GaN-based half bridge power conversion circuits employ control, support and logic functions that are monolithically integrated on the same devices as the power transistors. The polishing pad includes a top layer formed of a porous soft material, a bottom layer formed of a rubber elastomer, and an intermediate layer formed of a hard plastic sheet. In one embodiment, a server receives a request for an application from the wireless device, enables the application, and tracks the time for which the application is enabled. The pool of liquid 18 may include solid matter 19, or even contain substances such as dissolved solids or salts. Thus, when the second etch stop layer is patterned using an etchant, the first etch stop layer covers the semiconductor pattern, thereby preventing the semiconductor pattern from being etched by the etchant. For a reduction of loading times, the transfer table retains substantially in its operating position during the loading of the charging plates and at least the region of the loading aperture above the transfer table is closed following each loading push.