The reaction center (RC) protein from photosynthetic purple bacteria is an organic structure with the capability of absorbing photons at low light intensities and generating electron-hole pairs with a high efficiency. Application of this biomaterial for energy harvesting and sensor devices has been studied before. A key in employing RCs in an electrochemical device is to immobilize the proteins on an electrode. In this work, ion-sensitive field-effect transistors with Si<sub>3</sub>N<sub>4</sub> and TaO<sub>2</sub> gate insulator were tested to measure the success rate in immobilizing the proteins. The results show that by far Si<sub>3</sub>N<sub>4</sub> is a better choice than TaO<sub>2</sub>, due to the effective self-assembly of the linker molecules. The density of the attached proteins to the Si<sub>3</sub>N<sub>4</sub> transistor was estimated to be 5×10<sup>9</sup> proteins/cm<sup>2</sup> by analyzing the drift in the threshold voltage of the transistor. The fabricated device also presented the feasibility of using the RCs in an integrated photo-transistor.