The effects of in‐plane lattice mismatch have been studied for Ga0.92In0.08As(p+)/GaAs(n)/GaAs(n+) diodes. Different in‐plane mismatch at thep–njunction was introduced by a variation of the GaInAs layer thickness (h=0.1, 0.25, 0.5, and 1 &mgr;m). Capacitance‐voltage (C‐V) measurements with different frequencies show a higher‐frequency dispersion for a greater lattice‐mismatched sample. From the frequency dependence of theC‐Vcurve, single‐level charged interface‐state density (Ns) was estimated using the effective parallel capacitance and conductance components. The average charged interface densityNsswas also estimated using Voltage‐intercept (Vint) method.Nssshows a linear dependence on the in‐plane mismatch. The charged interface state density is approximately 2.7 &Dgr;a∥/a30for partially lattice‐relaxed heterojunctions. For the 1 &mgr;m sample, the forwardI‐Vcharacteristic shows quasi‐Fermi level pinning effect. Admittance spectroscopy measurement gives an equilibrium Fermi energy at aboutEv+0.36 eV with hole capture cross sectioncp=2.7×10−15cm2for the 1 &mgr;m sample and atEv+0.21 eV andcp=2.4×10−16cm2for the 0.5 &mgr;m sample.