Nanowire Diameter Dependency of the Variability in n/p Silicon Nanowire FETs With Ultrashort Gate Length of 15 nm

In this study, the nanowire diameter (
${D}_{\text{NW}}$
) dependency of the variability of n/p silicon nanowire field-effect transistors was experimentally investigated in silicon nanowire field-effect transistors (SNWFETs) with an ultrashort gate length (
${L}_{\text{G}}$
) of 15 nm and
${D}_{\text{NW}}$
of 7, 9, and 12 nm. The ON current at linear region (
${I}_{\text{on}\_{}\text{lin}}$
) variation was decomposed into three performance parameters—threshold voltage (
${V}_{\text{th}}$
), carrier mobility (
$\mu _{0}$
), and source/drain series resistance (
${R}_{\text{SD}}$
)—using the error propagation equation. The results indicated that a moderate decrease in
${D}_{\text{NW}}$
improves gate controllability, reducing variation in
${V}_{\text{th}}$
and
${I}_{\text{on}\_{}\text{lin}}$
. However, when
${D}_{\text{NW}}$
was excessively reduced, the
${I}_{\text{on}\_{}\text{lin}}$
variation increased because random variations and quantum confinement (QC) effects caused fluctuations in
${V}_{\text{th}}$
and
${R}_{\text{SD}}$
. The effect of gate-controllability improvement was dominant in p-SNWFETs, whereas the effect of QC and
${R}_{\text{SD}}$
variation was dominant in n-SNWFETs. Consequently, the
${R}_{\text{SD}}$
(
${V}_{\text{th}}$
) contribution to
${I}_{\text{on}\_{}\text{lin}}$
variation rapidly increased (decreased) from 15% (64%) at
${D}_{\text{NW}}$
of 7 nm to 54% (41%) at
${D}_{\text{NW}}$
of 12 nm in n-(p-)SNWFETs. Overall, the
${I}_{\text{on}\_{}\text{lin}}$
variation was smallest at
${D}_{\text{NW}}$
of 9 nm (7 nm) in n-(p-) SNWFET. The optimal
${D}_{\text{NW}}$
occurred at a point where
${V}_{\text{th}}$
and
${R}_{\text{SD}}$
variations were balanced, and this point depended on the dopant profile of SNWFETs. This suggests that the variability improvement strategy should differ depending on the
${D}_{\text{NW}}$
and dopant profile of ultrascaled channel FETs.