Extended microtunnels with triangular cross sections in thick GaN films were demonstrated using wet chemical etching on specially designed epitaxial lateral overgrowth structures. For tunnels along the < 1 (1) over bar 00 > and < 11 (2) over bar0 > directions of GaN, the {11 (22) over bar} and {10 (11) over bar} facets are the etch stop planes with activation energies of 23 kcal/mol determined by wet chemical etching. The axial etching rate of the tunnels in the < 1 (1) over bar 00 > direction is twice as large than that along the < 11 (2) over bar0 > direction. The highest etching rate of the tunnels in the axial direction is 1000 mu m/h. (C) 2008 The Electrochemical Society.