The electric field dependence of the microwave noise temperature and t
he diffusion coefficient are found to be significantly different for t
wo sets of AlGaAs/GaAs structures with different spacer thickness and
aluminium mole fraction. In the samples with a 'wide' quantum well (QW
), one maximum of the diffusion coefficient is observed at fields belo
w the threshold for the intervalley diffusion, but two maxima are char
acteristic of a 'narrow' QW with an intersubband gap wider than the k(
B)T0. A microscopic explanation of the observed noise spectra of AlGaA
s//GaAs is given and used to determine kinetic parameters of the heter
ostructures.