Contamination of ions causes the largest problem in semiconductors. For example, sodium interferes with the normal operation of semiconductors devices by rapidly drifting through silicon dioxide towards a region with a negative voltage, which then gives rise to excessive leakage of current (current moves where it is not wanted).

The process of ion exchange, or deionization was used almost exclusively in water purification prior to the last few years. Ion exchange is the removal of positive and negative ions using activated resins. A typical ion exchanger contains the following elements:

1. Chemical treatment (often chlorination) to kill organism present in feed water.
2. Sand filter to remove particles from the incoming water.
3. Activating charcoal filter to remove free chlorine and traces of organic matter.
4. Diatomaceous earth filter to retain additional contaminants.
5. Anion exchange to remove strongly ionized acids such as sulfuric, hydrochloric, and nitric acid.
6. Mixed bed polisher that contains both cation and anion resins to remove any ions missed by previous exchange filters.
7. Sterilization to control growth of bacteria; often achieved by chlorination or ultraviolet light.
8. Filter to removes any residual particles left in the wafer prior to its first use.

© 2007 Arizona Board of Regents for The University of Arizona