We have investigated the first in a series of compositionally graded quasi‐insulator/semiconductor heterojunction (HJ) capacitors, intended to provide low interface state densities, low dc leakage currents, and high effective surface barriers. Molecular‐beam epitaxy (MBE) was used to grow compositionally graded InxAl1−xAs quasi‐insulators withx≤0.52 deposited on semiconducting In0.53Ga0.47As layers lattice matched ton+‐InP substrates. Interface states derived from electric field and temperature‐dependent capacitance‐voltage measurements have densities in the order of ≊1011/cm2 eV and have time constant distributions which are strongly dependent on compositional gradient. The surface barrier height also depends on the compositional gradient and the leakage current is decreased by three orders of magnitude from 10−6to 10−9A for a reverse bias of 1 V. Heterojunction insulated gate field‐effect transistors (HIGFETs) with an intrinsic transconductancegm=125 mS/mm were obtained on a 5‐μm‐long gate structure.