A method for making three-dimensional structures comprises providing a plurality of components, each of which is moveably coupled to a substrate. According to one aspect, a material property gauge includes a nuclear density gauge for measuring the density of a material. The hydrogen-containing substance has a hydrogen gas equivalent concentration in the hydrogen-containing atmosphere of 3 to 20%. In other embodiments, the wheels are pivoted through 90 to the stowed state thereof. The donor is conveyed to the reactor and dissociated. In particular, the 3-terminal harmonic mixer in accordance with the present invention provides harmonically pumped anti-parallel diode mixing operation by utilizing the inherent characteristics of complementary transistors.