We present experimental results of metamaterials operating at terahertz and mm-wave frequencies. Metamaterials consist of a single layer of 200nm thick gold on a doped or undoped semiconducting substrate. By optical and electronic doping of supporting semiconducting substrates we show external control of planar arrays of metamaterials, characterized with terahertz time domain spectroscopy. Both methods yield metamaterial / semiconductor devices which can be utilized as switches or modulators, enabling modulation of THz Transmission by 50 percent. Experiments are supported by simulations and results agree well. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly play a key role in future demonstrations of novel high-performance devices.