We present first-principles density functional theory (DFT) based quantum transport simulations of nanoscale field effect transistors made of Ge, Si, strained-Si, and few-layer black phosphorus channels. The effects of atomistically modeled, crystalline/amorphous SiO2 gate dielectrics on device performance are investigated. A spectral adjustment technique is developed to overcome the band gap underestimation problem of DFT and applied to simulations of tunnel field effect transistors.