  When a high electron mobility transistor is illuminated, the absorbed photons
excite electron-hole pairs. The generated pairs are separated by the built in
field in such a way that the electrons end up in the quantum well generating a
photocurrent, while together with the holes that are swept towards the gate
they generate a surface photovoltage. Here, we define photon-induced
transconductance as the ratio between the surface photovoltage and the 2DEG
photocurrent under identical illumination conditions. We show that this ratio
directly yields the channel mobility and the 2DEG sheet charge density. The
photocurrent and photovoltage may vary with the wavelength of the exciting
photons. We examine and analyze optical spectra of this photon-induced
transconductance obtained from an AlGaN/GaN heterostructure for a range of
photon energies showing that the mobility is obtained only for excitation at
photon energies above the wide bandgap energy. The method offers an optical
alternative to Hall effect and to field effect mobility. Unlike Hall effect, it
may be measured in the transistor itself. The only alternative that can measure
mobility in the transistor itself measures field effect mobility, while the
proposed method mesures the same conductivity mobility as measured by Hall
effect.
