A dual-mode In0.34Al0.66Sb0.15/In0.75Ga0.25As/InP inverted delta-doped heterostructure field-effect transistor (HFET) with field-effect transistor mode (FET mode) and pronounced N-shaped negative-differential resistance mode (NDR mode) characteristics has been studied by low-pressure metalorganic chemical vapor deposition. By using the shallow ohmic source contact, an NDR mode HFET with a peak-to-valley current ratio of 5.8 (5.4) at 300 (77) K can be obtained. When the source is with deep ohmic contact, an FET mode HFET with a maximum extrinsic transconductance of 216 (250) mS/mm at 300 (77) K, as well as a high-breakdown voltage up to 40 V can be achieved for a gate length of 1.5 mu m by using the high-energy gap and high-barrier height (>0.73 eV) In0.34Al0.66As0.85Sb0.15 Schottky layer.