  We study magneto-electric properties of single electron traps in
metal-oxide-semiconductor field effect transistors. Using a microscopic
description of the system based on the single-site Anderson-Holstein model, we
derive an effective low energy action for the system. The behavior of the
system is characterized by simultaneous polaron tunneling (corresponding to the
charging and discharging of the trap) and Kondo screening of the trap spin in
the singly occupied state. Hence, the obtained state of the system is a hybrid
between the Kondo regime, typically associated with single electron occupancy,
and the mixed valence regime, associated with large charge fluctuations. In the
presence of a strong magnetic field, we demonstrate that the system is
equivalent to a two level-level system coupled to an Ohmic bath, with a bias
controlled by the applied magnetic field. Due to the Kondo screening, the
effect of the magnetic field is significantly suppressed in the singly occupied
state. We claim that this suppression can be responsible for the experimentally
observed anomalous magnetic field dependence of the average trap occupancy in
${\rm Si-Si0_2}$ field effect transistors.
