Residual gas inside field emission displays (FED) is the most important issue related to the device lifetime. Increasing the display area while maintaining the display thickness unchanged results in lifetime decrease, since the pressure gradient is fostered. Therefore, improvement of vacuum properties is a mandatory step towards large area displays. In a prior publication we have demonstrated that porous diamond membranes show good vacuum performance, while requiring low emitter switching voltage. In this work, we continue the porous membrane development by using polyimide as the base material for the membrane. The use of polyimide instead of diamond allows for easier production of large area porous FEDs. In addition, we present results of preliminary field emission experiments showing a direct correlation between the emitted current and the number of pores. This result strongly suggests that the emission sites are located at the pore edges in the polyimide membranes, similar to our observations for diamond membranes. From the theoretical point of view, we propose a new geometry: still based on the use of pores, but including a grid for triode mode operation. Finally, we present electron trajectory simulations that address some of the focusing issues in the proposed device.