In this study, a numerical simulation of the tunnel effect in the n-GaN/p-Si heterostructure has been performed. Variations of band diagrams, current-voltage characteristics and cutoff frequencies of the diode heterostructures under study were obtained depending on the doping levels of GaN and Si. The dopant concentration values were found for implementing backward and tunnel diode modes. In the tunnel diode mode, the peak current density and maximal generation frequency were 24.6 kA/cm2 and 17 GHz, respectively.