PIN is a common structure of electrical modulation in electro-optic modulator, and the performance of the electro-optic modulator is directly affected by the carrier injection in PIN structure. In previous studies, we have invented a novel structure of PIN electronic modulation based on SOI material. In the new structure, the SiGe material is adopted in the waveguide zone, therefore the double heterojunction PIN structure is formed in the horizontal direction. The carrier injection efficiency can be enhanced in the novel structure, and the power consumption could be reduced. In order to further study the modulation mechanism of the novel structure, based on the single heterojunction band theory, the barrier heights of the double heterojunction are analyzed, and the quantitative formulas of the barrier heights of the double heterojunction are given. It is shown that the barrier heights of the double heterojunction are related to the doping concentration, the band gap of material, the temperature, and the Ge content. The bands are compared between the novel structure, SiGe-OI structure and SOI structure to analyze the reason why the carrier injection of the novel structure could be enhanced. In the same conditions, the barrier heights of Si/SiGe/Si double heterojunction are minimal values, and those of SiGe and Si materials are second minimal value and maximal value, respectively. When the PIN device is set at a forward biased voltage (P region is the anode, and N region is the cathode), the balance between the carrier diffusion and the carrier drift is broken, and the PIN device is in a non-equilibrium state. According to the quantitative formula of the barrier heights of the double heterojunction, the barrier heights of Si/SiGe/Si double heterojunction are lower than that of SiGe-OI material, and the barrier height of SiGe material is lower than that of SOI material. It is shown that the barrier heights of Si/SiGe/Si double heterojunction could be flatten at first, so its PIN structure has the higher carrier injection than those of SiGe-OI and SOI under the same conditions. Finally, the band distribution of the novel structure and the relationships between the band distribution, the modulation voltage and the carrier injection are simulated. The results show that when the modulation voltage is 1 V, the carrier density of the novel structure arrives at 8×1018 cm-3, which is 800% higher than that of SOI structure, and 340% higher than that of SiGe-OI structure. The advantages of the novel structure are further indicated, and the correctness of the theoretical analysis is also verified.