The study of deep levels in semiconductors has seen considerable growth in recent years. Many new techniques have become available for investigating both the electronic properties of deep levels and the chemical nature of the defects from which they arise. This increasing interest has been stimulated by the importance of the subject to device technology, in particular those microwave and opto-electronic devices made from GaAs, InP and their alloys. While previous conferences have covered specialist areas of deep level technology, the meeting described here was arranged to draw together workers from these separate fields of study. The following papers reflect the breadth of interests represented at the conference. For the sake of uniformity we have chosen the English alternative where an American expression has been used. We have also sought to improve grammar, sometimes without the approval of the author in the interests of rapid publication. The Editor wishes to thank the referees for their ready advice at all stages, Paul Jay who helped with many of the editorial duties and Muriel Howes and Lorraine Jones for rapid and accurate typing.

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Section 1: Invited papers.- Whither chromium in gallium arsenide?.- Key electrical parameters in semi-insulating materials; the methods to determine them in GaAs.- Modelling of a multi-valent impurity, such as GaAs:Cr.- Review of techniques for epitaxial growth of high-resistivity GaAs - Growth systems, problems and substrate effects.- Section 2: Preparation and growth of semi-insulating GaAs and InP.- The growth and properties of large semi-insulating crystals of indium phosphide.- Low pressure organometallic growth of chromium-doped GaAs buffer layers.- Large diameter, undoped semi-insulating GaAs for high mobility direct ion implanted FET technology.- Growth and characterization of semi-insulating GaAs for use in ion implantation.- The influence of substrate properties on the electrical characteristics of ion implanted GaAs.- Heat treatment behaviour of Cr implanted in GaAs SI material.- Redistribution and vaporization of Cr impurities in semi-insulating GaAs.- A model for thermal conversion of semi-insulating GaAs.- Hydrogen ion bombardment of GaAs for device isolation.- Influence of chromium redistribution on the electrical properties of Se and Zn implanted Cr-doped substrates.- Influence of annealing on the electrical properties of semi-insulating GaAs.- Redistribution of S and Cr in thermally annealed, sulphur implanted, semi-insulating GaAs.- Section 3: Investigations of semi-insulating materials.- Photoconductivity, photoluminescence and Zeeman spectroscopy of Cr in GaAs, GaP and InP.- Photoconductivity studies of Cr deep acceptors in InP.- Photoluminescence studies of deep impurity states in Fe-doped InP.- Photoluminescence and photoconductivity study of the 1.10 eV energy level in Fe-doped InP.- The cathodoluminescence of Cr-doped GaAs prepared by MOCVDepitaxy.- True mobilities in semi-insulating O- and Cr-doped GaAs.- Magneto-optical measurements on the 0.839 eV emission in GaAs:Cr.- Charge-carrier transport in semi-insulating InP surface layers.- Cr redistribution in epitaxial and implanted GaAs layers.- Investigations of Cr in GaAs using phonons.- Correlation between EPR assessed Cr2+ and electrical compensation in semi-insulating GaAs:Cr.- Activation of Cr1+ (3d5) level in GaAs:Cr induced by hydrostatic pressure.- Optical and thermal properties of Cr in GaAs.- Observation of very deep levels by optical DLTS.- Photoconductivity of epitaxial GaAs:Cr.- A possible interpretation of the 0.839 eV line in GaAs:Cr.- Section 4: Theoretical consideration of deep level semi-insulating systems.- Procedures for comparing the theoretical and experimental positions of energy levels of multi-electron impurities in semiconductors.- Jahn-Teller model of Cr2+:GaAs.- The application of evanescent states to deep impurities.- Complexes and their effects on III-V compounds.- Acceptor states and core shifts in AlxGa1-xAs.- Photo-ionization with phonon participation in semiconductors.- Influence of deep levels on Schottky barrier formation.- Impact ionization and Augereffects involving traps.- Section 5: Influence of deep levels on device properties and material characterization.- Electron mobility calculations for Fe-doped InP.- Material and structure factors affecting the large-signal operation of GaAs MESFETs.- Deep level measurements of layers on semi-insulating GaAs substrates by means of the photofet method.- Dark capacitance, photocapacitance, dark conductance and photoconductance transients on GaAs MESFETs.- Semi-insulating GaAs - a user's view.- Interface effects on noise temperature of ungated GaAs-MESFETs.- Selenium ionimplanted GaAs MESFETs.- High resistivity layers in GaP, GaAs and GaAsP using electron bombardment.- Concluding remarks.- Author index.