Mihail Nedjalkov D.Sc. Publications

Charge trapping in the gate oxide of nanoscale Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) featuring an ‘atomistic’ channel doping profile has been revealed as a key concept to explain the Random Telegraph Noise (RTN) and Bias Temperature Instability (BTI) phenomena strongly affecting contemporary-technology transistors’ performance. By means of a 2D Wigner function approach we investigate the trapping of a single electron in the gate oxide of a 25nm transistor as well as the scattering effects due to discrete dopants in the channel. We demonstrate the ability of our simulation methodology to capture not only the quantum nature but also the transient behavior of charge-trapping and scattering phenomena.
The trap is introduced by means of an approximated 2D square quantum well at the channel/oxide interface. The simulation consists of an initial 2D Gaussian wave packet with low initial energy and an initial velocity oblique to the direction of the silicon oxide. The trap inside the oxide is of rectangular shape with zero bottom energy. A high barrier is placed at the interface between the channel and the oxide. Thus, a packet can enter the trap zone only by a quantum tunnelling process. The dynamics of the charging process provides information about the time constants of the phenomenon and the dependence on the size and shape of the trap.
A single dopant affects the device characteristics in a way depending on the position inside the channel. When evaluating this dependence, drift-diffusion models account for the long range Coulombic forces via the Poisson equation. Short range forces may be accounted by Boltzmann or Wigner models via a local potential barrier (scattering centre) associated with the dopant. Simulations demonstrate a striking difference between the two approaches. In the former case, only particles in the proximity of the barrier are affected, namely, only those hitting it’s surface are scattered back. In contrast, in the quantum case, the vicinity of the centre remains barely populated. and the transport in the whole cross section of the channel is affected. This difference is associated with the nonlocal character of the quantum interaction. In contrast to classical particles driven by the first derivative of the electric potential, according to the Wigner theory, quantum particles are affected by the rest of the potential derivatives. The quantum non-locality makes the interaction with the scattering centre a prominent effect that eventually reduces the current in the device.