Page 39 - Epistle 2018 ISSUE 1

The negative capacitance FET (NCFET) proposed by Supriyo
Datta et. al is an alternative steep slope device to

overcome the classical limit of 60mV/dec. The switching

energy of the device can be reduced by replacing the
insulator with ferroelectric (FE) materials. The basic

structure of NCFET with ferroelectric material as gate
oxide . The underlying mechanism for sub-60 mV/decade

operation of an NCFET is the passive amplification of the

gate voltage at the interface between the ferroelectric gate
oxide and the semiconductor channel. The voltage drop

across the ferroelectric gate oxide is a negative quantity as

ferroelectric capacitance is negative (CF<0) which increases
the charge density in the channel. Thus it acts as an

external amplifier to MOSFETs and also achieves

subthreshold slope of less than 60mV/decade. This
principle reduces the gate voltage required to turn the

transistor ON the NCFET. Von,D is the turn-ON voltage for

a conventional MOSFET and Von,F for an NCFET.

One of the key challenges of NCFET technology is to
achieve steep sub-threshold swing due to the variations in

substrate capacitance, CS. To achieve steep SS, the

capacitance CF and CS shown in Fig. 1 should be closer ,
however the variation of CS is large compared with CF in

the sub-threshold regime. The scaling will necessitate

thinner FE layer and resulting FE capacitance is too large
and hence there will barely be any NC effects . To improve

SS, it is required to use thicker FE which is difficult to

realize in the scaled FETs .