Terahertz (THz) waves refer to the electromagnetic radiation in the frequency range from 0.1 to 10 THz, which corresponds to the wavelengths from 3 mm to 30 μm, respectively. This spectral region, called also as “T-gap”, is important for many practical applications, including THz imaging, chemical and biological sensing, high-speed telecommunication, security and medical applications. Here we report the results of a numerical study of quantum transport in ZnO-based resonant-tunneling diodes (RTDs) and quantum cascade lasers (QCLs) with different design schemes. We found that by varying and optimizing constituent layer widths and doping level these quantum structures, high power performance of THz RTDs and QCLs can be achieved at room temperature. Moreover, it was established also that the ZnO-based terahertz sources can cover the spectral region of 5-12 THz, which is very important for THz imaging and detection of explosive materials, and which could be not covered by conventional GaAs-based terahertz devices.