  We present a method to obtain quantitative profiles of surface state charge
density and monitor its dynamics under various stress conditions in high
electron mobility transistor (HEMT) devices. The method employs an optical
spectroscopy of the channel current at various bias conditions. We test the
method on a classical AlGaN/GaN HEMT structure. To analyze the results, we
propose a model, according to which the energy distribution of the surface
charge density may be obtained from the derivative of the channel photocurrent.
The proposed method is applied to fully fabricated transistors and can be
measured under any device bias combination. This way, it is possible to explore
the effect of device operating conditions on the surface state charge. This
feature should be especially useful in studies of the various surface charge
migration effects in nitride HEMTs. An important byproduct of the method is a
quantitative assessment of the energy position of the surface Fermi level and
its dynamics under various bias conditions.
