  Field-effect transistors coupled to integrated antennas (TeraFETs) are
photodetectors being actively developed for the THz frequency range ($\sim$ 100
GHz - 10 THz). Among them, Graphene TeraFETs (G-TeraFETs) have demonstrated
distinctive photoresponse features compared to those made from elementary
semiconductors. For instance, previous studies have shown that G-TeraFETs
exhibit a THz response that comprises two components: the resistive self-mixing
(RSM) and photothermoelectric effect (PTE). The RSM and PTE arise from carrier
density oscillations and carrier heating, respectively. In this work, we
confirm that the photoresponse can be considered a combination of RSM and PTE,
with PTE being the dominant rectification mechanism at higher frequencies. For
our CVD G-TeraFETs with asymmetric antenna coupling, the PTE response dominates
over the RSM at frequencies above 100 GHz. We find that relative contribution
of RSM and PTE to the photoresponse is strongly frequency dependent.
Electromagnetic wave simulations show that this behavior is due to the relative
change in the total dissipated power between the gated and ungated channel
regions of the G-TeraFET as the frequency increases. The simulations also
indicate that the channel length over which the PTE contributes to the
photoresponse below the gate electrode is approximately the same as the
electronic cooling length. Finally, we identify a PTE contribution that can be
attributed to the contact doping effect in graphene close to the metal
contacts. Our detectors achieve a minimum optical noise-equivalent power of 101
(114) pW/$\sqrt{Hz}$ for asymmetric (symmetric) THz antenna coupling conditions
at 400 GHz. This work demonstrates how the PTE response can be used to optimize
the THz responsivity of G-TeraFETs.
