A novel selective etching method was employed to construct monolithic GaAs/AlGaAs microvacuum structures for transmission-mode photoemitters. These emitters consisted of a photon absorption layer with a window and top contact layers supported by micropillars of about 140 mu m(2) in cross section areas, a 2-mu m-wide vacuum spate, and an electron collector. Part of the photoelectrons generated in the absorption layer were emitted into the vacuum space inside the microvacuum structure. They were subsequently accelerated by an applied reverse bias across the vacuum space and collected by a conductive GaAs substrate. The electron emitting surface was protected against contaminants from air by sulfur radicals by a H2SO4 solution treatment. After thermal cleaning at 475 degrees C and activation with Cs, the microvacuum emitter exhibited an external quantum efficiency of 0.33% at 650 nm wavelength. Photocurrent was a linear function of the power of incident light, and the dark current of the microvacuum emitters remained below 90 pA/cm(2) at 20 degrees C. Experimentally observed spectral quantum efficiencies of the microvacuum and reflection-mode emitters are compared with theoretical diffusion model calculations.