In nanoscale devices, electron transport tends to become ballistic. Then, the current and the noise are mainly determined by the injection process. An electron injection model suitable for the semi-classical Monte Carlo (or time-dependent quantum) simulation of nanoscale devices with (or without electron) confinement is presented. While the injection model is conceptually quite similar to the boundary conditions used in the Landauer formalism, its mathematical description is quite different because it is developed for time-dependent scenarios. As an application, numerical data show that the signal-to-noise ratio and the bit-error probability are degraded in nanoscale transistors because of electron confinement.