Tunnel field-effect transistors have the potential to overcome the subthreshold slope limit of the conventional metal-oxide-semiconductor field effect transistors at room temperature. They allow to further scale down the supply voltage, threshold voltage and power consumption of the integrated circuits. The one of the main challenges of group - I V - based TFETs is the unsatisfactoryon-current due to the phonon - assisted tunneling through the large indirect bandgaps. This article is devoted to discuss the direct and indirect band-to-band tunneling in group-IV semiconductors.