A silicon purity of 12N only exists in the world of marketers

Although claimed again and again, a purity of 99.9999999999% (twelve nines or briefly 12N) is not achieved in the production of electronic-grade polysilicon for semiconductors.

At the CRU Silicon Market Forum 2025 in the last week of September, it happened once again: U.S.-based polysilicon manufacturer Hemlock Semiconductor claimed a purity of “up to 12 nines” (99.9999999999% – a number with twelve nines, briefly called 12N) for its electronic-grade polysilicon used in semiconductors – see the image above. To make a long story short: Such a purity does only exist on Powerpoint slides and websites, not in the real world of polysilicon production.

First of all, when market participants talk about a silicon purity of x nines, they only refer to the purity regarding the concentration of dopants; other impurities (carbon, oxygen, hydrogen and metals) are not taken into account in this nomenclature.

Silicon can be doped negatively with so-called donors, which are elements with five electrons in the outer shell of the atom, such as phosphorus, arsenic or antimony. When silicon, which has four electrons in its outer shell, is doped with such an element, four of the donor’s five outer electrons form bonds with neighboring silicon atoms in the crystal lattice, but the fifth electron becomes free and creates an n-type semiconductor.

Conversely, silicon is doped positively with so-called acceptors, which are elements with three electrons in the outer shell, such as boron or aluminum. When integrated in the silicon crystal lattice, acceptors cause electron holes and create a p-type semiconductor.

A dopant concentration of only one part per trillion atoms does not exist

In the production of semiconductors, you want to dope silicon as precisely as possible; hence; the concentration of already existing doping elements in the silicon material must be extremely low.

A purity of 12N is equivalent to a dopant concentration of one part per trillion atoms (1 ppt_a). However, measurement data of electronic-grade material from Wacker, the world’s largest manufacturer of polysilicon for semiconductors, shows that the lowest sum of donors and acceptors amounts to 9 ppt_a and the lowest net dopant concentration (donor concentration minus acceptor concentration) is 5 ppt_a.

So, if one refers to the net dopant concentration, one might talk of a purity of 11.5N in the best case; more realistic is a level of 11N. Yet, Wacker boasts of a silicon purity of 12N on its website.

If you hear or read the marketing claim of 12N, don’t believe it. Such a silicon purity simply does not exist. Even more out of touch with reality is the designation of “12N/12N+”, which price data provider PVinsights uses for the highest quality of solar-grade polysilicon. And when a Chinese author babbles about a polysilicon purity of “13 to 15 nines” as seen recently, it becomes entirely ridiculous.