Simulation studies for the field electron emission from double-gate emitters with planar cathodes were carried out using the finite-element method, the Fowler-Nordheim field-emission equation, and the equation of motion for electrons. We systematically investigated radial position dependence of electric field, radial distribution of emission current, and trajectories of emitted electrons for various double-gate geometries and bias configurations. In particular, we studied the simplest operation mode of double-gate emitters, grounding both the cathode and the second gate adjacent to it, and the dependence of emission-electron focusing on the vertical position and the negative biasing of the second gate in detail. The flexibility in double-gate-emitter operation due to the separate control of electron emission and focusing was also discussed. (C) 2004 American Vacuum Society.