We discuss the basis of a set of quantum hydrodynamic equations and the use of this set of equations in the two-dimensional simulation of quantum effects in deep submicron semiconductor devices. The equations are obtained from the Wigner function equation-of-motion. Explicit quantum correction is built into these equations by using the quantum mechanical expression of the moments of the Wigner function, and its physical implication is clearly explained. These equations are then applied to numerical simulation of various small semiconductor devices, which demonstrate expected quantum effects, such as barrier penetration and repulsion. These effects modify the electron density distribution and current density distribution, and consequently cause a change of the total current flow by 10-15 per cent for the simulated HEMT devices. Our work suggests that the inclusion of quantum effects into the simulation of deep submicron and ultra-submicron semiconductor devices is necessary.