Carrier density saturation in a image heterostructure

We observe a strong saturation of the carrier density in the quantum well of a Ga0.25In0.75As/InP MISFET at positive gate voltages. Using a self-consistent Schrödinger/Poisson solver, we model the band structure and find that the saturation is caused by the population of charge states between the gate and the quantum well. We discuss the impact of these charge states on the transport properties, and present a fabrication method that avoids parallel conduction in this heterostructure.