This article provides a significant contribution to understanding current transport in boron-doped diamond under high electric fields, with direct implications for diamond-based power electronics. Using pulsed measurements and finite element modeling, the study reveals that the observed exponential and super-exponential current-voltage (I-V) behavior—including negative differential resistance (NDR)—results from a combination of impurity impact ionization (III) and self-heating effects (SHE). Experimental techniques, including transient interferometric thermal mapping, confirm thermal runaway phenomena and localized heating. Simulation results incorporating III and SHE accurately replicate experimental I-V curves, establishing a comprehensive model for charge transport in highly doped diamond. These findings advance the design of diamond-based electronic devices by clarifying high-field conduction mechanisms, offering new insights into achieving reliable high-performance operation in extreme conditions.