PIN tunneling field effect transistor (TFET) is one of the most promising devices due to its low sub-threshold swing. In this paper, using TCAD simulation, we investigate the doping and structure dependence of the electric field in PIN TFET. We show that an insertion of a thin N layer into PIN structure (i.e., PNIN TFET) not only enhances the drive current but also improves the reliability of the device. We also show that the device characteristics can be further improved by properly aligning the gate electrodes with respect to the tunneling junction.