More researchers have recently paid much attention to the fabrication of field emission triode with a volcano-type gate on the silicon substrate because of their ease of fabrication and low cost. In this article, five different structures of this triode are presented. The electric field on the tip of field emitter (E(tip)) is calculated for these structures by using the EMAS software, and the different potential distribution and electric field distribution are obtained from these calculations. The results show that the diameter of the gate hole is important in determining E(tip) for this triode, Because the volcano-type gate holes fabricated by wet or dry etching have a very sharp rim, when the distance between anode and gate is small, a high electric field more than 1x10(7) V/cm is formed on the rim of gate hole at the given gate voltage V-g and anode voltage V-a.