Printable semiconductors have recently gained widespread attention due to the potential they engender in enabling the fabrication of revolutionary new forms of lightweight, mechanically flexible circuits. [1][2][3][4] A primary attraction of these materials resides in the fact that they can be deposited in patterned forms onto various substrates, including low-cost plastics and even paper, by using intrinsically low-cost means of fabrication. This ability, and the novel form factors of the devices they serve to enable, may lead to new opportunities in broad areas of commercial electronics technology. [5][6][7] However, the inherent properties, notably the mobilities of carriers in materials that have been explored most extensively, serve to limit the range of these applications. Small-molecule and polymeric semiconductors [8] can be used to form transistors whose mobilities typically fall in the range of 0.1-1 cm 2 V À1 s À1 . Nanowire and nanoparticle semiconductors, [9] another printable class of semiconductor material, yield devices with mobilities of % 1-3 cm 2 V À1 s À1 .Single-walled carbon nanotubes have exceptional intrinsic transport properties [10] and, in the form of networks and arrays, have been used to form devices with more promising mobilities, ranging from % 10 to 40 cm 2 V À1 s À1 . [11] Often, these nanowire-or nanotube-based devices show effective device mobilities that are too low for many classes of circuits, due primarily to the relatively low filling factor of semiconducting elements in the channel. [6] We recently described a complementary approach based on printable forms of single-crystalline inorganic semiconductors (Si, GaAs, InP), which we refer to as microstructured semiconductors (ms-SC), for these types of applications. Here, micrometer-scale elements, which have some advantages compared to the types of nanometer-scale objects explored previously, are derived by patterning and etching high-quality, wafer-scale sources of the semiconductor. [1] This "top-down" approach to printable inorganic semiconductors provides deterministic control over the resulting shapes of the objects-wires, ribbons, rectangles, etc.-and their spatial organization across the wafer. [1,2] High fill factors, and correspondingly high effective device mobilities, can be achieved readily. This attribute enables a multitude of possibilities for new applications that use device substrates (e.g., plastics, paper, etc.) which are incompatible with the high-temperature steps that are generally required to grow and subsequently process these semiconductors.Herein, we describe procedures suitable for generating and printing collections of microribbons of GaN, or ms-GaN, onto plastic substrates. [2, 12] GaN is a material that is not generally available in the form of bulk single-crystalline wafers, the preferred forms of the semiconductors used in our earlier work. To fabricate ms-GaN, we employ instead high-quality GaN thin films that are supported on sacrificial handle wafers, materials grown by using a metal-organi...
