  Modern bandgap engineered electronic devices are typically made of
multi-semiconductor multi-layer heterostructures that pose a major challenge to
silicon-era characterization methods. As a result, contemporary bandgap
engineering relies mostly on simulated band structures that are hardly ever
verified experimentally. Here, we present a method that experimentally
evaluates bandgap, band offsets, and electric fields, in complex
multi-semiconductor layered structures and it does so simultaneously in all the
layers. The method uses a modest optical photocurrent spectroscopy setup at
ambient conditions. The results are analyzed using a simple model for
electro-absorption. As an example, we apply the method to a typical GaN high
electron mobility transistor structure. Measurements under various external
electric fields allow us to experimentally construct band diagrams, not only at
equilibrium, but also under any other working conditions of the device. The
electric fields are then used to obtain the charge carrier density and mobility
in the quantum well as a function of the gate voltage over the entire range of
operating conditions of the device. The principles exemplified here may serve
as guidelines for the development of methods for simultaneous characterization
of all the layers in complex, multi-semiconductor structures.
