Effects of strong correlations in single electron traps in field-effect transistors

We study effects of strong electron-electron and electron-phonon correlations in single electron traps in metal-oxide-semiconductor field effect transistors (FETs). In order to explain the strong suppression of single electron tunneling in the trap, we introduce a model in which the excess charge of the trap couples to a local lattice deformation. By using nonperturbative techniques, 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 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 Si-Si0₂ FETs.