In this article, a first-principles based simulation framework is presented to project the performance of a novel 2-D field-effect transistor (FET) under the ballistic limit. Our framework consists of: 1) density functional theory modeling of the novel 2-D material that gives accurate electronic structure without requiring parameters; 2) mode-space transformation; 3) spectral adjustment to maximize computational efficiency; 4) extraction of the dielectric constant of the novel 2-D material using a first-principles approach; and 5) nonorthogonal nonequilibrium Green's function method for accurate quantum transport simulations. We have applied our framework to evaluate the device performance of novel silicene/gallium phosphide (Si/GaP) heterobilayer FETs. Our results reveal that Si/GaP FETs have a great potential for high-performance logic devices, with high ON-state current, low subthreshold swing, and high speed with small dynamic power consumption.