In this article, we report a Si/Ge waveguide phototransistor with high responsivity and low dark current under low bias voltages, due to an engineered electric field distribution. The photodetector consists of n-i-p-i-n doping regions and shows a responsivity of 606 A/W at 1 V bias, and 1032 A/W at 2.8V bias with an input optical power of -50 dBm, and dark current of 4 µA and 42 µA respectively. This is achieved by placing two p+-doped regions in the silicon slab region beneath the Ge epitaxial layer. A measured small signal -3 dB bandwidth of 1.5 GHz with a -80 dBc/Hz phase noise response at 1 KHz frequency offset were demonstrated experimentally.