Semiconductor quantum dots are nano-particles showing remarkable optical properties from ultraviolet (UV) to infrared (IR) regions. Most of the quantum dots operating in visible or near-infrared regions utilize the electronic transitions between the valence and conduction sub-bands across the fundamental band-gap of the semiconductor material. In the infrared region, quantum dots are operated utilizing the electronic transitions between the sub-bands within the conduction (or valence) band. Here, various quantum dot infrared photodetector (QDIP) structures are presented. These can be operated as multi-band detectors in the mid-long-and far-infrared regions (terahertz). Specific QDIP architectures include dots-in-a-well (DWELL), tunneling quantum dot (T-QDIP), and superlattice quantum dot (SL-QDIP) structures. DWELL exhibits multi-color characteristics and offers wavelength tunability based on well parameters, instead of the quantum dot size. The main idea of T-QDIP is to reduce the dark current without reducing the photocurrent but can also be used for bias-selectable multi-band detectors, as well as for terahertz detectors. Similar to quantum dots, quantum ring photodetector structures (QRIPs) will also be presented, which were specifically designed for terahertz detection. As a second approach to achieve bias-selectable response peaks, a quantum well photodetector (QWIP) structure, known as npn-QWIP which also assembles two back-to-back connected p-i-n diodes with the p-region being common is discussed. A theoretical work supported by some preliminary experiments on the polarization sensitivity of QWIP coupled to 1D metal grids will also be discussed presented. The polarization extinction ratio is based on the polarization sensitivity of the diffraction grid, which depends on grid parameters, as well as the intrinsic polarization sensitivity of the photodetector itself. A low-cost photoconductive dual-band detector based on a ZnO film sensitized with lead sulfide quantum dots (PbS-QDs) is also presented. The UV response arises from the interband absorption of UV radiation by ZnO, and the IR response is due to the absorption in the PbS-QDs. Finally, as time permits, homojunction/heterojunction detector structures will be discussed with an emphasis on spin split-off detectors, which are operated based on light/heavy hole to split-off band transitions in p-type doped emitter layers at the interface of a heterojunction structure.