The traditional test for assessing the quality of a semiconductor, called the Hall method, measures the number of freely moving charge carriers (electrons and holes) in a material. But it is fairly time-consuming to perform.
A new, quicker technique makes this measurement by exposing the semiconductor to terahertz (THz) light, which is much redder than the human eye can see. The THz light shines straight through pure silicon and other semiconductor materials. But it is absorbed by the freely moving electrons and holes (added to the material by doping it with impurities or by exposing it to certain frequencies of light).
The more charge carriers in the material, the less THz light shines through the other side. The method measures not only how many charge carriers there are in the material, but also how easily they move around.