boron into a p-type wafer, and deposit a Q₀ of 5×10¹³ phosphorus atoms/cm² atoms. We then perform an anneal diffusion at 1100°C for 60 minutes. At 1150°C, D for phosphorus seems to be about 2×10^-13 cm²/sec. In Fig. 3.41 a plot of n(x) for various times, to see how the impurities move into the semiconductor, and how the concentration at the surface, N(0) decreases as more and more of the impurities moves deeper into the wafer.

If the substrate had been doped at 10¹⁶ acceptors/cm³ where would be the location of the p-n junction between the implanted phosphorus layer, and the background boron?

Solution

About 1.2 m after 1 hour of diffusion time. You know this because for x<1.2m the phosphorus concentration is greater than that of boron, and so the material is n-type. For x>1.2m, the boron concentration exceeds that of the phosphorous, and so the material is now p-type.