A field-plated (FP) AlGaN/GaN high electron mobility transistor (HEMT) was fabricated. Investigations on the linearity characteristics were performed through two-tone and wide band code division multiple access (WCDMA) modulated excitations. The FP-HEMT exhibited an improved breakdown voltage of 160 V compared with that of the conventional HEMT. Additionally, a higher output power of 25.4 dBm with 43% power added efficiency at a 30V drain bias at 2 GHz was achieved. When biased at 30 V and 15mA/mm current density, the third-order intermodulation (IM3) level was measured to be -27.1 dBc (at P1dB) and the adjacent channel power rejection (ACPR) was -33.8dBc (at P 1dB) under WCDMA modulation at 2GHz. Measurement results revealed that the field-plated structure improved the linearity performance over the conventional structure at high output power levels even beyond P1dB.