A novel Zn diffusion technique in n-GaSb substrate from a low temperature chemical bath deposited ZnS layer has been developed to obtain high breakdown voltages. Junctions formed by this technique have breakdown voltages of similar to 18.5V, low reverse leakage current (0.01-0.03 A/cm(2) at -3 V), excellent reverse current saturation and ideality factor of similar to 1.3. The high breakdown voltages obtained are due to the co-doping of zinc and sulfur from the ZnS film. Sulfur forms shallow and deep levels that compensate the p-doping of zinc. The non-linear relation of the inverse of the zero-bias resistance area product (1/R(0)A) versus perimeter to area ratio (P/A) in these diodes indicates surface leakage is the dominant leakage mechanism. US has been used to passivate the mesa photodiodes. After passivation, the 1/R(0)A product reduces from 0.3 to 0.02 Omega(-1) cm(-2) for a 150 mu m diameter device. The 1/R(0)A product is also independent of the diode dimension confirming effective passivation. ZnS surface passivation on the mesa walls is not effective and is found to increase the leakage current. (C) 2007 Elsevier B.V. All rights reserved.