IMPATT and Gunn diodes; light sources, bipolar electronics and optoelectronic circuits; FET; HEMT; integrated optoelectronics; solar cells; mechanical, thermal, piezoelectric and electro-optic properties; carrier transport; band structure; offsets; linear and nonlinear optical constants; photoluminescence; Raman and IR reflection spectra; photoconductivity; defects; DLTS; passivation; etching; ion implantation; metallisation; diffusion; oxidation; plasma and ion beam processing.