A comparison of wave function penetration effects on ballistic drain current (I-D) in nanoscale double gate (DG) MOSFETs fabricated on (100) and (110) silicon surfaces is presented. MOS electrostatics is determined from the self consistent solution of 2D Schrodinger and Poisson equations and I-D is calculated assuming that the ballistic transport through each subband is independent. Numerical results show that I-D is much more sensitive to wave function penetration in (I 10) DG MOSFETs than in (100) DG MOSIFETs. This difference is due to the much heavier quantization effective mass in the longitudinal valley of (100) silicon relative to that in the fourfold valley (Delta(4)) of(110) silicon. It is also observed that in (100) devices, penetration effect is not sensitive to device scaling, whereas in (110) devices, penetration effect increases significantly with device scaling. The wave function penetration effect on I-D may be represented in terms of a reduction of the threshold voltage. (C) 2009 Elsevier Ltd. All rights reserved.