Abstract : Low pump power varactor diodes have been fabricated using epitaxial GaAs. Frequency cut-offs as high as 785 kmc have been obtained at break down. A detailed analysis of the parameters in influencing point-contact GaAs varactors is given. The results of a basic program to determine the feasibility of employing high energy gap semi conductors in varactor diodes are presented. The voltage breakdown and capacitance of abrupt large area P on N GaAs diodes have been deter mined as a function of resistivity. Square law behavior is observed up to breakdown. The electrical properties of P on N large area GaP diodes are also given. The HCl carrier gas tech technique and the travelling solvent method of crystal growth have been employed in the epitaxial growth of GaAs and GaP on GaAs, GaP, and Ge substrates. The properties of GaAs-GaP, GaP-Ge, and GaAs-Ge heterojunctions are presented. Low pump power varactor diodes have been fabricated using epitaxial GaP as the active semiconductor (GaP deposited on 0007 ohm-cm GaAs). Frequency cut-offs of 187 kmc at-2v have been obtained using this N on N++ configuration.