We examine the injection of electron spins from ferromagnets (F) into quasi-two-dimensional electron systems (2DES’s) of semiconductors and employ the transfer-matrix formalism to obtain the carrier-density dependence of the conductances across F/InAs(2DES) single as well as F/InAs(2DES)/F double junctions in the ballistic limit. The Rashba spin-orbit interaction in the semiconductor and oblique modes in devices of finite widths are taken into account. We distinguish between spin-valve and spin-transistor geometry, in which the in-plane magnetization vectors in the ferromagnetic electrodes point perpendicular and parallel to the current direction, respectively. In case of the spin-transistor geometry, we find optimum coupling with the Rashba spin-precession state for injection straight along the channel. The distinct mismatch of majority and minority spin subbands in the ferromagnet to the band structure of the semiconductor causes spin-dependent scattering at the interface. This results in spin filtering. Within a Stoner model for ferromagnets spin filtering is stronger for iron than for Permalloy parameters and can be enhanced by an additional elastic scattering potential at the interface. In F/InAs(2DES)/F double junctions Fabry-Perot-type interferences are obtained. In case of spin-valve geometry they are due to particle interference, whereas in spin-transistor geometry spin itself is involved.