Terahertz (THz) spectroscopy is a great tool not only to study fundamental physical processes such as many-body Coulomb interaction but also to develop ultrafast electronic devices. More specifically, intense THz fields interacting with semiconductors have exhibited strong nonlinear effect involving extreme carrier dynamics. In this experiment, intense THz radiation was applied to nanoantennas on a gallium arsenide wafer. The nanoantenna structure causes the incident THz field to be tremendously enhanced (field enhancement factor superior to 50). This huge field enhancement allows the THz fields to generate charge carriers of high density (Ne > 10^20 cm^-3) via interband excitation associated with Zener tunneling and impact ionization. The high-density free electrons induce strong THz absorption, up to 35%. This shows the potential application of metal-semiconductor hybrid nanostructure in the development of optical modulators and active switching devices.