Protrusions 2 each having a shape of a pyramid or a truncated pyramid are regularly arranged on a growing face 1a of a seed crystal 1 composed of gallium nitride single crystal. The mixer has first and second inputs and has first and second outputs. It is preferable that the loading density of the catalyst is 40 to 70%. The first contact is arranged at the first main surface area and the second contact is arranged at the second main surface area. A wall between the ends includes first and second re-entrant wall portions facing the one end, and the grommet can be provided with a recess in the wall adjacent the flange for receiving the peripheral edge of the opening through the substrate andor can be adhesively bonded to the substrate by an adhesive interposed between the flange and substrate. The matrix which has a fine grain crystalline structure of predominantly 20 microns or less in size.