As outlined in Section 7.2, the FEMFET is one of the most attractive designs of ferroelectric non-volatile memory cells, especially because of its low space consumption. However this compact design also increases the complexity of and accordingly the demands on a simulation tool.
The insertion of a material with a high permittivity has strong impact on the device performance. This became one of the key issues of device design and is being investigated heavily, with respect to material issues [LWK+98][MOS+99] and the performance point of view [CCR+99][CCV+99][IRCW99][KYY+98][YKL98].
The modification of the dielectric properties of the gate insulator
has visible consequences for device performance. Similarly to regular
CMOS devices an increase of the relative dielectric constant
shifts the
characteristics to the right. This
can be avoided by an increase of the oxide thickness which has in
turn a significant impact on the electrical behavior of the
device. Since the distance from the gate to the channel is
larger, this leads to a loss of control over the channel. The most
serious consequence of this effect is a significant increase of the drain
current in the OFF state of the transistor.
Next will be demonstrated that the simulation tool can keep up with the demands of TCAD by extraction of the significant device information like transfer characteristics, field distribution, and space charge density.
For the simulation an NMOS with
gate length was transformed
into a FEMFET, by entering a ferroelectric segment under the gate. In
order to keep parasitic effects between gate and drain/source and
between drain/source and channel low, two non-ferroelectric spacers are
placed on both sides of the gate. The geometry of the simulated
device is outlined in Fig. 8.14. Acceptor- and Donor
doping distribution are shown in Fig. 8.15
and Fig. 8.16, respectively.
The operating conditions for the transistor were chosen at values
similar to those of a CMOS transistor of the same gate length.
was
, bulk and source bias
, respectively.