Investigation of impact ionization in strained-Si n-channel metal-oxide-semiconductor field-effect transistors

In this study, we have systematically re-investigated impact ionization (II) characteristics in strained-Si n-channel metal-oxide-semiconductor field-effect transistors (nMOSFETs) with different strained-Si cap layers at two Ge contents. The strained-Si nMOSFETs can supply further II experimental conditions with band-gap energy narrowing, higher electron mobility, and greater scattering caused by the Ge out-diffusion effect. Despite such II conditions, no marked difference in the II multiplication coefficient as a function of drain voltage, M - 1(V-D), between unstrained- and strained-Si nMOSFETs is found for widely accepted strain-enhanced II efficiency, implying that II efficiency depends on the maximum channel electric field E-m in the pinch-off region. Through the translation of M - 1(V-D) into M - 1(E-m), it is found that strain-enhanced II efficiency is attributed to the narrowing of band-gap energy, taking into account the difference in source/drain junction depth between unstrained- and strained-Si nMOSFETs.