  We present a contactless method that is capable of characterizing a high
electron mobility transistor 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-dimensional
electron gas in the quantum well. 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
quantum well 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 high electron mobility transistor
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.
