Bell Labs zone melting semiconductor purification (1950s)

Bell Labs technician Pfann invents zone melting in the 1950s to purify silicon and germanium to 6-9s (one part per million) for early semiconductors. Still the basis of high-purity silicon production.

It was a big deal when I was a student because that was what happened in the 1950s when they first invented semiconductors, at Bell Labs. You needed very pure silicon or germanium. By very pure, we mean six or seven nines pure. What do we mean by 6-9s pure? 99.9999% — one part per million in purity. Seven nines is a tenth of a part per million, 100 parts per billion. That's what you need in silicon or germanium to make good semiconductors. How could you make that? The things will start to react with the wall of the crucible and give you that type of impurity level.