Voltage bias-enhanced photoelectrochemical (PEC) etching of GaN is demonstrated for fabrication of undercut III-V microelectromechanical system (MEMS) structures. The use of voltage bias was found to enhance lateral etching of GaN under opaque Ti etch masks, resulting in a regular and uniform undercut. The etch profiles obtained using bias-enhanced PEC etching are dramatically different from those obtained with either conventional unbiased PEC etching or through-wafer illuminated etching. Bias-enhanced etching was observed to result in improved morphology for typical MEMS applications. Bias-enhanced etching is demonstrated for the formation of fully released Ti cantilevers on GaN, as well as undercut structures. (c) 2006 American Vacuum Society.