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We describe information which has been obtained on point defects detected in various types of GaAs materials using electron paramagnetic resonance as well as electrical and optical techniques. From a comparison of their characteristics and those of simple intrinsic defects (As and Ga interstitials, vacancies and antisites) it is concluded that native defects are not simple intrinsic defects, with the exception of the antisites, but complexes formed by the interaction of such defects between themselves or with impurities. Particular emphasis is given to the As antisite complexed with an As interstitial, the so-called EL2 defect which plays a major role in the electrical properties of bulk materials. Differential thermal analysis, positron annihilation, and x-ray diffraction demonstrate that bulk materials contain a large concentration of vacancy-related defects and As precipitates located along dislocations which play the role of gettering centers. Presumably, bulk materials also contain other As clusters of various sizes although only the smallest ones (EL2) have been detected. All these As clusters are sources of As interstitials which play an important role in thermal treatments. As to semi-insulating materials, their electrical properties result mainly from the compensation between the double donor, called EL2, associated with the As antisite and the double acceptor ascribed to the Ga antisite.

The electronic conductivity of synthetic boron-doped diamonds is studied in the temperature range 12-1300 K. It is shown that in the low-temperature range {below 150-100'K) the results are best interpreted in terms of variable-range hopping. An attempt to evaluate quantitatively the parameters which characterize this hopping mechanism is made using the boron concentration and the degree of compensation determined from the variation of conductivity in the high-temperature range, coupled with optical-absorption measurements. The efFect of irradiation with energetic electrons, which introduced new compensating centers, is investigated and can also be explained in terms of the theory for the hopping mechanism.