The electronic noise of the front-end preamplifier can potentially limit the energy and position resolutions of radiation sensors, a detrimental effect that can be diminished by the use of on-chip amplification. The implementation of an avalanching structure upon a direct-conversion semiconductor-based radiation detector is modeled and demonstrated using high-resistivity silicon. The avalanche particle sensor configuration is designed analytically, and we validated the design with numerical process and device simulations. Process and device simulation results from the sensors modeled with various geometries, doping profile arrangements, and fabrication conditions are reported. We tested the refined numerical design by fabricating and testing diagnostic sensors. Multiplication junctions, based on a junction termination extension design, were created from micrometer-scale highly-doped layers on silicon wafers using both diffusion and ion implantation techniques. For k α =0.2 device), the degree of improvement limited by the leakage current of the devices, and within a factor of 2 of the ultimate Fano limit (0.53%).