MOSFET Modeling into the Ballistic Regime

Based on the first moment of the Boltzmann equation, drift-diffusion (DD) is only strictly valid for the low field near-equilibrium conditions found in long channel MOSFETs. However, despite its well-documented inadequacies, it has been found to be surprisingly accurate for channel lengths (L sub CH) as short as 0.1 micron. Although full CMOS processes with effective channel lengths (L sub eff) 90 nm and experimental nMOS/pMOS down to 25 nm which exhibit near ballistic transport have recently become available, no systematic assessment of the applicability of the DD equations at these extremes exists. It is the purpose of this paper to clarify the accuracy of DD models as L sub eff is scaled as short as 30 nm by means of comparison to self-consistent Monte Carlo (MC) simulations. The MC model has been carefully calibrated to yield bulk field dependent velocities which are nearly identical to those in the DD model, thereby directly highlighting the errors in the DD transport model. Different scaling trends and discrepancies as large as 40% in I sub on and 50% in g sub m in nMOS devices have been found, and the sources of these errors are identified. Results for pMOS will also be presented.