Observation of mobility and velocity behaviors in ultra-scaled L-G=15 nm silicon nanowire field-effect transistors with different channel diameters

Abstract

Experimentally, two critical device performance factors, apparent mobility (mu(app)) and virtual source velocity (v(x0)) were investigated down to effective channel length (L-eff) = 15 nm silicon nanowire field-effect transistors (SNWFETs) by using virtual source (VS) model. Both mu(app) and v(x0) decreased in n-SNWFETs but increased in p-SNWFETs as the nanowire diameter (D-NW) shrank because of opposite effective mass (m(eff)) dependency. The critical on-current booster, v(x0) rather than mu(app) increased monotonically as L-eff shrank, and it showed that v(x0) boosting by L-eff scaling is still valid to L-eff = 15 nm in SNWFETs. Furthermore, p-SNWFETs had higher mu(app) and v(x0) than n-SNWFETs because compressive stress from SiGe layer below source/drain improved the performance of p-SNWFETs. Interestingly, unpredicted non-linearity of L-eff/mu(app) vs. 1/v(x0) plot was observed in short channel p-SNWFETs and its origin was discussed. Finally, thermal limit velocity (v(Tx)) and ballistic efficiency (B-sat) consisting v(x0) were extracted from experimental data. The D-NW dependence of v(Tx) and B-sat was analyzed using stress effect, m(eff), critical length (L-C), and mean free path (lambda), which provides the way of v(x0) boosting.