In this paper, we report a reduction in the access resistance of InP/InGaAs composite-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a back-source electrode. The source region has two electrodes. The source electrode on the surface side is connected to the channel through a doped layer and supplies the electrons. The back-source electrode is constructed under the channel layer and is insulated from the doped layer in order to avoid current leakage. The function of the back-source electrode is to increase the carrier concentration in the channel layer of the source region. In the simulation, the electron density in the channel layer is almost doubled by the effect of the back-source voltage. The fabricated III-V MOSFET has a channel length of 6 µm. A 6% increase in the maximum drain current density (I_d) and a 6.8% increase in the transconductance (g_m) (V_d=2V) are observed. The increase in the carrier density in the channel is estimated to be 20% when the applied voltage of the back-source electrode is 6V.