The purpose of the present invention is to improve the efficiency of conversion between terahertz electromagnetic wave energy and direct current energy via plasma waves in a terahertz electromagnetic wave conversion device with a field effect transistor structure. This invention has an HEMT structure having a substrate, an electron transit layer, an electron supply layer, a source and a drain, and includes a first and second group of gates. The gate length of each finger of the first group of gates is narrower than the gate length of each finger of the second group of gates, and each finger of each group of gates is disposed between the source and the drain on the same cycle. A first and second distance from each finger of the first group of gates to two fingers of the second group of gates adjacent to each finger are unequal lengths.