Basic electronic properties of semiconductors are determined by defects and
impurities. Extremely small relative concentrations may have an effect, if
the impurity in question gives rise to a localized electron state having a
n energy within the band gap of the semiconductor. Among the techniques ava
ilable to characterize band gap states, the present paper focuses on Deep L
evel Transient Spectroscopy (DLTS). To derive a definite chemical identific
ation of the band gap states detected, radioactive isotopes are used as a t
racer. Characteristic concentration changes of band gap states (detected by
repeated DLTS measurements during the elemental transmutation) clearly rev
eal the involvement of a radioisotope in the formation of a certain defect
level. The key issues of a radiotracer experiment are the radioactive dopin
g process and the interpretation of transmutation-induced phenomena. Critic
al aspects are illustrated on the basis of recent radiotracer-DLTS studies
in the semiconductors silicon and silicon carbide.