Physics · Ch 14 — Semiconductors
Diffusion
Diffusion
When n-type and p-type semiconductor materials are first fused together, the number of free electrons on the n-side is initially far larger than on the p-side, and, symmetrically, the number of holes on the p-side is initially far larger than on the n-side. This produces a large DENSITY GRADIENT for each carrier type across the junction, and this gradient is what drives DIFFUSION: electrons migrate from the n-side across the junction into the p-side, where they fill up existing holes there (this recombination effectively produces negative ions on the p-side); at the same time, holes effectively migrate from the p-side into the n-side (Fig. 14.16). …
What this figure shows. A diagram of the p-n junction (as in Fig. 14.15) with arrows drawn ONLY in the narrow region immediately straddling the junction line, showing the initial diffusion process: one set of arrows shows free electrons crossing FROM the n-side INTO the p-side (down the electron concentration gradient), and a second set of arrows shows holes crossing FROM the p-side INTO the n-side (down the hole concentration gradient) -- the two carrier flows drawn moving in opposite physical directions across the same junction boundary. Away from this narrow junction region the bulk p-side and n-side are left largely undisturbed, emphasising that diffusion is a localised, jun …