Trap Assisted Tunneling and band interaction using the non-radiative multi phonon model

Abstract

Bias temperature instability (BTI) and trap assisted tunneling (TAT) play an important role for the reliability of semiconductor devices. Both phenomena can be traced back to defects. BTI denotes the reversible and irreversible change in threshold voltage with temperature and bias stress, TAT denotes leakage currents resulting from electron capture and emission of defects. A number of models aiming to describe BTI have been developed. Initially, it was assumed that BTI is a result of dangling bonds at silicon atoms near the silicon-silicon dioxide interface and the hydrogen species dissociating from these atoms. However, newer results show that at least the reversible part of BTI is caused by the capture of charge in pre-existing defects and that the charge transfer can be described by non-radiative multi phonon (NMP) transitions. This work is based on an existing NMP model. The existing rate equations of the NMP model describe the charge exchange between semiconductors or metals and insulators. Additional, field dependent rate equations to conduction and valence band near the defect are necessary for simulation of traps in semiconducting materials, e.g. for GaN/AlGaN high electron mobility transistors (HEMTs) and transistors with highk dielectrics. An extension to the model enables the simulation of static tunnel and transient displacement currents for individual traps. Mathematical approximations in the rate equations allow an estimation of the rates for different regimes of the electric field. The temperature dependence of the band rates follows an Arrhenius-law, whereby the apparent thermal barrier decreases with field strength. A comparison of simulations of capture and emission times with and without band rates shows for which situations the band rates can not be neglected. With the extended rates reverse leakage currents in a GaN/AlGaN HEMT could be simulated. The results show Frenkel-Poole like behaviour for low fields and high temperatures and Fowler-Nordheim like behaviour for high fields and low temperatures in good agreement with measurement results in literature. The transient simulation of a MOSFET shows the displacement and transport currents during stress and recovery cycles, and the contribution for each of the traps.