  Electro optic modulators being key for many signal processing systems must
adhere to requirements given by both electrical and optical constrains.
Distinguishing between charge driven (CD) and field driven (FD) designs, we
answer the question of whether fundamental performance benefits can be claimed
of modulators based on emerging electro-optic materials. Following primary
metrics, we compare the performance of emerging electro-optic and
electro-absorption modulators such as graphene, transparent conductive oxides,
and Si, based on charge injection with that of the legacy FD modulators, such
as those based on lithium niobate and quantum confined Stark effect. We show
that for rather fundamental reasons, FD modulators always outperform CD ones in
the conventional wavelength scale waveguides. However, for waveguide featuring
a sub-wavelength optical mode, such as those assisted by plasmonics, the
emerging CD devices are indeed highly competitive.
