  We report on extremely sensitive measurements of changes in the microwave
properties of high purity non-intentionally-doped single-crystal semiconductor
samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when
illuminated with light of different wavelengths at cryogenic temperatures.
Whispering gallery modes were excited in the semiconductors whilst they were
cooled on the coldfinger of a single-stage cryocooler and their frequencies and
Q-factors measured under light and dark conditions. With these materials, the
whispering gallery mode technique is able to resolve changes of a few parts per
million in the permittivity and the microwave losses as compared with those
measured in darkness. A phenomenological model is proposed to explain the
observed changes, which result not from direct valence to conduction band
transitions but from detrapping and retrapping of carriers from impurity/defect
sites with ionization energies that lay in the semiconductor band gap.
Detrapping and retrapping relaxation times have been evaluated from comparison
with measured data.
