We use the Hall effect and a new charge-transfer technique to study molecular beam epitaxial GaAs grown at the low substrate temperatures of 300-450 degrees C. Layers grown from 350-450 degrees C are semi-insulating (resistivity greater than 10(7) Omega-cm), as grown, because of an As-Ga-related donor (not EL2) at E(c)-0.65 eV. The donor concentrations are about 2 x 10(18) cm(-3) and 2 x 10(17) cm(-3) at growth temperatures of 300 and 400 degrees C, respectively, and acceptor concentrations are about an order of magnitude lower. Relatively high mobilities (similar to 5000 cm(2)/V s) along with the high resistivities make this material potentially useful for certain device applications.