We propose and evaluate the vertical cascade terahertz and infrared photodetectors based on multiple-graphene-layer (GL) structures with thin tunnel barrier layers (made of tungsten disulfide or related materials). The photodetector operation is asso
ciated with the cascaded radiative electron transitions from the valence band in GLs to the conduction band in the neighboring GLs (interband- and inter-GL transitions). We calculate the spectral dependences of the responsivity and detectivity for the vertical cascade interband GL- photodetectors (I-GLPDs) with different number of GLs and doping levels at different bias voltages in a wide temperature range. We show the possibility of an effective manipulation of the spectral characteristics by the applied voltage. The spectral characteristics depend also on the GL doping level that opens up the prospects of using I-GLPDs in the multi-color systems. The advantages of I-GLPDs under consideration are associated with their sensitivity to the normal incident radiation, weak temperature dependence of the dark current as well as high speed of operation. The comparison of the proposed I-GLDs with the quantum-well intersubband photodectors demonstrates the superiority of the former, including a better detectivity at room temperature and a higher speed. The vertical cascade I-GLDs can also surpass the lateral p-i-n GLDs in speed.
We study the processes of the electron and hole injection (double injection) into the i-region of graphene-layer and multiple graphene-layer p-i-n structures at the forward bias voltages. The hydrodynamic equations governing the electron and hole tra
nsport in graphene coupled with the two-dimensional Poisson equation are employed. Using analytical and numerical solutions of the equations of the model, we calculate the band edge profile, the spatial distributions of the quasi-Fermi energies, carrier density and velocity, and the current-voltage characteristics. In particular, we demonstrated that the electron and hole collisions can strongly affect these distributions. The obtained results can be used for the realization and optimization of graphene-based injection terahertz and infrared lasers.
We propose the concept of terahertz (THz) photomixing enabled by the interband electron transitions due to the absorption of modulated optical radiation in double-graphene layer (double-GL) structures and the resonant excitation of plasma oscillation
s. Using the developed double-GL photomixer (DG-PM) model, we describe its operation and calculate the device characteristics. The output power of the THz radiation exhibits sharp resonant peaks at the plasmonic resonant frequencies. The peak powers markedly exceed the output powers at relatively low frequencies. Due to relatively high quantum efficiency of optical absorption in GLs and short inter-GL transit time, the proposed DG-PM operating in the resonant plasma oscillation regime can surpass the photomixers based on the standard heterostructures .
A multiple-graphene-layer (MGL) structure with a stack of GLs and a highly conducting bottom GL on SiC substrate pumped by optical radiation is considered as an active region of terahertz (THz) and far infrared (FIR) lasers with external metal mirror
s. The dynamic conductivity of the MGL structure is calculated as a function of the signal frequency, the number of GLs, and the optical pumping intensity. The utilization of optically pumped MGL structures might provide the achievement of lasing with the frequencies of about 1 THz at room temperature due to a high efficiency.
We present an analytical device model for a graphene bilayer field-effect transistor (GBL-FET) with a graphene bilayer as a channel, and with back and top gates. The model accounts for the dependences of the electron and hole Fermi energies as well a
s energy gap in different sections of the channel on the bias back-gate and top-gate voltages. Using this model, we calculate the dc and ac source-drain currents and the transconductance of GBL-FETs with both ballistic and collision dominated electron transport as functions of structural parameters, the bias back-gate and top-gate voltages, and the signal frequency. It is shown that there are two threshold voltages, $V_{th,1}$ and $V_{th,2}$, so that the dc current versus the top-gate voltage relation markedly changes depending on whether the section of the channel beneath the top gate (gated section) is filled with electrons, depleted, or filled with holes. The electron scattering leads to a decrease in the dc and ac currents and transconductances, whereas it weakly affects the threshold frequency. As demonstrated, the transient recharging of the gated section by holes can pronouncedly influence the ac transconductance resulting in its nonmonotonic frequency dependence with a maximum at fairly high frequencies.
