Abstract: Using polycrystalline silicon (poly-Si) as the cathode material to prepare planar electron tubes shows the advantages of straightforward fabrication process and compatibility with integrated circuit (IC) technology. This approach simplifies the device manufacturing process, reduces production costs, and holds potential for advancing the development of planar electron tube ICs. To explore the field emission properties and modulation mechanisms of planar electron tubes with poly-Si cathodes, this study investigates the variation in electron density within poly-Si thin films under back-gate modulation by measuring the current passing through them. Additionally, the influence of the drain voltage on the modulation effect is examined. Based on these findings, the relationship between the field emission current of poly-Si cathodes and the back-gate modulation voltage is analyzed, demonstrating the ability to control the field emission current of poly-Si cathodes via the back-gate. A positive correlation is observed between the field emission current characteristics of poly-Si cathodes and their electron concentration. These results lay a foundation for the advancement of poly-Si-based planar electron tubes.