In this work， 100 nm gate-length InP-based high electron mobility transistors （HEMTs） with a composite InGaAs/InAs/InGaAs channel are fabricated. DC measurements indicate that the InAs channel enhances transconductance but shifts the peak point toward lower V_gs under high V_ds bias. Peak separation analysis reveals the DC transconductance curve is composed of two components： the gate-controlled transconductance and the impact-ionization-induced additional transconductance. Further analysis demonstrates that the anomalous shift originates from the channel impact ionization intensity variation， which is caused by changes in the gate-drain electric field rather than the carrier density in the channel. Two additional current sources are introduced in the small-signal model to characterize the impact-ionization-induced transconductance， and the numerical variation trends of their parameters are consistent with the peak separation results， which validate the mechanism''s correctness. RF measurements confirm that the DC transconductance enhancement does not effectively improve RF characteristics， which is attributed to the ionization-induced transconductance having a time constant significantly larger than that of conventional transconductance components. These findings provide a theoretical foundation for controlling the impact-ionization.