The generation of coherent terahertz radiation is an outstanding challenge at both scientific and technological levels. In one hand, the quest for table-top THz solutions based on integrated-circuit technology puts graphene and other bi-dimensional in the run. Plasma instabilities in bi-dimensional materials are an appealing mechanism for the production of low-power coherent THz signals. In this talk, we discuss how to produce plasmonic instabilities based on electronic injection only.