Microwave imaging of nanoelectronic devices has turned a simple reflection coefficient measurement, usually carried out by a 50-Ω vector-network analyzer, into a high-impedance instrumentation challenge. Interferometry-based reflectometers (IBR) have been found to be successful solutions in addressing this challenge. However, such solutions do not consider instrumentation of high impedance and high frequency as well as minimization of environment variations in a comprehensive manner. In this paper, these aspects are addressed jointly through the proposal of a fully integrated IBR in the STMicroelectronics BiCMOS 55-nm technology. Three varactor samples having a capacitance ranging from 0.65 to 0.95 fF are measured at 17.6 GHz for demonstration. The fully integrated IBR achieved a magnitude error below -35 dB, a phase error below 0.03°, and an accuracy better than 59.7 aF. Moreover, C-V slope measurement error is better than 2.8 aF, which is ten times smaller than found in the state-of-the-art IBR. Such betterment is explained by the monolithic integration of IBR and device-under-test as implemented in this paper.