Finger capacitance of a terahertz photomixer in low-temperature-grown GaAs using the finite element method

Interdigitated finger capacitance of a continuous-wave terahertz photomixer is calculated using the finite element method. For the frequently used electrode width (0.2 mu m) and gap width (1.8 mu m), the finger capacitance increases quasi-quadratically with the number of electrodes increasing. The quasi-quadratic dependence can be explained by a sequence of lumped capacitors connected in parallel. For a photomixer composed of 10 electrodes and 9 photoconductive gaps, the finger capacitance increases as the gap width increases at a small electrode width, and follows the reverse trend at a large electrode width. For a constant electrode width, the finger capacitance first decreases and then slightly increases as the gap broadens until the smallest finger capacitance is formed. We also investigate the finger capacitances at different electrode and gap configurations with the 8 mu m x 8 mu m photomixer commonly used in previous studies. These calculations lead to a better understanding of the finger capacitance affected by the finger parameters, and should lead to terahertz photomixer optimization.