The work focuses on ridge-geometry QCDs operating around 9.6 µm. This geometry is also compatible, in the longer term, with integrated photonic approaches. The objective is to better understand the interplay between the intrinsic carrier dynamics of the quantum cascade active region and the extrinsic limitations introduced by device geometry and associated parasitic elements. A first generation of devices is investigated through a comprehensive characterization combining current–voltage, capacitance–voltage, responsivity, noise, electrical rectification, optical frequency response, and data-transmission demonstrations. The results show that the QCD active region is intrinsically compatible with high-speed operation, but that the measured bandwidth is initially strongly limited by parasitic capacitances associated with the coplanar access. To overcome this limitation, a second generation of devices was designed on semi-insulating InP:Fe substrates, with redesigned coplanar waveguides and dedicated de-embedding structures. This new architecture strongly reduces radio-frequency parasitics and provides experimental access to multi-gigahertz bandwidths. The thesis combines analytical modeling, full-wave electromagnetic simulations, and calibrated microwave measurements to separate the intrinsic behavior of the device from effects related to its electrical environment. Beyond the specific devices studied, this work identifies the main design parameters for high-speed LWIR receivers based on QCDs.