Power and linearity characteristics of GaN MISFETs on sapphire substrate

The improvement of device performance arising from the adoption of a MIS gate structure in GaN field-effect transistor (FET) is presented. GaN MISFET/MESFET devices were fabricated on sapphire substrate with and without the insertion of a thin SiN layer on device surface. The MISFET device showed improved device characteristic due to significant reduction in device gate leakage with respect to the standard MESFET structure. Measured power and linearity performance showed promising results. Under single-tone testing at 4 GHz, device yielded saturated output power 6.2 W/mm with 55% peak power added efficiency. When tested with two-tone signal device maintained a carrier to third order intermodulation ratio of 30 dBc up to power levels of 1.8 W/mm with 40% power added efficiency.

GaN films were grown on c‐plane sapphire substrates by low pressure metalorganic chemical vapor deposition using a GaN buffer layer grown at lower temperatures. The quality and surface morphology of the GaN films were strongly affected by the surface pretreatment of the sapphire substrate before the growth of the GaN buffer layer and the GaN epitaxial layer. The films grown on pre‐nitrided sapphire substrates exhibit improved electrical and optical properties over those of the films grown on unnitrided substrates even though the surface morphology became rougher. We have achieved a carrier concentration in the range of 2×1016–5×1017 cm-3 and electron mobility in the range of 250–300 cm2/V s at room temperature. The GaN films were evaluated by photoluminescence, Hall measurement, x‐ray rocking curve, and transmission electron and scanning electron microscopies.


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