Thermoelectric energy conversion technology attracts attention to convert waste heats from various sources, e.g., electric power plants, factories, automobiles, computers, and even human bodies, into electricity. Since the power factor of a thermoelectric material must be maximized by means of carrier concentration, thermoelectrician usually fabricates many samples to find a good thermoelectric material. On the other hand, nanostructuring of thermoelectric materials such as superlattices with 2DEG is known as an excellent approach to enhance the thermopower. However, fabrication of superlattices using state-of-the-art thermoelectric materials is extremely difficult due to their complicated crystal structures. Further, the production cost of such superlattice materials is extremely high. To overcome this issue, we propose an epoch-making method to enhance thermopower of a thermoelectric material. We fabricated a field effect transistor structure and modulated the 2DEG thickness by applying a gate electric field. An electric field application provides an extremely thin (~2 nm) 2DEG, which exhibits unusually large thermopower, approximately five times larger than that of the bulk. This method does not depend on the crystal structure, and can truly applicable for exploration of high performance thermoelectric materials with complicated crystal structures. Additionally, this approach may accelerate the development of nanostructures of high performance thermoelectric materials.