  We have studied the electron transport in SiO${}_2$(Co)/GaAs and
SiO${}_2$(Co)/Si heterostructures, where the SiO${}_2$(Co) structure is the
granular SiO${}_2$ film with Co nanoparticles. In SiO${}_2$(Co)/GaAs
heterostructures giant magnetoresistance effect is observed. The effect has
positive values, is expressed, when electrons are injected from the granular
film into the GaAs semiconductor, and has the temperature-peak type character.
The temperature location of the effect depends on the Co concentration and can
be shifted by the applied electrical field. For the SiO${}_2$(Co)/GaAs
heterostructure with 71 at.% Co the magnetoresistance reaches 1000 ($10^5$ %)
at room temperature. On the contrary, for SiO${}_2$(Co)/Si heterostructures
magnetoresistance values are very small (4%) and for SiO${}_2$(Co) films the
magnetoresistance has an opposite value. High values of the magnetoresistance
effect in SiO${}_2$(Co)/GaAs heterostructures have been explained by
magnetic-field-controlled process of impact ionization in the vicinity of the
spin-dependent potential barrier formed in the semiconductor near the
interface. Kinetic energy of electrons, which pass through the barrier and
trigger the avalanche process, is reduced by the applied magnetic field. This
electron energy suppression postpones the onset of the impact ionization to
higher electric fields and results in the giant magnetoresistance. The
spin-dependent potential barrier is due to the exchange interaction between
electrons in the accumulation electron layer in the semiconductor and
$d$-electrons of Co.
