Efficiency of Ferroelectric Field-Effect Transistors: An Experimental Study

While the theoretical maximum of the memory window
$\Delta {V}_{t}$
in a ferroelectric field-effect transistor (FEFET) is
$2{E}_{C}{t}_{F}$
,
${E}_{C}$
and
${t}_{F}$
being coercive field and FE thickness, respectively, experimentally
$\Delta {V}_{t}$
is observed to be much less than that, even in the case of complete polarization switching in the ferroelectric layer. This occurs because trapped charges in the gate oxide stack partially or completely screen the ferroelectric polarization, only a fraction of which gets “electrostatically” reflected in the semiconductor channel. In this article, we provide a generalized experimental framework to quantify the efficiency of practical FEFETs in converting the switched ferroelectric polarization into the memory window. To that end, we propose three efficiency metrics: 1) switched ferroelectric polarization
$\Delta {P}_{F}$
to memory window conversion ratio,
$\eta _{MW}=\Delta {V}_{t}/\Delta {P}_{F}$
; 2) switched ferroelectric polarization to the semiconductor charge (
$\Delta {Q}_{S}$
) conversion efficiency or the charge conversion efficiency
$\eta _{c}=\Delta {Q}_{s}/\Delta {P}_{F}$
; and 3) the ratio of memory window to the width of polarization versus gate voltage hysteresis characteristics (
$\Delta {V}_{P}$
) or the voltage conversation efficiency
$\eta _{V}=\Delta {V}_{t}/\Delta {V}_{P}$
. We measure and compare these efficiency metrics in n-type and p-type FEFETs, fabricated in different facilities by different groups. In all the cases, we find that the maximum values of memory window efficiency
$(\eta _{MW})_{max}$
, charge conversion efficiency
$(\eta _{c}$
)
$_{max}$
, and voltage conversion efficiency (
$\eta _{V}$
)
$_{max}$
are in the range: 2.5–5.5 Vm
²
/C, 4%–10%, and 5%–20%, respectively.