New insights into carrier transport in n-MOSFETs

Abstract

This paper discusses recent experimental investigations of the relation between low-field effective mobility and effective injection velocity of electrons from the source into the channel, as manifested in current drive, of deeply scaled n-MOSFETs. It is first established that the effective velocity in electrostatically sound, “well-tempered” scaled devices, for example with drain-induced barrier lowering (DIBL) limited to 120 mV/V, is well below the theoretical fully ballistic injection velocity. This is consistent with the fact that, as the channel length is scaled and the longitudinal field increases, preservation of electrostatic integrity requires increasing transverse field, which leads to increased surface scattering and therefore decreased mobility. In addition, evidence is presented that the effective channel mobility in modern short-channel devices is further decreased, probably due to increased ionized dopant scattering in the heavily doped channel halos. Then a correlation range of 45–60% between effective injection velocity and low-field mobility is established experimentally in sub-50-nm-channel MOSFETs. All of these factors point to the possibility of increasing the performance of deeply scaled n-MOSFETs by pursuing enhanced channel-mobility device structures such as double-gate MOSFET, or materials such as strained Si on relaxed SiGe.