A radiation detector made of High Purity Germanium (HPGe) has been specially machined to be this invented multilayer Inter-Coaxial configuration. With this special configuration, extra large volume HPGe detectors of diameters to be 6 inches, 9 inches, and even 12 inches, can be produced with current achievable HPGe crystal purity and quality, in which the entire detector crystal will be depleted and properly over biased for effective photo-induced signal collection with just less than 5000V bias applied. This invention makes extra large efficiency of 200%, 300%, and maybe even higher than 500% possible with HPGe gamma ray detectors with reasonable great resolution performances procurable based on current HPGe crystal supply capability. The invention could also be applied to any other kind of semiconductor materials if any of them could be purified enough for this application in the future.