A comprehensive analysis and numerical simulation of continuous wave terahertz (THz) generation based on photomixing is performed in this paper. For numerical investigation, both semiconductor and electromagnetic equations are considered. The effects of photomixer parameters and geometry of the antenna on terahertz power generation (photocurrent and radiated power) are investigated. In this work, for improving performance of the integrated antenna, the fractal tree antenna for two different geometries (the narrow and wide configurations) using the Fibonacci sequence is presented. We demonstrate that the fractal tree antenna with Fibonacci series arrangement in the both cases of narrow and wide configurations increases the THz radiation power more than twice in comparison to usual dipole antenna.