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This letter focuses on the critical device-level linearity issues resulting from out-of-band terminations for reliable distortion characterization in future Universal Mobile Telecommunications System-Long Term Evolution (UMTS-LTE). Using Volterra series technique, the key distortion sources arising from the envelope and harmonic components in 0.5-mm GaN HEMT were identified using commercial and in-house bias tees. With the designed in-house bias tee, the baseband performance, in comparison with the commercial bias tee, is tested through drain-bias sensing. In reference to the commercial bias tee, up to 99.3% reduction in drain modulation is achieved using the in-house bias tee. Memory-effect characterization of GaN HEMT exemplified the implications of baseband and second-harmonic load terminations, which was theoretically confirmed through Volterra series technique. Using the in-house bias tee, under two-carrier wideband code-division multiple-access excitation, up to 47-dBc 3rd-order intermodulation ratio (IMR3) is achieved at 13.5-dB backoff. This has resulted in a 5-dB IMR suppression together with the minimization of intermodulation-distortion asymmetry, confirming the possibility to achieve the 3rd Generation Partnership Project linearity specification at the device level.