Positron mobility measurements carried out on semi-insulating GaAs, using the Doppler shift in annihilation radiation technique, show a sharp transition from a high mobility value -120 cm'-V ' s ' to a lower value -45 cm' V ' s ' just below room temperature. The temperature of the transition is found to be dependent on the frequency of the applied AC bias. We show that this effect is an artifact due to the thermal ionization of the EL2 deep donor state, which in its ionized state forms a positive

Positron lifetime measurements performed on Au/GaAs samples at room temperature with an applied square-wave ac bias show a frequency dependent interface related lifetime intensity that peaks around 0.4 Hz. The observation is explained by the ionization of the deep-donor level EL2 to EL2 ϩ in the GaAs region adjacent to the Au/GaAs interface, causing a transient electric field to be experienced by positrons drifting towards the interface. Without resorting to temperature scanning or any Arrhenius plot the EL2 donor level is found to be located 0.80Ϯ0.01Ϯ0.05 eV below the conduction-band minimum, where the first error estimate is statistical and the second systematic. The result suggests positron annihilation may, in some instances, act as an alternative to capacitance transient spectroscopies in characterizing deep levels in both semiconductors and semi-insulators.

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The I-V characteristics of Aulsemi-insulating InP (100) under reversed bias have been measured between 230 K and '290 K. A simple model based on the thermionic field emission theory (TFE) is proposed to describe this system. Based on this model, the value of the bandgap obtained from the C V characteristics is in reasonable agreement with the literature. At room temperature, the corresponding fitted values of the barrier height Qb and the series bulk resistance R, are 0.68 0.05V and 1.32 x IO'R respectively.

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