Book excerpt: Semiconductor devices have transformed the world through tremendous technological advances in all aspects of life imaginable. An important aspect of the research into improving these devices is computer-aided simulation and modeling of their electrical behavior. The ability to study theoretically semiconductor devices allows us to predict their behavior as well as optimize their performance before having to physically fabricate the device, saving us money and time. To this end, we have developed a novel approach, based on the effective mass approximation, to study theoretically quantum transport, both ballistic and dissipative, in realistic semiconductor devices. Our model takes into account quantum confinement and other non-local quantum effects affecting electronic transport in the current and near future generations of transistors. As an example of application, we have studied the electrical behavior of well-known silicon field-effect transistors (FETs) and the factors affecting their performance.