Integrated photonics is an emerging technology for many existing and future tele- and data communication applications. One platform of particular interest is Silicon Nitride (SiN) thanks to - amongst others - its very low-loss waveguides. However, it lacks active devices, such as lasers, amplifiers and photodiodes. For this, hybrid or heterogeneous integration is needed. Here, we bring high-speed uni-travelling-carrier photodiodes (UTC PDs) to a low-loss SiN-platform by means of micro-transfer-printing. This versatile technology for heterogeneous integration not only allows very dense and material-efficient III-V integration, it also eases fabrication yielding high-performance detectors. The waveguide-coupled photodiodes feature a responsivity of 0.3 A/W at 1550 nm, a dark current of 10 nA and a bandwidth of 155 GHz at a low bias. At zero bias, a record bandwidth of 135 GHz is achieved. We further demonstrate that this integrated detector can be used for direct photomixing at terahertz frequencies. A back-to-back communication link with a carrier frequency around 300 GHz is set up, and data rates up to 160 Gbit/s with low error vector magnitude (EVM) are shown, showcasing near-identical performance at zero bias.