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Theoretical and experimental investigations of integrated p-i-n structures designed for quesi-optical millimeter and submm modulators in oversized waveguides are presented. These structures possess the fransparency in regime without injection and good operational characteristics in presence of injecting current. Also a possibility to control the transverse structure of initial beam by means of non-un form injection current is numerically demonstrated.Silicon surface-oriented p-i-n-structures are well known as basic elements for microwave modulators in single-wave waveguide channel. These structures are made of silicon plate where the set of injecting pT4, n+-i junctions is formed in the one side of the plate (see Fig.1). Integrated p-i-n-structures with deep injecting junctions (b = 10 ... 25 mkm) are the most perspective because they combine the preferences of planar geometry (wide operating frequency range) and volume p-i-n diodes (the great value of modulation under moderate controlling currents). These structures demonstrate good operational Characteristics in 20 = (8 ... 2 ) mm wavelength region. But for shorter wavelengths 20 d 2 mm, the transverse sizes of single-mode metallic waveguides become smaller, and, therefore, the limitations connected with possible integral power levels and the degree of dissipation occur. Apparently, in the range of wavelengths 10 5 2 mm the perspective transmission line is multimode oversized metallic multimode waveguide with the typical transverse sizes 2 O.5cm. In this case, there are no serious limitations for integral microwave power levels transmitted by waveguide and the losses can be decreased too. Surface-oriented integrated p-i-nstructures also can be utilized in quasi-optical modulators placed into oversized waveguides but their characteristics differ from ones in single-mode waveguides (A, -8 mm).The main demands are the next: 1. The quasi-optical modulators must provide the transmission coefficients that are close to unity for all modes of oversized waveguide in wide frequency range, when the controlling current is absent. 2. The degree of modulation must be quite great for the moderate densities of controlling current. transmit microwave beams without essential distortion in regime without controlling current and provide modulation of these beams in the presence of it. This leads to limitations for transverse sizes and shape of injecting pc-i and n' -i junctions, and for the thickness of structure.By another words, the integrated p-i-n-structures designed for quasi-optical modulators must Of course, these integrated p-i-n-structures can effectively control only linearly polarized modes (U,) because when the incident wave beam has component of electric field directed along the electrodes the reflection coefficients can be quite large even without injection. Besides, the thickness of i-layer (base) w between injecting electrodes must be no greater than the diffusion length LD = (D.#I2: w I LD where D is the ambipolar diasion coefficient, z is the lifetime of carrier...