We report a promising N+-InGaP/δ(P+)-InGaP/n-GaAs hetero-planar-doped barrier, which isused to fabricate both high-breakdown and self-aligned T-gate(SAT-gate) field-effect transistors (FETs). The characteristics of the devices and comparisons with previous reports are discussed. The enhanced conduction- and valence-band offsets associated with the new hetero-planardoped barrier show high-breakdown behaviour. In addition, highselective etching between InGaP and GaAs layers togetherwith an ohmic gate allows the fabrication of a SAT-gate with areduced gate-length of 0.8 µm. In the case of ahigh-breakdown FET, the drain-source breakdown voltage is ashigh as 32 V. In addition to competitive direct-current (dc) performances, areduced knee voltage and improved frequency performances areobtained in SAT-gate FETs. The available unity-current-gainand unity-power-gain frequencies are, respectively, 19.5 and30 GHz achieved as a 0.6 µm gate is obtained by forming a 1 µmmetal gate. Furthermore, all the measured SAT-gate FETsexhibit high-linearity and high-uniformity dc andalternating-current performances.