Scanning transmission electron micrographs were used to investigate the gate metal sinking effect in 0.15 ?m GaAs PHEMT MMICs subjected to elevated temperature lifetesting. Gate metal sinking causes a decrease of the Schottky barrier height, therefore reducing the Schottky diode's forward turn-on voltage. The progressive gate metal sinking eventually leads to a drastic increase in the three-terminal forward gate current. Accordingly, a distinct degradation phenomenon in MMICs due to high forward gate current and gate resistors was observed. The degradation causes a decrease in transconductance and small signal gain, following the earlier gate sinking. In conclusion, a dependence of DC performance and small signal gain on the forward gate current was observed in GaAs PHEMT MMICs.