Saturation Overshoot Research Articles

Experimental measurements of saturation overshoot on infiltration

Gravity‐driven fingers in uniform porous medium are known to have a distinctive nonmonotonic saturation profile, with saturated (or nearly so) tips and much less saturation in the tails. In this work, constant‐flux infiltrations into confined porous media (laterally smaller than the finger diameter, thus essentially one‐dimensional) are found to produce saturation overshoot identical to that found in gravity‐driven fingers. Light transmission is used to measure the saturation profiles as a function of infiltrating flux, porous media grain size, grain sphericity, and initial water saturation. Saturation overshoot is found to cease below a certain minimum infiltrating flux. This minimum flux depends greatly on the grain sphericity and initial water content of the media and slightly on the mean grain size. The observed saturation overshoot is inconsistent with a continuum description of porous media but qualitatively matches well observations and predictions from discrete pore‐filling mechanisms. This suggests that pore‐scale physics controls saturation overshoot and in turn gravity‐driven fingering.

Energy-momentum transport-based simulator adapted to the general purpose semiconductor device simulation development tool CHORD

Application of the conventional drift–diffusion model to certain submicrometre devices may be inappropriate owing to hot carrier effects. These may include localized velocity saturation overshoot and impact ionization. Silicon device models based on the conservation of carrier momentum and energy have been shown to model such effects more accurately. However, to date, these energy-momentum transport (EMT)-based simulators have been limited in scope or designed for specific device application only. This paper describes the implementation of an EMT model into the modular semiconductor device simulation tool CHORD. The simulator fully couples energy-momentum transport equations for electrons and holes with the lattice heat equation in two space dimensions and time. The EMT model is designed to be consistent with the general purpose nature of the CHORD simulation system and thus can accommodate arbitrary device structure and bias conditions. A major strength in the CHORD system is that it allows for the addition of new models with minimal interface between the user and the simulator infrastructure. The addition of the EMT model to the existing set of models, already available to CHORD users, is a representative example of the procedures involved. The equations that define the EMT model are included along with results from simulations of an ultrashort channel MOSFET (metal oxide semiconductor field effect transistor) intended to highlight carrier heating and velocity saturation overshoot.