Current-Assistance (CA) has emerged as a popular pathway for high-performance time-resolved imaging in the near-infrared spectrum. It demodulates photogenerated charges classically by injecting Ohmic majority currents in a low-resistivity epilayer, obtaining a uniform drift field deep into the photodetection volume. To reduce its power consumption, recent implementations utilize high-resistivity epilayers, which introduce strong nonlinearities in the resulting drift fields. This work provides a physical description of the distribution of drift fields in high resistivity silicon substrates by considering that the injected majority currents are in fact Space-Charge-Limited (SCL). A two-dimensional analytical model for the potential and electric field in the epilayer is derived and validated using TCAD drift-diffusion simulations, introducing the concept of the Virtual Cathode (VC) for CA. The SCL nature of the injected majority current is verified through I-V measurements. These insights establish a physical understanding for the design of future current-assisted photodetectors in high-resistivity epilayers.