Contactless Method to Measure 2DEG Charge Density and Band Structure in HEMT Structures

We present a contactless method capable of characterizing a high electron mobility transistor (HEMT) heterostructure at the wafer stage, right after its growth, before any production process has been attempted, to provide the equilibrium band structure and the density of charge of the 2-D electron gas in the quantum well (QW). The method can thus evaluate critical transistor parameters and help to screen out low performance wafers before the actual fabrication. To this end, we use a simple optical spectroscopy at room temperature that measures the surface photovoltage band-edge responses in the heterostructure and uses a model that takes into account the effect of the built-in electric fields on optical absorption in the layers and heterojunctions to evaluate bandgaps, band offsets, and built-in fields. The QW charge is then calculated from the built-in fields. The main advantage of the method is in its capability to provide information on all the different layers in the typical heterostructure simultaneously in a single measurement. The method is not limited to the HEMT structure but may be used on any other heterostructure. It opens the door for a new type of characterization methods suitable for the post-silicon multi-layer multi-semiconductor heterostructure device era.