Non-degenerate semiconductor nanoparticles with bulk plasma frequency in the terahertz frequency range are of substantial current interest for device and sensing applications in that part of the electromagnetic spectrum. The penetration of electromagnetic field in a material containing mobile charges is governed by two length scales, namely the Debye length and the skin depth. Their relative influence on the response of a semiconductor nanoparticle to an external terahertz electric field is examined in this Letter. A transport-based formulation to describe charge-field interactions by coupling the Boltzmann equation with the field equations is employed to characterize space-charge effects. Numerical results computed with the charge transport formulation reveal that plasmonic interactions in a semiconductor nanoparticle remains a surface phenomenon up to the surface plasmon resonance frequency, beyond which it evolves into a bulk phenomenon as the inertia effect of the charge carriers subdues their response to the field, which penetrates deeper into the particle as the frequency is increased. Examination of the spectra of charge, field, and current distributions allows for the identification of the influence of particle size, Debye length and skin depth on space charge interactions in a semiconductor nanoparticle.