  We develop a device model for p-i-n tunneling transit-time diodes based on
single- and multiple graphene layer structures operating at the reverse bias
voltages. The model of the graphene tunneling transit-time diode (GTUNNETT)
accounts for the features of the interband tunneling generation of electrons
and holes and their ballistic transport in the device i-section, as well as the
effect of the self-consistent electric field associated with the charges of
propagating electrons and holes. Using the developed model, we calculate the dc
current-voltage characteristics and the small-signal ac frequency-dependent
admittance as functions of the GTUNNETT structural parameters, in particular,
the number of graphene layers and the dielectric constant of the surrounding
media. It is shown that the admittance real part can be negative in a certain
frequency range. As revealed, if the i-section somewhat shorter than one
micrometer, this range corresponds to the terahertz frequencies. Due to the
effect of the self-consistent electric field, the behavior of the GTUNNETT
admittance in the range of its negativity of its real part is rather sensitive
to the relation between the number of graphene layers and dielectric constant.
The obtained results demonstrate that GTUNNETTs with optimized structure can be
used in efficient terahertz oscillators.
